netfs: Fix io_uring based write-through

This can be triggered by mounting a cifs filesystem with a cache=strict
mount option and then, using the fsx program from xfstests, doing:

        ltp/fsx -A -d -N 1000 -S 11463 -P /tmp /cifs-mount/foo \
          --replay-ops=gen112-fsxops

Where gen112-fsxops holds:

        fallocate 0x6be7 0x8fc5 0x377d3
        copy_range 0x9c71 0x77e8 0x2edaf 0x377d3
        write 0x2776d 0x8f65 0x377d3

The problem is that netfs_io_request::len is being used for two purposes
and ends up getting set to the amount of data we transferred, not the
amount of data the caller asked to be transferred (for various reasons,
such as mmap'd writes, we might end up rounding out the data written to the
server to include the entire folio at each end).

Fix this by keeping the amount we were asked to write in ->len and using
->submitted to track what we issued ops for.  Then, when we come to calling
->ki_complete(), ->len is the right size.

This also required netfs_cleanup_dio_write() to change since we're no
longer advancing wreq->len.  Use wreq->transferred instead as we might have
done a short read.

With this, the generic/112 xfstest passes if cifs is forced to put all
non-DIO opens into write-through mode.

Fixes: 288ace2f ("netfs: New writeback implementation")
Signed-off-by: David Howells <dhowells@redhat.com>
Link: https://lore.kernel.org/r/295086.1716298663@warthog.procyon.org.uk
cc: Jeff Layton <jlayton@kernel.org>
cc: Steve French <stfrench@microsoft.com>
cc: Enzo Matsumiya <ematsumiya@suse.de>
cc: netfs@lists.linux.dev
cc: v9fs@lists.linux.dev
cc: linux-afs@lists.infradead.org
cc: linux-cifs@vger.kernel.org
cc: linux-fsdevel@vger.kernel.org
Signed-off-by: Christian Brauner <brauner@kernel.org>

fs/netfs/direct_write.c

@@ -12,7 +12,7 @@
 static void netfs_cleanup_dio_write(struct netfs_io_request *wreq)
 {
 	struct inode *inode = wreq->inode;
-	unsigned long long end = wreq->start + wreq->len;
+	unsigned long long end = wreq->start + wreq->transferred;
 
 	if (!wreq->error &&
 	    i_size_read(inode) < end) {

fs/netfs/write_collect.c

@@ -510,7 +510,7 @@ static void netfs_collect_write_results(struct netfs_io_request *wreq)
 	 * stream has a gap that can be jumped.
 	 */
 	if (notes & SOME_EMPTY) {
-		unsigned long long jump_to = wreq->start + wreq->len;
+		unsigned long long jump_to = wreq->start + READ_ONCE(wreq->submitted);
 
 		for (s = 0; s < NR_IO_STREAMS; s++) {
 			stream = &wreq->io_streams[s];
@@ -690,10 +690,11 @@ void netfs_write_collection_worker(struct work_struct *work)
 	wake_up_bit(&wreq->flags, NETFS_RREQ_IN_PROGRESS);
 
 	if (wreq->iocb) {
-		wreq->iocb->ki_pos += wreq->transferred;
+		size_t written = min(wreq->transferred, wreq->len);
+		wreq->iocb->ki_pos += written;
 		if (wreq->iocb->ki_complete)
 			wreq->iocb->ki_complete(
-				wreq->iocb, wreq->error ? wreq->error : wreq->transferred);
+				wreq->iocb, wreq->error ? wreq->error : written);
 		wreq->iocb = VFS_PTR_POISON;
 	}
 

fs/netfs/write_issue.c

@@ -254,7 +254,7 @@ static void netfs_issue_write(struct netfs_io_request *wreq,
 	stream->construct = NULL;
 
 	if (subreq->start + subreq->len > wreq->start + wreq->submitted)
-		wreq->len = wreq->submitted = subreq->start + subreq->len - wreq->start;
+		WRITE_ONCE(wreq->submitted, subreq->start + subreq->len - wreq->start);
 	netfs_do_issue_write(stream, subreq);
 }