[PATCH v4] 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")
Reviewed-by: Boris Burkov <[email protected]>
Signed-off-by: Yichong Chen <[email protected]>
---
v4:
- Add a comment explaining the locked uptodate recheck.
- Add Boris' Reviewed-by.

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 | 16 +++++++++++-----
 1 file changed, 11 insertions(+), 5 deletions(-)

diff --git a/fs/btrfs/verity.c b/fs/btrfs/verity.c
index 983365a73541..1133a56c0568 100644
--- a/fs/btrfs/verity.c
+++ b/fs/btrfs/verity.c
@@ -720,14 +720,18 @@ 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;
+		/* Another reader may have filled the folio while we waited. */
+		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 +748,7 @@ 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 +758,7 @@ 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