mm: revert KERNEL_DS buffered write optimisation

Revert the patch from Neil Brown to optimise NFSD writev handling.

Cc: Neil Brown <neilb@suse.de>
Signed-off-by: Nick Piggin <npiggin@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 13 insertions, 19 deletions

diff --git a/mm/filemap.c b/mm/filemap.c
index 7989c44cb293..c504db18ac26 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -1874,27 +1874,21 @@ generic_file_buffered_write(struct kiocb *iocb, const struct iovec *iov,
                 /* Limit the size of the copy to the caller's write size */
                 bytes = min(bytes, count);
 
-                /* We only need to worry about prefaulting when writes are from
-                 * user-space.  NFSd uses vfs_writev with several non-aligned
-                 * segments in the vector, and limiting to one segment a time is
-                 * a noticeable performance for re-write
+                /*
+                 * Limit the size of the copy to that of the current segment,
+                 * because fault_in_pages_readable() doesn't know how to walk
+                 * segments.
                  */
-                if (!segment_eq(get_fs(), KERNEL_DS)) {
-                        /*
-                         * Limit the size of the copy to that of the current
-                         * segment, because fault_in_pages_readable() doesn't
-                         * know how to walk segments.
-                         */
-                        bytes = min(bytes, cur_iov->iov_len - iov_base);
+                bytes = min(bytes, cur_iov->iov_len - iov_base);
+
+                /*
+                 * Bring in the user page that we will copy from _first_.
+                 * Otherwise there's a nasty deadlock on copying from the
+                 * same page as we're writing to, without it being marked
+                 * up-to-date.
+                 */
+                fault_in_pages_readable(buf, bytes);
 
-                        /*
-                         * Bring in the user page that we will copy from
-                         * _first_.  Otherwise there's a nasty deadlock on
-                         * copying from the same page as we're writing to,
-                         * without it being marked up-to-date.
-                         */
-                        fault_in_pages_readable(buf, bytes);
-                }
                 page = __grab_cache_page(mapping,index,&cached_page,&lru_pvec);
                 if (!page) {
                         status = -ENOMEM;