[PATCH] knfsd: stop NFSD writes from being broken into lots of little writes to filesystem

When NFSD receives a write request, the data is typically in a number of
1448 byte segments and writev is used to collect them together.

Unfortunately, generic_file_buffered_write passes these to the filesystem
one at a time, so an e.g.  32K over-write becomes a series of partial-page
writes to each page, causing the filesystem to have to pre-read those pages
- wasted effort.

generic_file_buffered_write handles one segment of the vector at a time as
it has to pre-fault in each segment to avoid deadlocks.  When writing from
kernel-space (and nfsd does) this is not an issue, so
generic_file_buffered_write does not need to break and iovec from nfsd into
little pieces.

This patch avoids the splitting when  get_fs is KERNEL_DS as it is
from NFSd.

This issue was introduced by commit 6527c2bdf1f833cc18e8f42bd97973d583e4aa83

Acked-by: Nick Piggin <nickpiggin@yahoo.com.au>
Cc: Norman Weathers <norman.r.weathers@conocophillips.com>
Cc: Vladimir V. Saveliev <vs@namesys.com>
Signed-off-by: Neil Brown <neilb@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 19 insertions, 13 deletions

diff --git a/mm/filemap.c b/mm/filemap.c
index 00414849a867..d1060b8d3cd6 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -2079,21 +2079,27 @@ 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);
 
-                /*
-                 * Limit the size of the copy to that of the current segment,
-                 * because fault_in_pages_readable() doesn't know how to walk
-                 * segments.
+                /* 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
                  */
-                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);
+                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);
 
+                        /*
+                         * 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;