[PATCH v2 6/7] netfs: Fix zeropoint update where i_size > remote_i_size

[David Howells <dhowells@redhat.com>]

Fix the update of the zero point[*] by netfs_release_folio() when there is
uncommitted data in the pagecache beyond the folio being released but the
on-server EOF is in this folio (ie. i_size > remote_i_size).  The update
needs to limit zero_point to remote_i_size, not i_size as i_size is a local
phenomenon reflecting updates made locally to the pagecache, not stuff
written to the server.  remote_i_size tracks the server's i_size.

[*] The zero point is the file position from which we can assume that the
    server will just return zeros, so we can avoid generating reads.

Note that netfs_invalidate_folio() probably doesn't need fixing as
zero_point should be updated by setattr after truncation.

Found with:

    fsx -q -N 1000000 -p 10000 -o 128000 -l 600000 \
        /xfstest.test/junk --replay-ops=junk.fsxops

using the following as junk.fsxops:

    truncate 0x0 0x1bbae 0x82864
    write 0x3ef2e 0xf9c8 0x1bbae
    write 0x67e05 0xcb5a 0x4e8f6
    mapread 0x57781 0x85b6 0x7495f
    copy_range 0x5d3d 0x10329 0x54fac 0x7495f
    write 0x64710 0x1c2b 0x7495f
    mapread 0x64000 0x1000 0x7495f

on cifs with the default cache option.

It shows read-gaps on folio 0x64 failing with a short read (ie. it hits
EOF) if the FMODE_READ check is commented out in netfs_perform_write():

                if (//(file->f_mode & FMODE_READ) ||
                    netfs_is_cache_enabled(ctx)) {

and no fscache.  This was initially found with the generic/522 xfstest.

Fixes: cce6bfa6ca0e ("netfs: Fix trimming of streaming-write folios in netfs_inval_folio()")
Signed-off-by: David Howells <dhowells@redhat.com>
cc: Paulo Alcantara <pc@manguebit.org>
cc: Matthew Wilcox <willy@infradead.org>
cc: netfs@lists.linux.dev
cc: linux-fsdevel@vger.kernel.org
---
 fs/netfs/misc.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/netfs/misc.c b/fs/netfs/misc.c
index d8e8a4b59768..e386cf31eb1e 100644
--- a/fs/netfs/misc.c
+++ b/fs/netfs/misc.c
@@ -302,7 +302,7 @@ bool netfs_release_folio(struct folio *folio, gfp_t gfp)
 	if (folio_test_dirty(folio))
 		return false;
 
-	end = umin(folio_next_pos(folio), i_size_read(&ctx->inode));
+	end = umin(folio_next_pos(folio), ctx->remote_i_size);
 	if (end > ctx->zero_point)
 		ctx->zero_point = end;