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Message-ID: <873caf750d495a1850839f30fb120be7c6b5fd36.camel@kernel.org>
Subject: Re: [PATCH] netfs: Fix the pre-flush when appending to a file in
 writethrough mode
From: Jeffrey Layton <jlayton@kernel.org>
To: David Howells <dhowells@redhat.com>, Christian Brauner
 <brauner@kernel.org>
Cc: Eric Van Hensbergen <ericvh@kernel.org>, Latchesar Ionkov
 <lucho@ionkov.net>,  Dominique Martinet <asmadeus@codewreck.org>, Christian
 Schoenebeck <linux_oss@crudebyte.com>, Marc Dionne
 <marc.dionne@auristor.com>, netfs@lists.linux.dev, 
 linux-fsdevel@vger.kernel.org, linux-mm@kvack.org, v9fs@lists.linux.dev, 
 linux-afs@lists.infradead.org, linux-cifs@vger.kernel.org, 
 linux-kernel@vger.kernel.org
Date: Fri, 26 Apr 2024 07:43:18 -0400
In-Reply-To: <2150448.1714130115@warthog.procyon.org.uk>
References: <2150448.1714130115@warthog.procyon.org.uk>
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On Fri, 2024-04-26 at 12:15 +0100, David Howells wrote:
> In netfs_perform_write(), when the file is marked NETFS_ICTX_WRITETHROUGH
> or O_*SYNC or RWF_*SYNC was specified, write-through caching is performed
> on a buffered file.  When setting up for write-through, we flush any
> conflicting writes in the region and wait for the write to complete,
> failing if there's a write error to return.
>=20
> The issue arises if we're writing at or above the EOF position because we
> skip the flush and - more importantly - the wait.  This becomes a problem
> if there's a partial folio at the end of the file that is being written o=
ut
> and we want to make a write to it too.  Both the already-running write an=
d
> the write we start both want to clear the writeback mark, but whoever is
> second causes a warning looking something like:
>=20
>     ------------[ cut here ]------------
>     R=3D00000012: folio 11 is not under writeback
>     WARNING: CPU: 34 PID: 654 at fs/netfs/write_collect.c:105
>     ...
>     CPU: 34 PID: 654 Comm: kworker/u386:27 Tainted: G S ...
>     ...
>     Workqueue: events_unbound netfs_write_collection_worker
>     ...
>     RIP: 0010:netfs_writeback_lookup_folio
>=20
> Fix this by making the flush-and-wait unconditional.  It will do nothing =
if
> there are no folios in the pagecache and will return quickly if there are
> no folios in the region specified.
>=20
> Further, move the WBC attachment above the flush call as the flush is goi=
ng
> to attach a WBC and detach it again if it is not present - and since we
> need one anyway we might as well share it.
>=20
> Fixes: 41d8e7673a77 ("netfs: Implement a write-through caching option")
> Reported-by: kernel test robot <oliver.sang@intel.com>
> Closes: https://lore.kernel.org/oe-lkp/202404161031.468b84f-oliver.sang@i=
ntel.com
> Signed-off-by: David Howells <dhowells@redhat.com>
> cc: Eric Van Hensbergen <ericvh@kernel.org>
> cc: Latchesar Ionkov <lucho@ionkov.net>
> cc: Dominique Martinet <asmadeus@codewreck.org>
> cc: Christian Schoenebeck <linux_oss@crudebyte.com>
> cc: Marc Dionne <marc.dionne@auristor.com>
> cc: netfs@lists.linux.dev
> cc: linux-fsdevel@vger.kernel.org
> cc: linux-mm@kvack.org
> cc: v9fs@lists.linux.dev
> cc: linux-afs@lists.infradead.org
> cc: linux-cifs@vger.kernel.org
> ---
>  fs/netfs/buffered_write.c |   13 ++++++-------
>  1 file changed, 6 insertions(+), 7 deletions(-)
>=20
> diff --git a/fs/netfs/buffered_write.c b/fs/netfs/buffered_write.c
> index 9a0d32e4b422..07aff231926c 100644
> --- a/fs/netfs/buffered_write.c
> +++ b/fs/netfs/buffered_write.c
> @@ -172,15 +172,14 @@ ssize_t netfs_perform_write(struct kiocb *iocb, str=
uct iov_iter *iter,
>  	if (unlikely(test_bit(NETFS_ICTX_WRITETHROUGH, &ctx->flags) ||
>  		     iocb->ki_flags & (IOCB_DSYNC | IOCB_SYNC))
>  	    ) {
> -		if (pos < i_size_read(inode)) {
> -			ret =3D filemap_write_and_wait_range(mapping, pos, pos + iter->count)=
;
> -			if (ret < 0) {
> -				goto out;
> -			}
> -		}
> -
>  		wbc_attach_fdatawrite_inode(&wbc, mapping->host);
> =20
> +		ret =3D filemap_write_and_wait_range(mapping, pos, pos + iter->count);
> +		if (ret < 0) {
> +			wbc_detach_inode(&wbc);
> +			goto out;
> +		}
> +
>  		wreq =3D netfs_begin_writethrough(iocb, iter->count);
>  		if (IS_ERR(wreq)) {
>  			wbc_detach_inode(&wbc);
>=20

Reviewed-by: Jeffrey Layton <jlayton@kernel.org>