Ross Zwisler <ross.zwis...@linux.intel.com> writes:
> To properly handle fsync/msync in an efficient way DAX needs to track dirty
> pages so it is able to flush them durably to media on demand.
Please see coments below
>
> The tracking of dirty pages is done via the radix tree in struct
> address_space. This radix tree is already used by the page writeback
> infrastructure for tracking dirty pages associated with an open file, and
> it already has support for exceptional (non struct page*) entries. We
> build upon these features to add exceptional entries to the radix tree for
> DAX dirty PMD or PTE pages at fault time.
>
> Signed-off-by: Ross Zwisler <ross.zwis...@linux.intel.com>
> ---
> fs/dax.c | 194
> ++++++++++++++++++++++++++++++++++++++++++++++++++--
> include/linux/dax.h | 2 +
> mm/filemap.c | 6 ++
> 3 files changed, 196 insertions(+), 6 deletions(-)
>
> diff --git a/fs/dax.c b/fs/dax.c
> index 5b84a46..0db21ea 100644
> --- a/fs/dax.c
> +++ b/fs/dax.c
> @@ -24,6 +24,7 @@
> #include <linux/memcontrol.h>
> #include <linux/mm.h>
> #include <linux/mutex.h>
> +#include <linux/pagevec.h>
> #include <linux/pmem.h>
> #include <linux/sched.h>
> #include <linux/uio.h>
> @@ -324,6 +325,174 @@ static int copy_user_bh(struct page *to, struct inode
> *inode,
> return 0;
> }
>
> +#define NO_SECTOR -1
> +
> +static int dax_radix_entry(struct address_space *mapping, pgoff_t index,
IMHO it would be sane to call that function as dax_radix_entry_insert()
> + sector_t sector, bool pmd_entry, bool dirty)
> +{
> + struct radix_tree_root *page_tree = &mapping->page_tree;
> + int type, error = 0;
> + void *entry;
> +
> + __mark_inode_dirty(mapping->host, I_DIRTY_PAGES);
> +
> + spin_lock_irq(&mapping->tree_lock);
> + entry = radix_tree_lookup(page_tree, index);
> +
> + if (entry) {
> + type = RADIX_DAX_TYPE(entry);
> + if (WARN_ON_ONCE(type != RADIX_DAX_PTE &&
> + type != RADIX_DAX_PMD)) {
> + error = -EIO;
> + goto unlock;
> + }
> +
> + if (!pmd_entry || type == RADIX_DAX_PMD)
> + goto dirty;
> + radix_tree_delete(&mapping->page_tree, index);
> + mapping->nrexceptional--;
> + }
> +
> + if (sector == NO_SECTOR) {
> + /*
> + * This can happen during correct operation if our pfn_mkwrite
> + * fault raced against a hole punch operation. If this
> + * happens the pte that was hole punched will have been
> + * unmapped and the radix tree entry will have been removed by
> + * the time we are called, but the call will still happen. We
> + * will return all the way up to wp_pfn_shared(), where the
> + * pte_same() check will fail, eventually causing page fault
> + * to be retried by the CPU.
> + */
> + goto unlock;
> + }
> +
> + error = radix_tree_insert(page_tree, index,
> + RADIX_DAX_ENTRY(sector, pmd_entry));
> + if (error)
> + goto unlock;
> +
> + mapping->nrexceptional++;
> + dirty:
> + if (dirty)
> + radix_tree_tag_set(page_tree, index, PAGECACHE_TAG_DIRTY);
> + unlock:
> + spin_unlock_irq(&mapping->tree_lock);
> + return error;
> +}
> +
> +static int dax_writeback_one(struct block_device *bdev,
> + struct address_space *mapping, pgoff_t index, void *entry)
> +{
> + struct radix_tree_root *page_tree = &mapping->page_tree;
> + int type = RADIX_DAX_TYPE(entry);
> + struct radix_tree_node *node;
> + struct blk_dax_ctl dax;
> + void **slot;
> + int ret = 0;
> +
> + spin_lock_irq(&mapping->tree_lock);
> + /*
> + * Regular page slots are stabilized by the page lock even
> + * without the tree itself locked. These unlocked entries
> + * need verification under the tree lock.
> + */
> + if (!__radix_tree_lookup(page_tree, index, &node, &slot))
> + goto unlock;
> + if (*slot != entry)
> + goto unlock;
> +
> + /* another fsync thread may have already written back this entry */
> + if (!radix_tree_tag_get(page_tree, index, PAGECACHE_TAG_TOWRITE))
> + goto unlock;
> +
> + radix_tree_tag_clear(page_tree, index, PAGECACHE_TAG_TOWRITE);
> +
> + if (WARN_ON_ONCE(type != RADIX_DAX_PTE && type != RADIX_DAX_PMD)) {
> + ret = -EIO;
> + goto unlock;
> + }
> +
> + dax.sector = RADIX_DAX_SECTOR(entry);
> + dax.size = (type == RADIX_DAX_PMD ? PMD_SIZE : PAGE_SIZE);
> + spin_unlock_irq(&mapping->tree_lock);
> +
> + /*
> + * We cannot hold tree_lock while calling dax_map_atomic() because it
> + * eventually calls cond_resched().
> + */
> + ret = dax_map_atomic(bdev, &dax);
> + if (ret < 0)
> + return ret;
> +
> + if (WARN_ON_ONCE(ret < dax.size)) {
> + ret = -EIO;
> + goto unmap;
> + }
> +
> + wb_cache_pmem(dax.addr, dax.size);
> + unmap:
> + dax_unmap_atomic(bdev, &dax);
> + return ret;
> +
> + unlock:
> + spin_unlock_irq(&mapping->tree_lock);
> + return ret;
> +}
> +
> +/*
> + * Flush the mapping to the persistent domain within the byte range of
> [start,
> + * end]. This is required by data integrity operations to ensure file data is
> + * on persistent storage prior to completion of the operation.
> + */
> +int dax_writeback_mapping_range(struct address_space *mapping, loff_t start,
> + loff_t end)
> +{
> + struct inode *inode = mapping->host;
> + struct block_device *bdev = inode->i_sb->s_bdev;
> + pgoff_t indices[PAGEVEC_SIZE];
> + pgoff_t start_page, end_page;
> + struct pagevec pvec;
> + void *entry;
> + int i, ret = 0;
> +
> + if (WARN_ON_ONCE(inode->i_blkbits != PAGE_SHIFT))
> + return -EIO;
> +
> + rcu_read_lock();
> + entry = radix_tree_lookup(&mapping->page_tree, start & PMD_MASK);
> + rcu_read_unlock();
> +
> + /* see if the start of our range is covered by a PMD entry */
> + if (entry && RADIX_DAX_TYPE(entry) == RADIX_DAX_PMD)
> + start &= PMD_MASK;
> +
> + start_page = start >> PAGE_CACHE_SHIFT;
> + end_page = end >> PAGE_CACHE_SHIFT;
> +
> + tag_pages_for_writeback(mapping, start_page, end_page);
> +
> + pagevec_init(&pvec, 0);
> + while (1) {
> + pvec.nr = find_get_entries_tag(mapping, start_page,
> + PAGECACHE_TAG_TOWRITE, PAGEVEC_SIZE,
> + pvec.pages, indices);
> +
> + if (pvec.nr == 0)
> + break;
> +
> + for (i = 0; i < pvec.nr; i++) {
> + ret = dax_writeback_one(bdev, mapping, indices[i],
> + pvec.pages[i]);
> + if (ret < 0)
> + return ret;
> + }
I think it would be more efficient to use batched locking like follows:
spin_lock_irq(&mapping->tree_lock);
for (i = 0; i < pvec.nr; i++) {
struct blk_dax_ctl dax[PAGEVEC_SIZE];
radix_tree_tag_clear(page_tree, indices[i],
PAGECACHE_TAG_TOWRITE);
/* It is also reasonable to merge adjacent dax
* regions in to one */
dax[i].sector = RADIX_DAX_SECTOR(entry);
dax[i].size = (type == RADIX_DAX_PMD ? PMD_SIZE :
PAGE_SIZE);
}
spin_unlock_irq(&mapping->tree_lock);
if (blk_queue_enter(q, true) != 0)
goto error;
for (i = 0; i < pvec.nr; i++) {
rc = bdev_direct_access(bdev, dax[i]);
wb_cache_pmem(dax[i].addr, dax[i].size);
}
ret = blk_queue_exit(q, true)
> + }
> + wmb_pmem();
> + return 0;
> +}
> +EXPORT_SYMBOL_GPL(dax_writeback_mapping_range);
> +
> static int dax_insert_mapping(struct inode *inode, struct buffer_head *bh,
> struct vm_area_struct *vma, struct vm_fault *vmf)
> {
> @@ -363,6 +532,11 @@ static int dax_insert_mapping(struct inode *inode,
> struct buffer_head *bh,
> }
> dax_unmap_atomic(bdev, &dax);
>
> + error = dax_radix_entry(mapping, vmf->pgoff, dax.sector, false,
> + vmf->flags & FAULT_FLAG_WRITE);
> + if (error)
> + goto out;
> +
> error = vm_insert_mixed(vma, vaddr, dax.pfn);
>
> out:
> @@ -487,6 +661,7 @@ int __dax_fault(struct vm_area_struct *vma, struct
> vm_fault *vmf,
> delete_from_page_cache(page);
> unlock_page(page);
> page_cache_release(page);
> + page = NULL;
> }
I've realized that I do not understand why dax_fault code works at all.
During dax_fault we want to remove page from mapping and insert dax-entry
Basically code looks like follows:
0 page = find_get_page()
1 lock_page(page)
2 delete_from_page_cache(page);
3 unlock_page(page);
4 dax_insert_mapping(inode, &bh, vma, vmf);
BUT what on earth protects us from other process to reinsert page again
after step(2) but before (4)?
Imagine we do write to file-hole which result in to dax_fault(write), but
another task also does read fault and reinsert deleted page via dax_hole_load
As result dax_tree_entry will fail with EIO
Testcase looks very trivial, but i can not reproduce this.
>
> /*
> @@ -591,7 +766,7 @@ int __dax_pmd_fault(struct vm_area_struct *vma, unsigned
> long address,
> pgoff_t size, pgoff;
> loff_t lstart, lend;
> sector_t block;
> - int result = 0;
> + int error, result = 0;
>
> /* dax pmd mappings require pfn_t_devmap() */
> if (!IS_ENABLED(CONFIG_FS_DAX_PMD))
> @@ -733,6 +908,16 @@ int __dax_pmd_fault(struct vm_area_struct *vma, unsigned
> long address,
> }
> dax_unmap_atomic(bdev, &dax);
>
> + if (write) {
> + error = dax_radix_entry(mapping, pgoff, dax.sector,
> + true, true);
> + if (error) {
> + dax_pmd_dbg(&bh, address,
> + "PMD radix insertion failed");
> + goto fallback;
> + }
> + }
> +
> dev_dbg(part_to_dev(bdev->bd_part),
> "%s: %s addr: %lx pfn: %lx sect: %llx\n",
> __func__, current->comm, address,
> @@ -791,15 +976,12 @@ EXPORT_SYMBOL_GPL(dax_pmd_fault);
> * dax_pfn_mkwrite - handle first write to DAX page
> * @vma: The virtual memory area where the fault occurred
> * @vmf: The description of the fault
> - *
> */
> int dax_pfn_mkwrite(struct vm_area_struct *vma, struct vm_fault *vmf)
> {
> - struct super_block *sb = file_inode(vma->vm_file)->i_sb;
> + struct file *file = vma->vm_file;
>
> - sb_start_pagefault(sb);
> - file_update_time(vma->vm_file);
> - sb_end_pagefault(sb);
> + dax_radix_entry(file->f_mapping, vmf->pgoff, NO_SECTOR, false, true);
> return VM_FAULT_NOPAGE;
> }
> EXPORT_SYMBOL_GPL(dax_pfn_mkwrite);
> diff --git a/include/linux/dax.h b/include/linux/dax.h
> index e9d57f68..8204c3d 100644
> --- a/include/linux/dax.h
> +++ b/include/linux/dax.h
> @@ -41,4 +41,6 @@ static inline bool dax_mapping(struct address_space
> *mapping)
> {
> return mapping->host && IS_DAX(mapping->host);
> }
> +int dax_writeback_mapping_range(struct address_space *mapping, loff_t start,
> + loff_t end);
> #endif
> diff --git a/mm/filemap.c b/mm/filemap.c
> index 1e215fc..2e7c8d9 100644
> --- a/mm/filemap.c
> +++ b/mm/filemap.c
> @@ -482,6 +482,12 @@ int filemap_write_and_wait_range(struct address_space
> *mapping,
> {
> int err = 0;
>
> + if (dax_mapping(mapping) && mapping->nrexceptional) {
> + err = dax_writeback_mapping_range(mapping, lstart, lend);
> + if (err)
> + return err;
> + }
> +
> if (mapping->nrpages) {
> err = __filemap_fdatawrite_range(mapping, lstart, lend,
> WB_SYNC_ALL);
> --
> 2.5.0
>
> --
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