[PATCH v2 4/4] btrfs: zoned: don't clobber the extent buffer when zeroing it out

On a zoned filesystem a freed-but-still-dirty tree block is written out
as zeros (EXTENT_BUFFER_ZONED_ZEROOUT) only to keep the zone write
pointer advancing. btree_csum_one_bio() implemented this by memzeroing
the extent buffer's own folios before submission.

That destroys the in-memory buffer while it may still be referenced. In
particular btrfs_free_tree_block() can run on it afterwards and reads
the header to add a delayed reference; once the header has been zeroed
it frees bytenr 0 and corrupts the extent tree (the
btrfs_header_bytenr(buf) != 0 ASSERT in btrfs_free_tree_block(), or an
"unable to find ref" abort). It is flaky and reproduces under fsstress,
e.g. generic/461 and generic/013.

Write the zeros to disk from the shared zero page instead and leave the
extent buffer content untouched, so any later reference - including the
delayed reference from btrfs_free_tree_block() - still sees a valid
header. end_bbio_meta_write() now clears writeback on the buffer's own
folios, as the bio no longer carries them.

Fixes: aa6313e6ff2b ("btrfs: zoned: don't clear dirty flag of extent buffer")
Assisted-by: LLM (debugging, commit message)
Signed-off-by: Johannes Thumshirn <[email protected]>
---
 fs/btrfs/disk-io.c   | 13 +++++++------
 fs/btrfs/extent_io.c | 31 ++++++++++++++++++++++++-------
 2 files changed, 31 insertions(+), 13 deletions(-)

diff --git a/fs/btrfs/disk-io.c b/fs/btrfs/disk-io.c
index 01d01e10d000..a1277c8f2871 100644
--- a/fs/btrfs/disk-io.c
+++ b/fs/btrfs/disk-io.c
@@ -271,14 +271,15 @@ int btree_csum_one_bio(struct btrfs_bio *bbio)
 		return -EIO;
 
 	/*
-	 * If an extent_buffer is marked as EXTENT_BUFFER_ZONED_ZEROOUT, don't
-	 * checksum it but zero-out its content. This is done to preserve
-	 * ordering of I/O without unnecessarily writing out data.
+	 * An extent_buffer marked EXTENT_BUFFER_ZONED_ZEROOUT is written out as
+	 * zeros to preserve ordering of I/O without persisting the now
+	 * unnecessary block. The bio is fed from the shared zero page (see
+	 * write_one_eb()), so there is nothing to checksum here. Crucially, the
+	 * buffer's own content is left intact: it may still be referenced, e.g.
+	 * btrfs_free_tree_block() reads its header to add a delayed reference.
 	 */
-	if (test_bit(EXTENT_BUFFER_ZONED_ZEROOUT, &eb->bflags)) {
-		memzero_extent_buffer(eb, 0, eb->len);
+	if (test_bit(EXTENT_BUFFER_ZONED_ZEROOUT, &eb->bflags))
 		return 0;
-	}
 
 	if (WARN_ON_ONCE(found_start != eb->start))
 		return -EIO;
diff --git a/fs/btrfs/extent_io.c b/fs/btrfs/extent_io.c
index fa473476b7db..e88381a40600 100644
--- a/fs/btrfs/extent_io.c
+++ b/fs/btrfs/extent_io.c
@@ -2266,14 +2266,17 @@ static struct extent_buffer *find_extent_buffer_nolock(
 static void end_bbio_meta_write(struct btrfs_bio *bbio)
 {
 	struct extent_buffer *eb = bbio->private;
-	struct folio_iter fi;
 
 	if (bbio->bio.bi_status != BLK_STS_OK)
 		set_btree_ioerr(eb);
 
-	bio_for_each_folio_all(fi, &bbio->bio) {
-		btrfs_meta_folio_clear_writeback(fi.folio, eb);
-	}
+	/*
+	 * Clear writeback on the buffer's own folios. The bio may carry the
+	 * shared zero page instead (EXTENT_BUFFER_ZONED_ZEROOUT), so iterate
+	 * the extent buffer folios rather than the bio folios.
+	 */
+	for (int i = 0; i < num_extent_folios(eb); i++)
+		btrfs_meta_folio_clear_writeback(eb->folios[i], eb);
 
 	buffer_tree_clear_mark(eb, PAGECACHE_TAG_WRITEBACK);
 	clear_and_wake_up_bit(EXTENT_BUFFER_WRITEBACK, &eb->bflags);
@@ -2314,7 +2317,8 @@ static noinline_for_stack void write_one_eb(struct extent_buffer *eb,
 	struct btrfs_fs_info *fs_info = eb->fs_info;
 	struct btrfs_bio *bbio;
 
-	prepare_eb_write(eb);
+	if (!test_bit(EXTENT_BUFFER_ZONED_ZEROOUT, &eb->bflags))
+		prepare_eb_write(eb);
 
 	bbio = btrfs_bio_alloc(INLINE_EXTENT_BUFFER_PAGES,
 			       REQ_OP_WRITE | REQ_META | wbc_to_write_flags(wbc),
@@ -2334,8 +2338,21 @@ static noinline_for_stack void write_one_eb(struct extent_buffer *eb,
 		btrfs_meta_folio_set_writeback(folio, eb);
 		if (!folio_test_dirty(folio))
 			wbc->nr_to_write -= folio_nr_pages(folio);
-		bio_add_folio_nofail(&bbio->bio, folio, range_len,
-				     offset_in_folio(folio, range_start));
+		if (test_bit(EXTENT_BUFFER_ZONED_ZEROOUT, &eb->bflags)) {
+			u32 off = 0;
+
+			while (off < range_len) {
+				u32 add = min_t(u32, PAGE_SIZE, range_len - off);
+
+				bio_add_folio_nofail(&bbio->bio,
+						     page_folio(ZERO_PAGE(0)),
+						     add, 0);
+				off += add;
+			}
+		} else {
+			bio_add_folio_nofail(&bbio->bio, folio, range_len,
+					     offset_in_folio(folio, range_start));
+		}
 		wbc_account_cgroup_owner(wbc, folio, range_len);
 		folio_unlock(folio);
 	}
-- 
2.55.0