Hi Chao,

This is really hard to review and risky a lot to apply it shortly. Do we have a
strong reason we have to do this? The original design goal was to minimize
allocation delay which is almost zero for now. Of course, I agreed that there'd
be some trade-off though, we don't have a critical issue with this, FWIW.
Can we expect to see some speed gains with this? How much?

Thanks,

On 11/30, Chao Yu wrote:
> Previously, we use free nid list to manage free nid entry, so during nid
> allocation, we can just pick up one entry from list header, which has
> quite low overhead.
> 
> But sadly, during initialization of free nid list, we should do lookup
> combining with lots of different inner caches, including NAT page cache,
> nat entry cache, curseg journal cache and free nid bitmap, so flow became
> quite complicated.
> 
> In this patch, we choose to obsolete old free nid management approach,
> instead, we use existing free nid bitmap which has the same functionality
> to manage free nid, in order to make free nid management codes more easy
> to maintain.
> 
> Signed-off-by: Chao Yu <yuch...@huawei.com>
> ---
> v4: clean up codes.
>  fs/f2fs/checkpoint.c |   1 -
>  fs/f2fs/debug.c      |   7 +-
>  fs/f2fs/f2fs.h       |   9 +-
>  fs/f2fs/inode.c      |   2 +
>  fs/f2fs/node.c       | 487 
> +++++++++++++++++----------------------------------
>  fs/f2fs/node.h       |  22 ---
>  fs/f2fs/segment.c    |   5 -
>  fs/f2fs/shrinker.c   |  14 --
>  8 files changed, 164 insertions(+), 383 deletions(-)
> 
> diff --git a/fs/f2fs/checkpoint.c b/fs/f2fs/checkpoint.c
> index d1f160ae4959..885525a0d981 100644
> --- a/fs/f2fs/checkpoint.c
> +++ b/fs/f2fs/checkpoint.c
> @@ -1024,7 +1024,6 @@ static void __prepare_cp_block(struct f2fs_sb_info *sbi)
>       struct f2fs_nm_info *nm_i = NM_I(sbi);
>       nid_t last_nid = nm_i->next_scan_nid;
>  
> -     next_free_nid(sbi, &last_nid);
>       ckpt->valid_block_count = cpu_to_le64(valid_user_blocks(sbi));
>       ckpt->valid_node_count = cpu_to_le32(valid_node_count(sbi));
>       ckpt->valid_inode_count = cpu_to_le32(valid_inode_count(sbi));
> diff --git a/fs/f2fs/debug.c b/fs/f2fs/debug.c
> index a66107b5cfff..413e031b10c4 100644
> --- a/fs/f2fs/debug.c
> +++ b/fs/f2fs/debug.c
> @@ -100,9 +100,8 @@ static void update_general_status(struct f2fs_sb_info 
> *sbi)
>       si->dirty_nats = NM_I(sbi)->dirty_nat_cnt;
>       si->sits = MAIN_SEGS(sbi);
>       si->dirty_sits = SIT_I(sbi)->dirty_sentries;
> -     si->free_nids = NM_I(sbi)->nid_cnt[FREE_NID];
> +     si->free_nids = NM_I(sbi)->available_free_nids;
>       si->avail_nids = NM_I(sbi)->available_nids;
> -     si->alloc_nids = NM_I(sbi)->nid_cnt[PREALLOC_NID];
>       si->bg_gc = sbi->bg_gc;
>       si->util_free = (int)(free_user_blocks(sbi) >> sbi->log_blocks_per_seg)
>               * 100 / (int)(sbi->user_block_count >> sbi->log_blocks_per_seg)
> @@ -233,10 +232,6 @@ static void update_mem_info(struct f2fs_sb_info *sbi)
>                       atomic_read(&SM_I(sbi)->dcc_info->discard_cmd_cnt);
>       }
>  
> -     /* free nids */
> -     si->cache_mem += (NM_I(sbi)->nid_cnt[FREE_NID] +
> -                             NM_I(sbi)->nid_cnt[PREALLOC_NID]) *
> -                             sizeof(struct free_nid);
>       si->cache_mem += NM_I(sbi)->nat_cnt * sizeof(struct nat_entry);
>       si->cache_mem += NM_I(sbi)->dirty_nat_cnt *
>                                       sizeof(struct nat_entry_set);
> diff --git a/fs/f2fs/f2fs.h b/fs/f2fs/f2fs.h
> index d92eba66263c..f08e0feb38c1 100644
> --- a/fs/f2fs/f2fs.h
> +++ b/fs/f2fs/f2fs.h
> @@ -723,14 +723,13 @@ struct f2fs_nm_info {
>       unsigned int nat_blocks;        /* # of nat blocks */
>  
>       /* free node ids management */
> -     struct radix_tree_root free_nid_root;/* root of the free_nid cache */
> -     struct list_head free_nid_list;         /* list for free nids excluding 
> preallocated nids */
> -     unsigned int nid_cnt[MAX_NID_STATE];    /* the number of free node id */
> -     spinlock_t nid_list_lock;       /* protect nid lists ops */
> +     spinlock_t free_nid_lock;       /* protect nid lists ops */
>       struct mutex build_lock;        /* lock for build free nids */
>       unsigned char (*free_nid_bitmap)[NAT_ENTRY_BITMAP_SIZE];
> +     unsigned char (*prealloc_nid_bitmap)[NAT_ENTRY_BITMAP_SIZE];
>       unsigned char *nat_block_bitmap;
>       unsigned short *free_nid_count; /* free nid count of NAT block */
> +     unsigned int available_free_nids;       /* available free nid count in 
> bitmaps */
>  
>       /* for checkpoint */
>       char *nat_bitmap;               /* NAT bitmap pointer */
> @@ -2622,11 +2621,9 @@ int fsync_node_pages(struct f2fs_sb_info *sbi, struct 
> inode *inode,
>                       struct writeback_control *wbc, bool atomic);
>  int sync_node_pages(struct f2fs_sb_info *sbi, struct writeback_control *wbc,
>                       bool do_balance, enum iostat_type io_type);
> -void build_free_nids(struct f2fs_sb_info *sbi, bool sync, bool mount);
>  bool alloc_nid(struct f2fs_sb_info *sbi, nid_t *nid);
>  void alloc_nid_done(struct f2fs_sb_info *sbi, nid_t nid);
>  void alloc_nid_failed(struct f2fs_sb_info *sbi, nid_t nid);
> -int try_to_free_nids(struct f2fs_sb_info *sbi, int nr_shrink);
>  void recover_inline_xattr(struct inode *inode, struct page *page);
>  int recover_xattr_data(struct inode *inode, struct page *page);
>  int recover_inode_page(struct f2fs_sb_info *sbi, struct page *page);
> diff --git a/fs/f2fs/inode.c b/fs/f2fs/inode.c
> index 9684d53563f1..82f543e5c35b 100644
> --- a/fs/f2fs/inode.c
> +++ b/fs/f2fs/inode.c
> @@ -559,7 +559,9 @@ void f2fs_evict_inode(struct inode *inode)
>                       add_ino_entry(sbi, inode->i_ino, UPDATE_INO);
>       }
>       if (is_inode_flag_set(inode, FI_FREE_NID)) {
> +             f2fs_lock_op(sbi);
>               alloc_nid_failed(sbi, inode->i_ino);
> +             f2fs_unlock_op(sbi);
>               clear_inode_flag(inode, FI_FREE_NID);
>       } else {
>               f2fs_bug_on(sbi, err &&
> diff --git a/fs/f2fs/node.c b/fs/f2fs/node.c
> index 80c37a094631..d2c9dcb0cbf8 100644
> --- a/fs/f2fs/node.c
> +++ b/fs/f2fs/node.c
> @@ -23,10 +23,7 @@
>  #include "trace.h"
>  #include <trace/events/f2fs.h>
>  
> -#define on_build_free_nids(nmi) mutex_is_locked(&(nm_i)->build_lock)
> -
>  static struct kmem_cache *nat_entry_slab;
> -static struct kmem_cache *free_nid_slab;
>  static struct kmem_cache *nat_entry_set_slab;
>  
>  bool available_free_memory(struct f2fs_sb_info *sbi, int type)
> @@ -43,13 +40,9 @@ bool available_free_memory(struct f2fs_sb_info *sbi, int 
> type)
>       avail_ram = val.totalram - val.totalhigh;
>  
>       /*
> -      * give 25%, 25%, 50%, 50%, 50% memory for each components respectively
> +      * give 25%, 50%, 50%, 50% memory for each components respectively
>        */
> -     if (type == FREE_NIDS) {
> -             mem_size = (nm_i->nid_cnt[FREE_NID] *
> -                             sizeof(struct free_nid)) >> PAGE_SHIFT;
> -             res = mem_size < ((avail_ram * nm_i->ram_thresh / 100) >> 2);
> -     } else if (type == NAT_ENTRIES) {
> +     if (type == NAT_ENTRIES) {
>               mem_size = (nm_i->nat_cnt * sizeof(struct nat_entry)) >>
>                                                       PAGE_SHIFT;
>               res = mem_size < ((avail_ram * nm_i->ram_thresh / 100) >> 2);
> @@ -1774,259 +1767,171 @@ const struct address_space_operations 
> f2fs_node_aops = {
>  #endif
>  };
>  
> -static struct free_nid *__lookup_free_nid_list(struct f2fs_nm_info *nm_i,
> -                                             nid_t n)
> -{
> -     return radix_tree_lookup(&nm_i->free_nid_root, n);
> -}
> -
> -static int __insert_free_nid(struct f2fs_sb_info *sbi,
> -                     struct free_nid *i, enum nid_state state)
> -{
> -     struct f2fs_nm_info *nm_i = NM_I(sbi);
> -
> -     int err = radix_tree_insert(&nm_i->free_nid_root, i->nid, i);
> -     if (err)
> -             return err;
> -
> -     f2fs_bug_on(sbi, state != i->state);
> -     nm_i->nid_cnt[state]++;
> -     if (state == FREE_NID)
> -             list_add_tail(&i->list, &nm_i->free_nid_list);
> -     return 0;
> -}
> -
> -static void __remove_free_nid(struct f2fs_sb_info *sbi,
> -                     struct free_nid *i, enum nid_state state)
> -{
> -     struct f2fs_nm_info *nm_i = NM_I(sbi);
> -
> -     f2fs_bug_on(sbi, state != i->state);
> -     nm_i->nid_cnt[state]--;
> -     if (state == FREE_NID)
> -             list_del(&i->list);
> -     radix_tree_delete(&nm_i->free_nid_root, i->nid);
> -}
> -
> -static void __move_free_nid(struct f2fs_sb_info *sbi, struct free_nid *i,
> -                     enum nid_state org_state, enum nid_state dst_state)
> -{
> -     struct f2fs_nm_info *nm_i = NM_I(sbi);
> -
> -     f2fs_bug_on(sbi, org_state != i->state);
> -     i->state = dst_state;
> -     nm_i->nid_cnt[org_state]--;
> -     nm_i->nid_cnt[dst_state]++;
> -
> -     switch (dst_state) {
> -     case PREALLOC_NID:
> -             list_del(&i->list);
> -             break;
> -     case FREE_NID:
> -             list_add_tail(&i->list, &nm_i->free_nid_list);
> -             break;
> -     default:
> -             BUG_ON(1);
> -     }
> -}
> -
> -static void update_free_nid_bitmap(struct f2fs_sb_info *sbi, nid_t nid,
> -                                                     bool set, bool build)
> +static bool update_free_nid_bitmap(struct f2fs_sb_info *sbi,
> +                                     nid_t nid, bool set)
>  {
>       struct f2fs_nm_info *nm_i = NM_I(sbi);
>       unsigned int nat_ofs = NAT_BLOCK_OFFSET(nid);
>       unsigned int nid_ofs = nid - START_NID(nid);
>  
>       if (!test_bit_le(nat_ofs, nm_i->nat_block_bitmap))
> -             return;
> +             return false;
>  
>       if (set) {
>               if (test_bit_le(nid_ofs, nm_i->free_nid_bitmap[nat_ofs]))
> -                     return;
> +                     return false;
>               __set_bit_le(nid_ofs, nm_i->free_nid_bitmap[nat_ofs]);
>               nm_i->free_nid_count[nat_ofs]++;
> +             nm_i->available_free_nids++;
>       } else {
>               if (!test_bit_le(nid_ofs, nm_i->free_nid_bitmap[nat_ofs]))
> -                     return;
> +                     return false;
>               __clear_bit_le(nid_ofs, nm_i->free_nid_bitmap[nat_ofs]);
> -             if (!build)
> -                     nm_i->free_nid_count[nat_ofs]--;
> +             nm_i->free_nid_count[nat_ofs]--;
> +             nm_i->available_free_nids--;
> +     }
> +     return true;
> +}
> +
> +static void update_prealloc_nid_bitmap(struct f2fs_sb_info *sbi,
> +                                     nid_t nid, bool set)
> +{
> +     struct f2fs_nm_info *nm_i = NM_I(sbi);
> +     unsigned int nat_ofs = NAT_BLOCK_OFFSET(nid);
> +     unsigned int nid_ofs = nid - START_NID(nid);
> +
> +     if (set) {
> +             f2fs_bug_on(sbi, test_bit_le(nid_ofs,
> +                             nm_i->prealloc_nid_bitmap[nat_ofs]));
> +             __set_bit_le(nid_ofs, nm_i->prealloc_nid_bitmap[nat_ofs]);
> +     } else {
> +             f2fs_bug_on(sbi, !test_bit_le(nid_ofs,
> +                             nm_i->prealloc_nid_bitmap[nat_ofs]));
> +             __clear_bit_le(nid_ofs, nm_i->prealloc_nid_bitmap[nat_ofs]);
>       }
>  }
>  
>  /* return if the nid is recognized as free */
> -static bool add_free_nid(struct f2fs_sb_info *sbi,
> -                             nid_t nid, bool build, bool update)
> +static bool add_free_nid(struct f2fs_sb_info *sbi, nid_t nid)
>  {
>       struct f2fs_nm_info *nm_i = NM_I(sbi);
> -     struct free_nid *i, *e;
>       struct nat_entry *ne;
> -     int err = -EINVAL;
> -     bool ret = false;
> +     unsigned int nat_ofs, nid_ofs;
>  
>       /* 0 nid should not be used */
>       if (unlikely(nid == 0))
>               return false;
>  
> -     i = f2fs_kmem_cache_alloc(free_nid_slab, GFP_NOFS);
> -     i->nid = nid;
> -     i->state = FREE_NID;
> -
> -     radix_tree_preload(GFP_NOFS | __GFP_NOFAIL);
> -
> -     spin_lock(&nm_i->nid_list_lock);
> +     ne = __lookup_nat_cache(nm_i, nid);
> +     if (ne && (!get_nat_flag(ne, IS_CHECKPOINTED) ||
> +                     nat_get_blkaddr(ne) != NULL_ADDR))
> +             return false;
>  
> -     if (build) {
> -             /*
> -              *   Thread A             Thread B
> -              *  - f2fs_create
> -              *   - f2fs_new_inode
> -              *    - alloc_nid
> -              *     - __insert_nid_to_list(PREALLOC_NID)
> -              *                     - f2fs_balance_fs_bg
> -              *                      - build_free_nids
> -              *                       - __build_free_nids
> -              *                        - scan_nat_page
> -              *                         - add_free_nid
> -              *                          - __lookup_nat_cache
> -              *  - f2fs_add_link
> -              *   - init_inode_metadata
> -              *    - new_inode_page
> -              *     - new_node_page
> -              *      - set_node_addr
> -              *  - alloc_nid_done
> -              *   - __remove_nid_from_list(PREALLOC_NID)
> -              *                         - __insert_nid_to_list(FREE_NID)
> -              */
> -             ne = __lookup_nat_cache(nm_i, nid);
> -             if (ne && (!get_nat_flag(ne, IS_CHECKPOINTED) ||
> -                             nat_get_blkaddr(ne) != NULL_ADDR))
> -                     goto err_out;
> -
> -             e = __lookup_free_nid_list(nm_i, nid);
> -             if (e) {
> -                     if (e->state == FREE_NID)
> -                             ret = true;
> -                     goto err_out;
> -             }
> -     }
> -     ret = true;
> -     err = __insert_free_nid(sbi, i, FREE_NID);
> -err_out:
> -     if (update) {
> -             update_free_nid_bitmap(sbi, nid, ret, build);
> -             if (!build)
> -                     nm_i->available_nids++;
> -     }
> -     spin_unlock(&nm_i->nid_list_lock);
> -     radix_tree_preload_end();
> +     nat_ofs = NAT_BLOCK_OFFSET(nid);
> +     nid_ofs = nid - START_NID(nid);
> +     if (test_bit_le(nid_ofs, nm_i->prealloc_nid_bitmap[nat_ofs]))
> +             return false;
>  
> -     if (err)
> -             kmem_cache_free(free_nid_slab, i);
> -     return ret;
> +     update_free_nid_bitmap(sbi, nid, true);
> +     return true;
>  }
>  
> -static void remove_free_nid(struct f2fs_sb_info *sbi, nid_t nid)
> +static void scan_curseg_cache(struct f2fs_sb_info *sbi, nid_t start_nid,
> +                                                             nid_t end_nid)
>  {
> -     struct f2fs_nm_info *nm_i = NM_I(sbi);
> -     struct free_nid *i;
> -     bool need_free = false;
> +     struct curseg_info *curseg = CURSEG_I(sbi, CURSEG_HOT_DATA);
> +     struct f2fs_journal *journal = curseg->journal;
> +     int i;
>  
> -     spin_lock(&nm_i->nid_list_lock);
> -     i = __lookup_free_nid_list(nm_i, nid);
> -     if (i && i->state == FREE_NID) {
> -             __remove_free_nid(sbi, i, FREE_NID);
> -             need_free = true;
> -     }
> -     spin_unlock(&nm_i->nid_list_lock);
> +     down_read(&curseg->journal_rwsem);
> +     for (i = 0; i < nats_in_cursum(journal); i++) {
> +             block_t addr;
> +             nid_t nid = le32_to_cpu(nid_in_journal(journal, i));
> +
> +             if (nid < start_nid || nid >= end_nid)
> +                     continue;
> +
> +             addr = le32_to_cpu(nat_in_journal(journal, i).block_addr);
> +
> +             f2fs_bug_on(sbi, addr == NEW_ADDR);
>  
> -     if (need_free)
> -             kmem_cache_free(free_nid_slab, i);
> +             spin_lock(&NM_I(sbi)->free_nid_lock);
> +             if (addr == NULL_ADDR)
> +                     add_free_nid(sbi, nid);
> +             else
> +                     update_free_nid_bitmap(sbi, nid, false);
> +             spin_unlock(&NM_I(sbi)->free_nid_lock);
> +     }
> +     up_read(&curseg->journal_rwsem);
>  }
>  
> -static void scan_nat_page(struct f2fs_sb_info *sbi,
> -                     struct page *nat_page, nid_t start_nid)
> +static void scan_nat_page(struct f2fs_sb_info *sbi, nid_t nid)
>  {
>       struct f2fs_nm_info *nm_i = NM_I(sbi);
> -     struct f2fs_nat_block *nat_blk = page_address(nat_page);
> +     struct page *page;
> +     struct f2fs_nat_block *nat_blk;
>       block_t blk_addr;
> -     unsigned int nat_ofs = NAT_BLOCK_OFFSET(start_nid);
> +     unsigned int nat_ofs = NAT_BLOCK_OFFSET(nid);
> +     nid_t start_nid = nid;
>       int i;
>  
> +     if (test_bit_le(nat_ofs, nm_i->nat_block_bitmap))
> +             return;
> +
> +     page = get_current_nat_page(sbi, nid);
> +     nat_blk = page_address(page);
> +
>       __set_bit_le(nat_ofs, nm_i->nat_block_bitmap);
>  
> -     i = start_nid % NAT_ENTRY_PER_BLOCK;
> +     i = nid % NAT_ENTRY_PER_BLOCK;
>  
> -     for (; i < NAT_ENTRY_PER_BLOCK; i++, start_nid++) {
> -             if (unlikely(start_nid >= nm_i->max_nid))
> +     for (; i < NAT_ENTRY_PER_BLOCK; i++, nid++) {
> +             if (unlikely(nid >= nm_i->max_nid))
>                       break;
>  
>               blk_addr = le32_to_cpu(nat_blk->entries[i].block_addr);
>               f2fs_bug_on(sbi, blk_addr == NEW_ADDR);
>               if (blk_addr == NULL_ADDR) {
> -                     add_free_nid(sbi, start_nid, true, true);
> -             } else {
> -                     spin_lock(&NM_I(sbi)->nid_list_lock);
> -                     update_free_nid_bitmap(sbi, start_nid, false, true);
> -                     spin_unlock(&NM_I(sbi)->nid_list_lock);
> +                     spin_lock(&nm_i->free_nid_lock);
> +                     add_free_nid(sbi, nid);
> +                     spin_unlock(&nm_i->free_nid_lock);
>               }
>       }
> -}
> -
> -static void scan_curseg_cache(struct f2fs_sb_info *sbi)
> -{
> -     struct curseg_info *curseg = CURSEG_I(sbi, CURSEG_HOT_DATA);
> -     struct f2fs_journal *journal = curseg->journal;
> -     int i;
>  
> -     down_read(&curseg->journal_rwsem);
> -     for (i = 0; i < nats_in_cursum(journal); i++) {
> -             block_t addr;
> -             nid_t nid;
> +     f2fs_put_page(page, 1);
>  
> -             addr = le32_to_cpu(nat_in_journal(journal, i).block_addr);
> -             nid = le32_to_cpu(nid_in_journal(journal, i));
> -             if (addr == NULL_ADDR)
> -                     add_free_nid(sbi, nid, true, false);
> -             else
> -                     remove_free_nid(sbi, nid);
> -     }
> -     up_read(&curseg->journal_rwsem);
> +     /* find free nids from current sum_pages */
> +     scan_curseg_cache(sbi, start_nid, start_nid + NAT_ENTRY_PER_BLOCK);
>  }
>  
> -static void scan_free_nid_bits(struct f2fs_sb_info *sbi)
> +static nid_t lookup_free_nid_bitmap(struct f2fs_sb_info *sbi)
>  {
>       struct f2fs_nm_info *nm_i = NM_I(sbi);
> -     unsigned int i, idx;
> -     nid_t nid;
> -
> -     down_read(&nm_i->nat_tree_lock);
> +     unsigned int i;
> +     nid_t nid = 0;
>  
>       for (i = 0; i < nm_i->nat_blocks; i++) {
> +             unsigned int idx = 0;
> +
>               if (!test_bit_le(i, nm_i->nat_block_bitmap))
>                       continue;
>               if (!nm_i->free_nid_count[i])
>                       continue;
> -             for (idx = 0; idx < NAT_ENTRY_PER_BLOCK; idx++) {
> -                     idx = find_next_bit_le(nm_i->free_nid_bitmap[i],
> -                                             NAT_ENTRY_PER_BLOCK, idx);
> -                     if (idx >= NAT_ENTRY_PER_BLOCK)
> -                             break;
>  
> -                     nid = i * NAT_ENTRY_PER_BLOCK + idx;
> -                     add_free_nid(sbi, nid, true, false);
> +             idx = find_next_bit_le(nm_i->free_nid_bitmap[i],
> +                                     NAT_ENTRY_PER_BLOCK, idx);
> +             if (idx >= NAT_ENTRY_PER_BLOCK)
> +                     continue;
>  
> -                     if (nm_i->nid_cnt[FREE_NID] >= MAX_FREE_NIDS)
> -                             goto out;
> -             }
> +             nid = i * NAT_ENTRY_PER_BLOCK + idx;
> +             break;
>       }
> -out:
> -     scan_curseg_cache(sbi);
>  
> -     up_read(&nm_i->nat_tree_lock);
> +     f2fs_bug_on(sbi, !nid);
> +     return nid;
>  }
>  
> -static void __build_free_nids(struct f2fs_sb_info *sbi, bool sync, bool 
> mount)
> +static void __build_free_nids(struct f2fs_sb_info *sbi, bool mount)
>  {
>       struct f2fs_nm_info *nm_i = NM_I(sbi);
>       int i = 0;
> @@ -2036,59 +1941,36 @@ static void __build_free_nids(struct f2fs_sb_info 
> *sbi, bool sync, bool mount)
>               nid = 0;
>  
>       /* Enough entries */
> -     if (nm_i->nid_cnt[FREE_NID] >= NAT_ENTRY_PER_BLOCK)
> -             return;
> -
> -     if (!sync && !available_free_memory(sbi, FREE_NIDS))
> +     if (!mount && nm_i->available_free_nids >= NAT_ENTRY_PER_BLOCK)
>               return;
>  
> -     if (!mount) {
> -             /* try to find free nids in free_nid_bitmap */
> -             scan_free_nid_bits(sbi);
> -
> -             if (nm_i->nid_cnt[FREE_NID] >= NAT_ENTRY_PER_BLOCK)
> -                     return;
> -     }
> -
>       /* readahead nat pages to be scanned */
>       ra_meta_pages(sbi, NAT_BLOCK_OFFSET(nid), FREE_NID_PAGES,
>                                                       META_NAT, true);
>  
>       down_read(&nm_i->nat_tree_lock);
>  
> -     while (1) {
> -             if (!test_bit_le(NAT_BLOCK_OFFSET(nid),
> -                                             nm_i->nat_block_bitmap)) {
> -                     struct page *page = get_current_nat_page(sbi, nid);
> -
> -                     scan_nat_page(sbi, page, nid);
> -                     f2fs_put_page(page, 1);
> -             }
> +     do {
> +             scan_nat_page(sbi, nid);
>  
>               nid += (NAT_ENTRY_PER_BLOCK - (nid % NAT_ENTRY_PER_BLOCK));
>               if (unlikely(nid >= nm_i->max_nid))
>                       nid = 0;
> -
> -             if (++i >= FREE_NID_PAGES)
> -                     break;
> -     }
> +     } while (++i < FREE_NID_PAGES);
>  
>       /* go to the next free nat pages to find free nids abundantly */
>       nm_i->next_scan_nid = nid;
>  
> -     /* find free nids from current sum_pages */
> -     scan_curseg_cache(sbi);
> -
>       up_read(&nm_i->nat_tree_lock);
>  
>       ra_meta_pages(sbi, NAT_BLOCK_OFFSET(nm_i->next_scan_nid),
>                                       nm_i->ra_nid_pages, META_NAT, false);
>  }
>  
> -void build_free_nids(struct f2fs_sb_info *sbi, bool sync, bool mount)
> +void build_free_nids(struct f2fs_sb_info *sbi, bool mount)
>  {
>       mutex_lock(&NM_I(sbi)->build_lock);
> -     __build_free_nids(sbi, sync, mount);
> +     __build_free_nids(sbi, mount);
>       mutex_unlock(&NM_I(sbi)->build_lock);
>  }
>  
> @@ -2100,7 +1982,6 @@ void build_free_nids(struct f2fs_sb_info *sbi, bool 
> sync, bool mount)
>  bool alloc_nid(struct f2fs_sb_info *sbi, nid_t *nid)
>  {
>       struct f2fs_nm_info *nm_i = NM_I(sbi);
> -     struct free_nid *i = NULL;
>  retry:
>  #ifdef CONFIG_F2FS_FAULT_INJECTION
>       if (time_to_inject(sbi, FAULT_ALLOC_NID)) {
> @@ -2108,32 +1989,34 @@ bool alloc_nid(struct f2fs_sb_info *sbi, nid_t *nid)
>               return false;
>       }
>  #endif
> -     spin_lock(&nm_i->nid_list_lock);
> +
> +     spin_lock(&nm_i->free_nid_lock);
>  
>       if (unlikely(nm_i->available_nids == 0)) {
> -             spin_unlock(&nm_i->nid_list_lock);
> +             spin_unlock(&nm_i->free_nid_lock);
>               return false;
>       }
>  
>       /* We should not use stale free nids created by build_free_nids */
> -     if (nm_i->nid_cnt[FREE_NID] && !on_build_free_nids(nm_i)) {
> -             f2fs_bug_on(sbi, list_empty(&nm_i->free_nid_list));
> -             i = list_first_entry(&nm_i->free_nid_list,
> -                                     struct free_nid, list);
> -             *nid = i->nid;
> +     if (nm_i->available_free_nids && mutex_trylock(&nm_i->build_lock)) {
> +             bool updated;
> +
> +             *nid = lookup_free_nid_bitmap(sbi);
>  
> -             __move_free_nid(sbi, i, FREE_NID, PREALLOC_NID);
> +             updated = update_free_nid_bitmap(sbi, *nid, false);
> +             f2fs_bug_on(sbi, !updated);
>               nm_i->available_nids--;
>  
> -             update_free_nid_bitmap(sbi, *nid, false, false);
> +             update_prealloc_nid_bitmap(sbi, *nid, true);
>  
> -             spin_unlock(&nm_i->nid_list_lock);
> +             spin_unlock(&nm_i->free_nid_lock);
> +             mutex_unlock(&nm_i->build_lock);
>               return true;
>       }
> -     spin_unlock(&nm_i->nid_list_lock);
> +     spin_unlock(&nm_i->free_nid_lock);
>  
>       /* Let's scan nat pages and its caches to get free nids */
> -     build_free_nids(sbi, true, false);
> +     build_free_nids(sbi, false);
>       goto retry;
>  }
>  
> @@ -2143,15 +2026,10 @@ bool alloc_nid(struct f2fs_sb_info *sbi, nid_t *nid)
>  void alloc_nid_done(struct f2fs_sb_info *sbi, nid_t nid)
>  {
>       struct f2fs_nm_info *nm_i = NM_I(sbi);
> -     struct free_nid *i;
> -
> -     spin_lock(&nm_i->nid_list_lock);
> -     i = __lookup_free_nid_list(nm_i, nid);
> -     f2fs_bug_on(sbi, !i);
> -     __remove_free_nid(sbi, i, PREALLOC_NID);
> -     spin_unlock(&nm_i->nid_list_lock);
>  
> -     kmem_cache_free(free_nid_slab, i);
> +     spin_lock(&nm_i->free_nid_lock);
> +     update_prealloc_nid_bitmap(sbi, nid, false);
> +     spin_unlock(&nm_i->free_nid_lock);
>  }
>  
>  /*
> @@ -2160,59 +2038,21 @@ void alloc_nid_done(struct f2fs_sb_info *sbi, nid_t 
> nid)
>  void alloc_nid_failed(struct f2fs_sb_info *sbi, nid_t nid)
>  {
>       struct f2fs_nm_info *nm_i = NM_I(sbi);
> -     struct free_nid *i;
> -     bool need_free = false;
>  
>       if (!nid)
>               return;
>  
> -     spin_lock(&nm_i->nid_list_lock);
> -     i = __lookup_free_nid_list(nm_i, nid);
> -     f2fs_bug_on(sbi, !i);
> -
> -     if (!available_free_memory(sbi, FREE_NIDS)) {
> -             __remove_free_nid(sbi, i, PREALLOC_NID);
> -             need_free = true;
> -     } else {
> -             __move_free_nid(sbi, i, PREALLOC_NID, FREE_NID);
> -     }
> -
> -     nm_i->available_nids++;
> -
> -     update_free_nid_bitmap(sbi, nid, true, false);
> -
> -     spin_unlock(&nm_i->nid_list_lock);
> -
> -     if (need_free)
> -             kmem_cache_free(free_nid_slab, i);
> -}
> -
> -int try_to_free_nids(struct f2fs_sb_info *sbi, int nr_shrink)
> -{
> -     struct f2fs_nm_info *nm_i = NM_I(sbi);
> -     struct free_nid *i, *next;
> -     int nr = nr_shrink;
> -
> -     if (nm_i->nid_cnt[FREE_NID] <= MAX_FREE_NIDS)
> -             return 0;
> -
> -     if (!mutex_trylock(&nm_i->build_lock))
> -             return 0;
> +     mutex_lock(&nm_i->build_lock);
> +     down_read(&nm_i->nat_tree_lock);
> +     spin_lock(&nm_i->free_nid_lock);
>  
> -     spin_lock(&nm_i->nid_list_lock);
> -     list_for_each_entry_safe(i, next, &nm_i->free_nid_list, list) {
> -             if (nr_shrink <= 0 ||
> -                             nm_i->nid_cnt[FREE_NID] <= MAX_FREE_NIDS)
> -                     break;
> +     update_prealloc_nid_bitmap(sbi, nid, false);
>  
> -             __remove_free_nid(sbi, i, FREE_NID);
> -             kmem_cache_free(free_nid_slab, i);
> -             nr_shrink--;
> -     }
> -     spin_unlock(&nm_i->nid_list_lock);
> +     if (add_free_nid(sbi, nid))
> +             nm_i->available_nids++;
> +     spin_unlock(&nm_i->free_nid_lock);
> +     up_read(&nm_i->nat_tree_lock);
>       mutex_unlock(&nm_i->build_lock);
> -
> -     return nr - nr_shrink;
>  }
>  
>  void recover_inline_xattr(struct inode *inode, struct page *page)
> @@ -2303,7 +2143,10 @@ int recover_inode_page(struct f2fs_sb_info *sbi, 
> struct page *page)
>       }
>  
>       /* Should not use this inode from free nid list */
> -     remove_free_nid(sbi, ino);
> +     spin_lock(&NM_I(sbi)->free_nid_lock);
> +     update_free_nid_bitmap(sbi, ino, false);
> +     NM_I(sbi)->available_nids--;
> +     spin_unlock(&NM_I(sbi)->free_nid_lock);
>  
>       if (!PageUptodate(ipage))
>               SetPageUptodate(ipage);
> @@ -2408,9 +2251,9 @@ static void remove_nats_in_journal(struct f2fs_sb_info 
> *sbi)
>                */
>               if (!get_nat_flag(ne, IS_DIRTY) &&
>                               le32_to_cpu(raw_ne.block_addr) == NULL_ADDR) {
> -                     spin_lock(&nm_i->nid_list_lock);
> +                     spin_lock(&nm_i->free_nid_lock);
>                       nm_i->available_nids--;
> -                     spin_unlock(&nm_i->nid_list_lock);
> +                     spin_unlock(&nm_i->free_nid_lock);
>               }
>  
>               __set_nat_cache_dirty(nm_i, ne);
> @@ -2519,11 +2362,14 @@ static void __flush_nat_entry_set(struct f2fs_sb_info 
> *sbi,
>               nat_reset_flag(ne);
>               __clear_nat_cache_dirty(NM_I(sbi), set, ne);
>               if (nat_get_blkaddr(ne) == NULL_ADDR) {
> -                     add_free_nid(sbi, nid, false, true);
> +                     spin_lock(&NM_I(sbi)->free_nid_lock);
> +                     add_free_nid(sbi, nid);
> +                     NM_I(sbi)->available_nids++;
> +                     spin_unlock(&NM_I(sbi)->free_nid_lock);
>               } else {
> -                     spin_lock(&NM_I(sbi)->nid_list_lock);
> -                     update_free_nid_bitmap(sbi, nid, false, false);
> -                     spin_unlock(&NM_I(sbi)->nid_list_lock);
> +                     spin_lock(&NM_I(sbi)->free_nid_lock);
> +                     update_free_nid_bitmap(sbi, nid, false);
> +                     spin_unlock(&NM_I(sbi)->free_nid_lock);
>               }
>       }
>  
> @@ -2647,10 +2493,10 @@ static inline void load_free_nid_bitmap(struct 
> f2fs_sb_info *sbi)
>               nid = i * NAT_ENTRY_PER_BLOCK;
>               last_nid = nid + NAT_ENTRY_PER_BLOCK;
>  
> -             spin_lock(&NM_I(sbi)->nid_list_lock);
> +             spin_lock(&NM_I(sbi)->free_nid_lock);
>               for (; nid < last_nid; nid++)
> -                     update_free_nid_bitmap(sbi, nid, true, true);
> -             spin_unlock(&NM_I(sbi)->nid_list_lock);
> +                     update_free_nid_bitmap(sbi, nid, true);
> +             spin_unlock(&NM_I(sbi)->free_nid_lock);
>       }
>  
>       for (i = 0; i < nm_i->nat_blocks; i++) {
> @@ -2660,6 +2506,8 @@ static inline void load_free_nid_bitmap(struct 
> f2fs_sb_info *sbi)
>  
>               __set_bit_le(i, nm_i->nat_block_bitmap);
>       }
> +
> +     scan_curseg_cache(sbi, 0, nm_i->max_nid);
>  }
>  
>  static int init_node_manager(struct f2fs_sb_info *sbi)
> @@ -2680,21 +2528,18 @@ static int init_node_manager(struct f2fs_sb_info *sbi)
>       /* not used nids: 0, node, meta, (and root counted as valid node) */
>       nm_i->available_nids = nm_i->max_nid - sbi->total_valid_node_count -
>                               sbi->nquota_files - F2FS_RESERVED_NODE_NUM;
> -     nm_i->nid_cnt[FREE_NID] = 0;
> -     nm_i->nid_cnt[PREALLOC_NID] = 0;
> +     nm_i->available_free_nids = 0;
>       nm_i->nat_cnt = 0;
>       nm_i->ram_thresh = DEF_RAM_THRESHOLD;
>       nm_i->ra_nid_pages = DEF_RA_NID_PAGES;
>       nm_i->dirty_nats_ratio = DEF_DIRTY_NAT_RATIO_THRESHOLD;
>  
> -     INIT_RADIX_TREE(&nm_i->free_nid_root, GFP_ATOMIC);
> -     INIT_LIST_HEAD(&nm_i->free_nid_list);
>       INIT_RADIX_TREE(&nm_i->nat_root, GFP_NOIO);
>       INIT_RADIX_TREE(&nm_i->nat_set_root, GFP_NOIO);
>       INIT_LIST_HEAD(&nm_i->nat_entries);
>  
>       mutex_init(&nm_i->build_lock);
> -     spin_lock_init(&nm_i->nid_list_lock);
> +     spin_lock_init(&nm_i->free_nid_lock);
>       init_rwsem(&nm_i->nat_tree_lock);
>  
>       nm_i->next_scan_nid = le32_to_cpu(sbi->ckpt->next_free_nid);
> @@ -2731,6 +2576,11 @@ static int init_free_nid_cache(struct f2fs_sb_info 
> *sbi)
>       if (!nm_i->free_nid_bitmap)
>               return -ENOMEM;
>  
> +     nm_i->prealloc_nid_bitmap = f2fs_kvzalloc(sbi, nm_i->nat_blocks *
> +                                     NAT_ENTRY_BITMAP_SIZE, GFP_KERNEL);
> +     if (!nm_i->prealloc_nid_bitmap)
> +             return -ENOMEM;
> +
>       nm_i->nat_block_bitmap = f2fs_kvzalloc(sbi, nm_i->nat_blocks / 8,
>                                                               GFP_KERNEL);
>       if (!nm_i->nat_block_bitmap)
> @@ -2763,14 +2613,13 @@ int build_node_manager(struct f2fs_sb_info *sbi)
>       /* load free nid status from nat_bits table */
>       load_free_nid_bitmap(sbi);
>  
> -     build_free_nids(sbi, true, true);
> +     build_free_nids(sbi, true);
>       return 0;
>  }
>  
>  void destroy_node_manager(struct f2fs_sb_info *sbi)
>  {
>       struct f2fs_nm_info *nm_i = NM_I(sbi);
> -     struct free_nid *i, *next_i;
>       struct nat_entry *natvec[NATVEC_SIZE];
>       struct nat_entry_set *setvec[SETVEC_SIZE];
>       nid_t nid = 0;
> @@ -2779,19 +2628,6 @@ void destroy_node_manager(struct f2fs_sb_info *sbi)
>       if (!nm_i)
>               return;
>  
> -     /* destroy free nid list */
> -     spin_lock(&nm_i->nid_list_lock);
> -     list_for_each_entry_safe(i, next_i, &nm_i->free_nid_list, list) {
> -             __remove_free_nid(sbi, i, FREE_NID);
> -             spin_unlock(&nm_i->nid_list_lock);
> -             kmem_cache_free(free_nid_slab, i);
> -             spin_lock(&nm_i->nid_list_lock);
> -     }
> -     f2fs_bug_on(sbi, nm_i->nid_cnt[FREE_NID]);
> -     f2fs_bug_on(sbi, nm_i->nid_cnt[PREALLOC_NID]);
> -     f2fs_bug_on(sbi, !list_empty(&nm_i->free_nid_list));
> -     spin_unlock(&nm_i->nid_list_lock);
> -
>       /* destroy nat cache */
>       down_write(&nm_i->nat_tree_lock);
>       while ((found = __gang_lookup_nat_cache(nm_i,
> @@ -2822,6 +2658,7 @@ void destroy_node_manager(struct f2fs_sb_info *sbi)
>  
>       kvfree(nm_i->nat_block_bitmap);
>       kvfree(nm_i->free_nid_bitmap);
> +     kvfree(nm_i->prealloc_nid_bitmap);
>       kvfree(nm_i->free_nid_count);
>  
>       kfree(nm_i->nat_bitmap);
> @@ -2840,19 +2677,12 @@ int __init create_node_manager_caches(void)
>       if (!nat_entry_slab)
>               goto fail;
>  
> -     free_nid_slab = f2fs_kmem_cache_create("free_nid",
> -                     sizeof(struct free_nid));
> -     if (!free_nid_slab)
> -             goto destroy_nat_entry;
> -
>       nat_entry_set_slab = f2fs_kmem_cache_create("nat_entry_set",
>                       sizeof(struct nat_entry_set));
>       if (!nat_entry_set_slab)
> -             goto destroy_free_nid;
> +             goto destroy_nat_entry;
>       return 0;
>  
> -destroy_free_nid:
> -     kmem_cache_destroy(free_nid_slab);
>  destroy_nat_entry:
>       kmem_cache_destroy(nat_entry_slab);
>  fail:
> @@ -2862,6 +2692,5 @@ int __init create_node_manager_caches(void)
>  void destroy_node_manager_caches(void)
>  {
>       kmem_cache_destroy(nat_entry_set_slab);
> -     kmem_cache_destroy(free_nid_slab);
>       kmem_cache_destroy(nat_entry_slab);
>  }
> diff --git a/fs/f2fs/node.h b/fs/f2fs/node.h
> index 0ee3e5ff49a3..d5cf8af70c90 100644
> --- a/fs/f2fs/node.h
> +++ b/fs/f2fs/node.h
> @@ -135,7 +135,6 @@ static inline bool excess_cached_nats(struct f2fs_sb_info 
> *sbi)
>  }
>  
>  enum mem_type {
> -     FREE_NIDS,      /* indicates the free nid list */
>       NAT_ENTRIES,    /* indicates the cached nat entry */
>       DIRTY_DENTS,    /* indicates dirty dentry pages */
>       INO_ENTRIES,    /* indicates inode entries */
> @@ -151,27 +150,6 @@ struct nat_entry_set {
>       unsigned int entry_cnt;         /* the # of nat entries in set */
>  };
>  
> -struct free_nid {
> -     struct list_head list;  /* for free node id list */
> -     nid_t nid;              /* node id */
> -     int state;              /* in use or not: FREE_NID or PREALLOC_NID */
> -};
> -
> -static inline void next_free_nid(struct f2fs_sb_info *sbi, nid_t *nid)
> -{
> -     struct f2fs_nm_info *nm_i = NM_I(sbi);
> -     struct free_nid *fnid;
> -
> -     spin_lock(&nm_i->nid_list_lock);
> -     if (nm_i->nid_cnt[FREE_NID] <= 0) {
> -             spin_unlock(&nm_i->nid_list_lock);
> -             return;
> -     }
> -     fnid = list_first_entry(&nm_i->free_nid_list, struct free_nid, list);
> -     *nid = fnid->nid;
> -     spin_unlock(&nm_i->nid_list_lock);
> -}
> -
>  /*
>   * inline functions
>   */
> diff --git a/fs/f2fs/segment.c b/fs/f2fs/segment.c
> index 176a2b97e6d3..c9a0563907aa 100644
> --- a/fs/f2fs/segment.c
> +++ b/fs/f2fs/segment.c
> @@ -482,11 +482,6 @@ void f2fs_balance_fs_bg(struct f2fs_sb_info *sbi)
>       if (!available_free_memory(sbi, NAT_ENTRIES))
>               try_to_free_nats(sbi, NAT_ENTRY_PER_BLOCK);
>  
> -     if (!available_free_memory(sbi, FREE_NIDS))
> -             try_to_free_nids(sbi, MAX_FREE_NIDS);
> -     else
> -             build_free_nids(sbi, false, false);
> -
>       if (!is_idle(sbi) && !excess_dirty_nats(sbi))
>               return;
>  
> diff --git a/fs/f2fs/shrinker.c b/fs/f2fs/shrinker.c
> index 0b5664a1a6cc..7123bcb3cb62 100644
> --- a/fs/f2fs/shrinker.c
> +++ b/fs/f2fs/shrinker.c
> @@ -26,13 +26,6 @@ static unsigned long __count_nat_entries(struct 
> f2fs_sb_info *sbi)
>       return count > 0 ? count : 0;
>  }
>  
> -static unsigned long __count_free_nids(struct f2fs_sb_info *sbi)
> -{
> -     long count = NM_I(sbi)->nid_cnt[FREE_NID] - MAX_FREE_NIDS;
> -
> -     return count > 0 ? count : 0;
> -}
> -
>  static unsigned long __count_extent_cache(struct f2fs_sb_info *sbi)
>  {
>       return atomic_read(&sbi->total_zombie_tree) +
> @@ -64,9 +57,6 @@ unsigned long f2fs_shrink_count(struct shrinker *shrink,
>               /* shrink clean nat cache entries */
>               count += __count_nat_entries(sbi);
>  
> -             /* count free nids cache entries */
> -             count += __count_free_nids(sbi);
> -
>               spin_lock(&f2fs_list_lock);
>               p = p->next;
>               mutex_unlock(&sbi->umount_mutex);
> @@ -111,10 +101,6 @@ unsigned long f2fs_shrink_scan(struct shrinker *shrink,
>               if (freed < nr)
>                       freed += try_to_free_nats(sbi, nr - freed);
>  
> -             /* shrink free nids cache entries */
> -             if (freed < nr)
> -                     freed += try_to_free_nids(sbi, nr - freed);
> -
>               spin_lock(&f2fs_list_lock);
>               p = p->next;
>               list_move_tail(&sbi->s_list, &f2fs_list);
> -- 
> 2.15.0.55.gc2ece9dc4de6

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