On 2021/1/6 上午9:01, Qu Wenruo wrote:
For subpage case, we need to allocate new memory for each metadata page.
So we need to:
- Allow attach_extent_buffer_page() to return int
To indicate allocation failure
- Prealloc page->private for alloc_extent_buffer()
We don't want to call memory allocation with spinlock hold, so
do preallocation before we acquire the spin lock.
- Handle subpage and regular case differently in
attach_extent_buffer_page()
For regular case, just do the usual thing.
For subpage case, allocate new memory and update the tree_block
bitmap.
The bitmap update will be handled by new subpage specific helper,
btrfs_subpage_set_tree_block().
Signed-off-by: Qu Wenruo <[email protected]>
---
fs/btrfs/extent_io.c | 74 ++++++++++++++++++++++++++++++++++----------
fs/btrfs/subpage.h | 50 ++++++++++++++++++++++++++++++
2 files changed, 108 insertions(+), 16 deletions(-)
diff --git a/fs/btrfs/extent_io.c b/fs/btrfs/extent_io.c
index d60f1837f8fb..2eeff925450f 100644
--- a/fs/btrfs/extent_io.c
+++ b/fs/btrfs/extent_io.c
@@ -24,6 +24,7 @@
#include "rcu-string.h"
#include "backref.h"
#include "disk-io.h"
+#include "subpage.h"
static struct kmem_cache *extent_state_cache;
static struct kmem_cache *extent_buffer_cache;
@@ -3140,22 +3141,41 @@ static int submit_extent_page(unsigned int opf,
return ret;
}
-static void attach_extent_buffer_page(struct extent_buffer *eb,
+static int attach_extent_buffer_page(struct extent_buffer *eb,
struct page *page)
{
- /*
- * If the page is mapped to btree inode, we should hold the private
- * lock to prevent race.
- * For cloned or dummy extent buffers, their pages are not mapped and
- * will not race with any other ebs.
- */
- if (page->mapping)
- lockdep_assert_held(&page->mapping->private_lock);
+ struct btrfs_fs_info *fs_info = eb->fs_info;
+ int ret;
- if (!PagePrivate(page))
- attach_page_private(page, eb);
- else
- WARN_ON(page->private != (unsigned long)eb);
+ if (fs_info->sectorsize == PAGE_SIZE) {
+ /*
+ * If the page is mapped to btree inode, we should hold the
+ * private lock to prevent race.
+ * For cloned or dummy extent buffers, their pages are not
+ * mapped and will not race with any other ebs.
+ */
+ if (page->mapping)
+ lockdep_assert_held(&page->mapping->private_lock);
+
+ if (!PagePrivate(page))
+ attach_page_private(page, eb);
+ else
+ WARN_ON(page->private != (unsigned long)eb);
+ return 0;
+ }
+
+ /* Already mapped, just update the existing range */
+ if (PagePrivate(page))
+ goto update_bitmap;
+
+ /* Do new allocation to attach subpage */
+ ret = btrfs_attach_subpage(fs_info, page);
+ if (ret < 0)
+ return ret;
+
+update_bitmap:
+ btrfs_subpage_set_tree_block(fs_info, page, eb->start, eb->len);
+ return 0;
}
void set_page_extent_mapped(struct page *page)
@@ -5063,21 +5083,29 @@ struct extent_buffer *btrfs_clone_extent_buffer(const
struct extent_buffer *src)
if (new == NULL)
return NULL;
+ set_bit(EXTENT_BUFFER_UPTODATE, &new->bflags);
+ set_bit(EXTENT_BUFFER_UNMAPPED, &new->bflags);
+
for (i = 0; i < num_pages; i++) {
+ int ret;
+
p = alloc_page(GFP_NOFS);
if (!p) {
btrfs_release_extent_buffer(new);
return NULL;
}
- attach_extent_buffer_page(new, p);
+ ret = attach_extent_buffer_page(new, p);
+ if (ret < 0) {
+ put_page(p);
+ btrfs_release_extent_buffer(new);
+ return NULL;
+ }
WARN_ON(PageDirty(p));
SetPageUptodate(p);
new->pages[i] = p;
copy_page(page_address(p), page_address(src->pages[i]));
}
- set_bit(EXTENT_BUFFER_UPTODATE, &new->bflags);
- set_bit(EXTENT_BUFFER_UNMAPPED, &new->bflags);
return new;
}
@@ -5316,6 +5344,18 @@ struct extent_buffer *alloc_extent_buffer(struct
btrfs_fs_info *fs_info,
goto free_eb;
}
+ /*
+ * Preallocate page->private for subpage case, so that
+ * we won't allocate memory with private_lock hold.
+ */
+ ret = btrfs_attach_subpage(fs_info, p);
Although we try to preallocate the subpage structure here, it's not
reliable.
I have hit a case just minutes before, where we still try to allocate
memory inside that private_lock spinlock, and causes sleep inside atomic
warning.
The problem is, we can have a race where the page has one existing eb,
and it's being freed.
At this point, we still have page::private.
But before we acquire that private_lock, the eb get freed and since it's
the only eb of that page (our eb hasn't yet being added to the page), it
detach page private.
+ if (ret < 0) {
+ unlock_page(p);
+ put_page(p);
+ exists = ERR_PTR(-ENOMEM);
+ goto free_eb;
+ }
+
spin_lock(&mapping->private_lock);
exists = grab_extent_buffer(p);
if (exists) {
@@ -5325,8 +5365,10 @@ struct extent_buffer *alloc_extent_buffer(struct
btrfs_fs_info *fs_info,
mark_extent_buffer_accessed(exists, p);
goto free_eb;
}
+ /* Should not fail, as we have attached the subpage already */
attach_extent_buffer_page(eb, p);
So here we can not rely on any result before we acquire private_lock.
Thus I guess we have to pre-allocate the memory manually and pass the
pointer in.
Why I didn't hit the bug before sending the patches...
Thanks,
Qu
spin_unlock(&mapping->private_lock);
+
WARN_ON(PageDirty(p));
eb->pages[i] = p;
if (!PageUptodate(p))
diff --git a/fs/btrfs/subpage.h b/fs/btrfs/subpage.h
index 96f3b226913e..e49d4a7329e1 100644
--- a/fs/btrfs/subpage.h
+++ b/fs/btrfs/subpage.h
@@ -23,9 +23,59 @@
struct btrfs_subpage {
/* Common members for both data and metadata pages */
spinlock_t lock;
+ union {
+ /* Structures only used by metadata */
+ struct {
+ u16 tree_block_bitmap;
+ };
+ /* structures only used by data */
+ };
};
int btrfs_attach_subpage(struct btrfs_fs_info *fs_info, struct page *page);
void btrfs_detach_subpage(struct btrfs_fs_info *fs_info, struct page *page);
+/*
+ * Convert the [start, start + len) range into a u16 bitmap
+ *
+ * E.g. if start == page_offset() + 16K, len = 16K, we get 0x00f0.
+ */
+static inline u16 btrfs_subpage_calc_bitmap(struct btrfs_fs_info *fs_info,
+ struct page *page, u64 start, u32 len)
+{
+ int bit_start = offset_in_page(start) >> fs_info->sectorsize_bits;
+ int nbits = len >> fs_info->sectorsize_bits;
+
+ /* Basic checks */
+ ASSERT(PagePrivate(page) && page->private);
+ ASSERT(IS_ALIGNED(start, fs_info->sectorsize) &&
+ IS_ALIGNED(len, fs_info->sectorsize));
+
+ /*
+ * The range check only works for mapped page, we can
+ * still have unampped page like dummy extent buffer pages.
+ */
+ if (page->mapping)
+ ASSERT(page_offset(page) <= start &&
+ start + len <= page_offset(page) + PAGE_SIZE);
+ /*
+ * Here nbits can be 16, thus can go beyond u16 range. Here we make the
+ * first left shift to be calculated in unsigned long (u32), then
+ * truncate the result to u16.
+ */
+ return (u16)(((1UL << nbits) - 1) << bit_start);
+}
+
+static inline void btrfs_subpage_set_tree_block(struct btrfs_fs_info *fs_info,
+ struct page *page, u64 start, u32 len)
+{
+ struct btrfs_subpage *subpage = (struct btrfs_subpage *)page->private;
+ unsigned long flags;
+ u16 tmp = btrfs_subpage_calc_bitmap(fs_info, page, start, len);
+
+ spin_lock_irqsave(&subpage->lock, flags);
+ subpage->tree_block_bitmap |= tmp;
+ spin_unlock_irqrestore(&subpage->lock, flags);
+}
+
#endif /* BTRFS_SUBPAGE_H */
