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

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Message-ID: <b3e8129937055ff8971d8be44286f0b8@3xo.fr>
Date: Mon, 06 Jan 2025 13:07:44 +0100
From: nicolas.baranger@....fr
To: David Howells <dhowells@...hat.com>
Cc: 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

Hi David

Thanks for the job !
I will buid Linux 6.10 and mainline with the provided change and I'm 
comming here as soon as I get results from tests (CET working time).

Thanks again for help in this issue
Nicolas

Le 2025-01-06 12:37, David Howells a écrit :

> Hi Nicolas,
> 
> Does the attached fix your problem?
> 
> David
> ---
> 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(-)
> 
> diff --git a/fs/netfs/direct_write.c b/fs/netfs/direct_write.c
> index eded8afaa60b..42ce53cc216e 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->buffer.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->buffer.iter = *iter;
> }
> }