Donate to e Foundation | Murena handsets with /e/OS | Own a part of Murena! Learn more

Commit 29dbb3fc authored by NeilBrown's avatar NeilBrown Committed by Linus Torvalds
Browse files

[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 6527c2bd

Acked-by: default avatarNick Piggin <nickpiggin@yahoo.com.au>
Cc: Norman Weathers <norman.r.weathers@conocophillips.com>
Cc: Vladimir V. Saveliev <vs@namesys.com>
Signed-off-by: default avatarNeil Brown <neilb@suse.de>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent 3160a711

mm/filemap.c

+19 −13
Original line number Diff line number Diff line
@@ -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);



		/* 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

		 */

		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.

			 * 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.

			 * 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;