+ knfsd-stop-nfsd-writes-from-being-broken-into-lots-of-little-writes-to-filesystem.patch added to -mm tree

[akpm@linux-foundation.org]

The patch titled
     knfsd: stop NFSD writes from being broken into lots of little writes to filesystem
has been added to the -mm tree.  Its filename is
     knfsd-stop-nfsd-writes-from-being-broken-into-lots-of-little-writes-to-filesystem.patch

*** Remember to use Documentation/SubmitChecklist when testing your code ***

See http://www.zip.com.au/~akpm/linux/patches/stuff/added-to-mm.txt to find
out what to do about this

------------------------------------------------------
Subject: knfsd: stop NFSD writes from being broken into lots of little writes to filesystem
From: NeilBrown <neilb@suse.de>

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>
---

 mm/filemap.c |   32 +++++++++++++++++++-------------
 1 files changed, 19 insertions(+), 13 deletions(-)

diff -puN mm/filemap.c~knfsd-stop-nfsd-writes-from-being-broken-into-lots-of-little-writes-to-filesystem mm/filemap.c
--- a/mm/filemap.c~knfsd-stop-nfsd-writes-from-being-broken-into-lots-of-little-writes-to-filesystem
+++ a/mm/filemap.c
@@ -2033,21 +2033,27 @@ generic_file_buffered_write(struct kiocb
 		/* 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;
_

Patches currently in -mm which might be from neilb@suse.de are

origin.patch
knfsd-fix-return-value-for-writes-to-some-files-in-nfsd-filesystem.patch
knfsd-tidy-up-choice-of-filesystem-identifier-when-creating-a-filehandle.patch
knfsd-add-some-new-fsid-types.patch
knfsd-allow-the-server-to-provide-a-gid-list-when-using-auth_unix-authentication.patch
sysctl-remove-insert_at_head-from-register_sysctl.patch
revert-md-avoid-possible-bug_on-in-md-bitmap-handling-for-git-block.patch
replace-highest_possible_node_id-with-nr_node_ids.patch
convert-highest_possible_processor_id-to-nr_cpu_ids.patch
fix-quadratic-behavior-of-shrink_dcache_parent.patch
knfsd-nfsd4-fix-non-terminated-string.patch
knfsd-nfsd4-relax-checking-of-acl-inheritance-bits.patch
knfsd-nfsd4-simplify-nfsv4-posix-translation.patch
knfsd-nfsd4-represent-nfsv4-acl-with-array-instead-of-linked-list.patch
knfsd-nfsd4-fix-memory-leak-on-kmalloc-failure-in-savemem.patch
knfsd-nfsd4-fix-error-return-on-unsupported-acl.patch
knfsd-nfsd4-acls-dont-return-explicit-mask.patch
knfsd-nfsd4-acls-avoid-unnecessary-denies.patch
knfsd-nfsd4-fix-handling-of-directories-without-default-acls.patch
knfsd-stop-nfsd-writes-from-being-broken-into-lots-of-little-writes-to-filesystem.patch
readahead-nfsd-case.patch
readahead-nfsd-case-fix.patch
drivers-mdc-use-array_size-macro-when-appropriate.patch
md-dm-reduce-stack-usage-with-stacked-block-devices.patch