[PATCH v3] btrfs: retry verity reads for not-uptodate Merkle folios

btrfs_read_merkle_tree_page() can find a folio in the mapping that is
not uptodate.  After taking the folio lock, the current code treats that
state as a read error and returns -EIO.

That can make a previous transient read failure sticky.  If the failed
read left a not-uptodate folio in the mapping, later callers find that
folio and fail instead of retrying the read.

Keep the existing page-cache insertion and locking order, but retry the
Merkle item read when a not-uptodate folio is found in the mapping.  Also
unlock the folio when read_key_bytes() fails so that a later caller can
lock it and retry the read.

Fixes: 06ed09351b67 ("btrfs: convert btrfs_read_merkle_tree_page() to use a folio")
Signed-off-by: Yichong Chen <[email protected]>
---
v3:
- Keep the existing filemap_add_folio() and read ordering.
- Retry the Merkle item read when a not-uptodate folio is found, as
  suggested by Boris.
- Unlock the folio on read_key_bytes() failure so later callers can retry.

v2:
- Avoid calling filemap_remove_folio(), which is not exported.
- Add the folio to the page cache only after read_key_bytes() succeeds.

 fs/btrfs/verity.c | 15 ++++++++++-----
 1 file changed, 10 insertions(+), 5 deletions(-)

diff --git a/fs/btrfs/verity.c b/fs/btrfs/verity.c
index 983365a73541..25f021b04ce4 100644
--- a/fs/btrfs/verity.c
+++ b/fs/btrfs/verity.c
@@ -720,14 +720,17 @@ static struct page *btrfs_read_merkle_tree_page(struct inode *inode,
 			goto out;
 
 		folio_lock(folio);
-		/* If it's not uptodate after we have the lock, we got a read error. */
-		if (!folio_test_uptodate(folio)) {
+		/* Folio was truncated from mapping. */
+		if (!folio->mapping) {
 			folio_unlock(folio);
 			folio_put(folio);
-			return ERR_PTR(-EIO);
+			goto again;
 		}
-		folio_unlock(folio);
-		goto out;
+		if (folio_test_uptodate(folio)) {
+			folio_unlock(folio);
+			goto out;
+		}
+		goto read_folio;
 	}
 
 	folio = filemap_alloc_folio(mapping_gfp_constraint(inode->i_mapping, ~__GFP_FS),
@@ -744,6 +747,7 @@ static struct page *btrfs_read_merkle_tree_page(struct inode *inode,
 		return ERR_PTR(ret);
 	}
 
+read_folio:
 	/*
 	 * Merkle item keys are indexed from byte 0 in the merkle tree.
 	 * They have the form:
@@ -753,6 +757,7 @@ static struct page *btrfs_read_merkle_tree_page(struct inode *inode,
 	ret = read_key_bytes(BTRFS_I(inode), BTRFS_VERITY_MERKLE_ITEM_KEY, off,
 			     folio_address(folio), PAGE_SIZE, folio);
 	if (ret < 0) {
+		folio_unlock(folio);
 		folio_put(folio);
 		return ERR_PTR(ret);
 	}
-- 
2.51.0