[PATCH RFC 2/8] btrfs: make btrfs_subpage_end_and_test_writer() to handle pages not locked by btrfs_page_start_writer_lock()
From: Qu Wenruo <hidden>
Date: 2021-06-23 05:55:39
Subsystem:
btrfs file system, filesystems (vfs and infrastructure), the rest · Maintainers:
Chris Mason, David Sterba, Alexander Viro, Christian Brauner, Linus Torvalds
Normally if a page is locked by page_lock() other than btrfs_page_start_writer_lock(), it should be passed as @locked_page for various extent_clear_unlock_delalloc() call sites. But there are quite some call sites in compression path, where we intentionally call extent_clear_unlock_delalloc() with @locked_page == NULL. This will abuse extent_clear_unlock_delalloc() to unlock @locked_page. This works fine for regular page size, but not really for subpage, as if a page is locked by btrfs_page_start_writer_lock() it will have proper @writers value increased. If a page not locked by btrfs_page_start_writer_lock() we will underflow the value. But thankfully, for such pages its @writers value should be zero, and we can use that to distinguish page locked by btrfs_page_start_writer_lock() and @locked_page by delalloc. Signed-off-by: Qu Wenruo <redacted> --- fs/btrfs/subpage.c | 24 +++++++++++++++++++++++- 1 file changed, 23 insertions(+), 1 deletion(-)
diff --git a/fs/btrfs/subpage.c b/fs/btrfs/subpage.c
index a61aa33aeeee..72d5d4712933 100644
--- a/fs/btrfs/subpage.c
+++ b/fs/btrfs/subpage.c@@ -245,11 +245,33 @@ bool btrfs_subpage_end_and_test_writer(const struct btrfs_fs_info *fs_info, { struct btrfs_subpage *subpage = (struct btrfs_subpage *)page->private; const int nbits = (len >> fs_info->sectorsize_bits); + unsigned long flags; + bool ret; btrfs_subpage_assert(fs_info, page, start, len); + /* Will do two atomic checks, no longer atomic and need spinlock */ + spin_lock_irqsave(&subpage->lock, flags); + + /* + * In compression path, we have extent_clear_unlock_delalloc() call + * sites which intentionally pass @locked_page == NULL to unlock + * the locked page. + * + * In that case, @locked_page should has no writer counts, as it's + * not locked by btrfs_page_start_writer_lock(). + * For such case, we just return true so that + * btrfs_page_end_writer_lock() will unlock the page. + */ + if (atomic_read(&subpage->writers) == 0) { + ret = true; + spin_unlock_irqrestore(&subpage->lock, flags); + return ret; + } ASSERT(atomic_read(&subpage->writers) >= nbits); - return atomic_sub_and_test(nbits, &subpage->writers); + ret = atomic_sub_and_test(nbits, &subpage->writers); + spin_unlock_irqrestore(&subpage->lock, flags); + return ret; } /*
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2.32.0