[PATCH] netfs: Fix the pre-flush when appending to a file in writethrough mode

[Jeffrey Layton]

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.
> 
> 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 out
> and we want to make a write to it too.  Both the already-running write and
> the write we start both want to clear the writeback mark, but whoever is
> second causes a warning looking something like:
> 
>     ------------[ cut here ]------------
>     R=00000012: 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
> 
> 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.
> 
> Further, move the WBC attachment above the flush call as the flush is going
> 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.
> 
> Fixes: 41d8e7673a77 ("netfs: Implement a write-through caching option")
> Reported-by: kernel test robot <oliver.sang at intel.com>
> Closes: https://lore.kernel.org/oe-lkp/202404161031.468b84f-oliver.sang@intel.com
> Signed-off-by: David Howells <dhowells at redhat.com>
> cc: Eric Van Hensbergen <ericvh at kernel.org>
> cc: Latchesar Ionkov <lucho at ionkov.net>
> cc: Dominique Martinet <asmadeus at codewreck.org>
> cc: Christian Schoenebeck <linux_oss at crudebyte.com>
> cc: Marc Dionne <marc.dionne at auristor.com>
> cc: netfs at lists.linux.dev
> cc: linux-fsdevel at vger.kernel.org
> cc: linux-mm at kvack.org
> cc: v9fs at lists.linux.dev
> cc: linux-afs at lists.infradead.org
> cc: linux-cifs at vger.kernel.org
> ---
>  fs/netfs/buffered_write.c |   13 ++++++-------
>  1 file changed, 6 insertions(+), 7 deletions(-)
> 
> 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, struct 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 = filemap_write_and_wait_range(mapping, pos, pos + iter->count);
> -			if (ret < 0) {
> -				goto out;
> -			}
> -		}
> -
>  		wbc_attach_fdatawrite_inode(&wbc, mapping->host);
>  
> +		ret = filemap_write_and_wait_range(mapping, pos, pos + iter->count);
> +		if (ret < 0) {
> +			wbc_detach_inode(&wbc);
> +			goto out;
> +		}
> +
>  		wreq = netfs_begin_writethrough(iocb, iter->count);
>  		if (IS_ERR(wreq)) {
>  			wbc_detach_inode(&wbc);
> 

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