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* [PATCH] netfs: Fix kernel async DIO
@ 2025-01-07 18:39 David Howells
  2025-01-09 16:19 ` Christian Brauner
  0 siblings, 1 reply; 14+ messages in thread
From: David Howells @ 2025-01-07 18:39 UTC (permalink / raw)
  To: Christian Brauner
  Cc: dhowells, Nicolas Baranger, Paulo Alcantara, Steve French,
	Jeff Layton, netfs, linux-cifs, linux-fsdevel, linux-kernel

Netfslib needs to be able to handle kernel-initiated asynchronous DIO that
is supplied with a bio_vec[] array.  Currently, because of the async flag,
this gets passed to netfs_extract_user_iter() which throws a warning and
fails because it only handles IOVEC and UBUF iterators.  This can be
triggered through a combination of cifs and a loopback blockdev with
something like:

        mount //my/cifs/share /foo
        dd if=/dev/zero of=/foo/m0 bs=4K count=1K
        losetup --sector-size 4096 --direct-io=on /dev/loop2046 /foo/m0
        echo hello >/dev/loop2046

This causes the following to appear in syslog:

        WARNING: CPU: 2 PID: 109 at fs/netfs/iterator.c:50 netfs_extract_user_iter+0x170/0x250 [netfs]

and the write to fail.

Fix this by removing the check in netfs_unbuffered_write_iter_locked() that
causes async kernel DIO writes to be handled as userspace writes.  Note
that this change relies on the kernel caller maintaining the existence of
the bio_vec array (or kvec[] or folio_queue) until the op is complete.

Fixes: 153a9961b551 ("netfs: Implement unbuffered/DIO write support")
Reported-by: Nicolas Baranger <nicolas.baranger@3xo.fr>
Closes: https://lore.kernel.org/r/fedd8a40d54b2969097ffa4507979858@3xo.fr/
Signed-off-by: David Howells <dhowells@redhat.com>
Tested-by: Nicolas Baranger <nicolas.baranger@3xo.fr>
Acked-by: Paulo Alcantara (Red Hat) <pc@manguebit.com>
cc: Steve French <smfrench@gmail.com>
cc: Jeff Layton <jlayton@kernel.org>
cc: netfs@lists.linux.dev
cc: linux-cifs@vger.kernel.org
cc: linux-fsdevel@vger.kernel.org
---
 fs/netfs/direct_write.c |    7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/fs/netfs/direct_write.c b/fs/netfs/direct_write.c
index 173e8b5e6a93..f9421f3e6d37 100644
--- a/fs/netfs/direct_write.c
+++ b/fs/netfs/direct_write.c
@@ -67,7 +67,7 @@ ssize_t netfs_unbuffered_write_iter_locked(struct kiocb *iocb, struct iov_iter *
 		 * allocate a sufficiently large bvec array and may shorten the
 		 * request.
 		 */
-		if (async || user_backed_iter(iter)) {
+		if (user_backed_iter(iter)) {
 			n = netfs_extract_user_iter(iter, len, &wreq->iter, 0);
 			if (n < 0) {
 				ret = n;
@@ -77,6 +77,11 @@ ssize_t netfs_unbuffered_write_iter_locked(struct kiocb *iocb, struct iov_iter *
 			wreq->direct_bv_count = n;
 			wreq->direct_bv_unpin = iov_iter_extract_will_pin(iter);
 		} else {
+			/* If this is a kernel-generated async DIO request,
+			 * assume that any resources the iterator points to
+			 * (eg. a bio_vec array) will persist till the end of
+			 * the op.
+			 */
 			wreq->iter = *iter;
 		}
 

^ permalink raw reply related	[flat|nested] 14+ messages in thread
end of thread, other threads:[~2025-03-20  8:56 UTC | newest]

Thread overview: 14+ messages (download: mbox.gz follow: Atom feed
-- links below jump to the message on this page --
     [not found] <669f22fc89e45dd4e56d75876dc8f2bf@3xo.fr>
2025-01-01 18:00 ` Losetup Direct I/O breaks BACK-FILE filesystem on CIFS share (Appears in Linux 6.10 and reproduced on mainline) nicolas.baranger
2025-01-06  7:20   ` Christoph Hellwig
2025-01-06  9:13   ` David Howells
2025-01-06  9:16     ` Christoph Hellwig
2025-01-06 11:37 ` [PATCH] netfs: Fix kernel async DIO David Howells
2025-01-06 12:07   ` nicolas.baranger
2025-01-07  8:26     ` nicolas.baranger
2025-01-07 14:49     ` David Howells
2025-01-07 18:08       ` Nicolas Baranger
2025-01-06 15:34   ` Christoph Hellwig
2025-03-20  8:46     ` [Linux 6.14 - netfs/cifs] loop on file cat + file copy Nicolas Baranger
2025-01-07 12:03   ` [PATCH] netfs: Fix kernel async DIO Paulo Alcantara
2025-01-07 18:39 David Howells
2025-01-09 16:19 ` Christian Brauner

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