Another nfsd patch for 2.6.21...
### Comments for Changeset
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
Cc: Nick Piggin <[email protected]>
Cc: Norman Weathers <[email protected]>
Cc: Vladimir V. Saveliev <[email protected]>
Signed-off-by: Neil Brown <[email protected]>
### Diffstat output
./mm/filemap.c | 32 +++++++++++++++++++-------------
1 file changed, 19 insertions(+), 13 deletions(-)
diff .prev/mm/filemap.c ./mm/filemap.c
--- .prev/mm/filemap.c 2007-02-13 16:53:19.000000000 +1100
+++ ./mm/filemap.c 2007-02-13 16:52:51.000000000 +1100
@@ -2160,21 +2160,27 @@ 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;
-
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