linux-kernel - Re: [PATCH] netfs: Fix kernel async DIO

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Message-ID: <14271ed82a5be7fcc5ceea5f68a10bbd@manguebit.com>
Date: Tue, 07 Jan 2025 09:03:11 -0300
From: Paulo Alcantara <pc@...guebit.com>
To: David Howells <dhowells@...hat.com>, nicolas.baranger@....fr
Cc: dhowells@...hat.com, Steve French <smfrench@...il.com>, Christoph
 Hellwig <hch@...radead.org>, Jeff Layton <jlayton@...nel.org>, Christian
 Brauner <brauner@...nel.org>, netfs@...ts.linux.dev,
 linux-cifs@...r.kernel.org, linux-fsdevel@...r.kernel.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH] netfs: Fix kernel async DIO

David Howells <dhowells@...hat.com> writes:

> netfs: Fix kernel async DIO
>
> 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@....fr>
> Closes: https://lore.kernel.org/r/fedd8a40d54b2969097ffa4507979858@3xo.fr/
> Signed-off-by: David Howells <dhowells@...hat.com>
> cc: Steve French <smfrench@...il.com>
> cc: Jeff Layton <jlayton@...nel.org>
> cc: netfs@...ts.linux.dev
> cc: linux-cifs@...r.kernel.org
> cc: linux-fsdevel@...r.kernel.org
> ---
>  fs/netfs/direct_write.c |    7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)

LGTM.  Feel free to add:

Acked-by: Paulo Alcantara (Red Hat) <pc@...guebit.com>

Thanks Christoph and Dave!