[PATCH 6.8 119/228] netfs: Fix the pre-flush when appending to a file in writethrough mode

[Greg Kroah-Hartman]

6.8-stable review patch.  If anyone has any objections, please let me know.

------------------

From: David Howells <dhowells at redhat.com>

[ Upstream commit c97f59e276d4e93480f29a70accbd0d7273cf3f5 ]

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>
Link: https://lore.kernel.org/r/2150448.1714130115@warthog.procyon.org.uk
Reviewed-by: Jeffrey Layton <jlayton at kernel.org>
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
Signed-off-by: Christian Brauner <brauner at kernel.org>
Signed-off-by: Sasha Levin <sashal at 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 8f13ca8fbc74d..267b622d923b1 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);
-- 
2.43.0