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

[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 6527c2bdAcked-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
...@@ -2079,21 +2079,27 @@ generic_file_buffered_write(struct kiocb *iocb, const struct iovec *iov, ...@@ -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 */ /* Limit the size of the copy to the caller's write size */
bytes = min(bytes, count); 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, * Limit the size of the copy to that of the current
* because fault_in_pages_readable() doesn't know how to walk * segment, because fault_in_pages_readable() doesn't
* segments. * 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_. * Bring in the user page that we will copy from
* Otherwise there's a nasty deadlock on copying from the * _first_. Otherwise there's a nasty deadlock on
* same page as we're writing to, without it being marked * copying from the same page as we're writing to,
* up-to-date. * without it being marked up-to-date.
*/ */
fault_in_pages_readable(buf, bytes); fault_in_pages_readable(buf, bytes);
}
page = __grab_cache_page(mapping,index,&cached_page,&lru_pvec); page = __grab_cache_page(mapping,index,&cached_page,&lru_pvec);
if (!page) { if (!page) {
status = -ENOMEM; status = -ENOMEM;
......
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