[PATCH] btrfs: fix u32 to s64 type conversion in dirty_metadata_bytes accounting

[Dave Chen <davechen@synology.com>]

The percpu_counter dirty_metadata_bytes is updated by negating eb->len
and passing it to percpu_counter_add_batch(), whose amount parameter is
s64.  Since commit 84cda1a6087d ("btrfs: cache folio size and shift in
extent_buffer"), eb->len is u32.  The u32 result of -eb->len, when
widened to the s64 parameter, becomes a large positive value instead of
the intended negative value.  For eb->len == 16384 the counter adds
+4294950912 instead of subtracting 16384.

The counter therefore grows on every metadata writeback instead of
shrinking by the extent buffer size, permanently exceeding
BTRFS_DIRTY_METADATA_THRESH and causing __btrfs_btree_balance_dirty()
to trigger balance_dirty_pages_ratelimited() unconditionally, adding
unnecessary writeback pressure.

Cast eb->len to s64 before negation at both call sites so the
subtraction is performed in signed 64-bit arithmetic.

Signed-off-by: Dave Chen <davechen@synology.com>
---
 fs/btrfs/extent_io.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/fs/btrfs/extent_io.c b/fs/btrfs/extent_io.c
index 7d604524e83c..de5785117a47 100644
--- a/fs/btrfs/extent_io.c
+++ b/fs/btrfs/extent_io.c
@@ -2004,7 +2004,7 @@ static noinline_for_stack bool lock_extent_buffer_for_io(struct extent_buffer *e
 
 		btrfs_set_header_flag(eb, BTRFS_HEADER_FLAG_WRITTEN);
 		percpu_counter_add_batch(&fs_info->dirty_metadata_bytes,
-					 -eb->len,
+					 -(s64)eb->len,
 					 fs_info->dirty_metadata_batch);
 		ret = true;
 	} else {
@@ -3774,7 +3774,7 @@ void btrfs_clear_buffer_dirty(struct btrfs_trans_handle *trans,
 		return;
 
 	buffer_tree_clear_mark(eb, PAGECACHE_TAG_DIRTY);
-	percpu_counter_add_batch(&fs_info->dirty_metadata_bytes, -eb->len,
+	percpu_counter_add_batch(&fs_info->dirty_metadata_bytes, -(s64)eb->len,
 				 fs_info->dirty_metadata_batch);
 
 	for (int i = 0; i < num_extent_folios(eb); i++) {
-- 
2.43.0


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