diff mbox series

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

Message ID 20230810071922.30229-2-russ@har.mn
State New
Headers show
Series cifs: Release folio lock on fscache read hit. | expand

Commit Message

Russell Harmon Aug. 10, 2023, 7:19 a.m. UTC 
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@har.mn>
---
 fs/smb/client/file.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
diff mbox series

Patch

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,
 
 io_error:
 	kunmap(page);
-	unlock_page(page);
 
 read_complete:
+	unlock_page(page);
 	return rc;
 }