[PATCH v1 1/1] cifs: Release folio lock on fscache read hit.

Russell Harmon russ at har.mn
Thu Aug 10 07:19:22 UTC 2023

Under the current code, when cifs_readpage_worker is called, the call
contract is that the callee should unlock the page. This is documented
in the read_folio section of Documentation/filesystems/vfs.rst as:

> The filesystem should unlock the folio once the read has completed,
> whether it was successful or not.

Without this change, when fscache is in use and cache hit occurs during
a read, the page lock is leaked, producing the following stack on
subsequent reads (via mmap) to the page:

$ cat /proc/3890/task/12864/stack
[<0>] folio_wait_bit_common+0x124/0x350
[<0>] filemap_read_folio+0xad/0xf0
[<0>] filemap_fault+0x8b1/0xab0
[<0>] __do_fault+0x39/0x150
[<0>] do_fault+0x25c/0x3e0
[<0>] __handle_mm_fault+0x6ca/0xc70
[<0>] handle_mm_fault+0xe9/0x350
[<0>] do_user_addr_fault+0x225/0x6c0
[<0>] exc_page_fault+0x84/0x1b0
[<0>] asm_exc_page_fault+0x27/0x30

This requires a reboot to resolve; it is a deadlock.

Note however that the call to cifs_readpage_from_fscache does mark the
page clean, but does not free the folio lock. This happens in
__cifs_readpage_from_fscache on success. Releasing the lock at that
point however is not appropriate as cifs_readahead also calls
cifs_readpage_from_fscache and *does* unconditionally release the lock
after its return. This change therefore effectively makes
cifs_readpage_worker work like cifs_readahead.

Signed-off-by: Russell Harmon <russ at har.mn>
 fs/smb/client/file.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/smb/client/file.c b/fs/smb/client/file.c
index fc5acc95cd13..767bcdd95b31 100644
--- a/fs/smb/client/file.c
+++ b/fs/smb/client/file.c
@@ -4681,9 +4681,9 @@ static int cifs_readpage_worker(struct file *file, struct page *page,
-	unlock_page(page);
+	unlock_page(page);
 	return rc;

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