NFS: Optimise append writes with holes

If a file is being extended, and we're creating a hole, we might as well
declare the entire page to be up to date.

This patch significantly improves the write performance for sparse files
in the case where lseek(SEEK_END) is used to append several non-contiguous
writes at intervals of < PAGE_SIZE.

Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>

1 files changed, 20 insertions, 0 deletions

diff --git a/fs/nfs/file.c b/fs/nfs/file.c
index 7c73f06692b6..7ac89a845a5e 100644
--- a/fs/nfs/file.c
+++ b/fs/nfs/file.c
@@ -344,6 +344,26 @@ static int nfs_write_end(struct file *file, struct address_space *mapping,
         unsigned offset = pos & (PAGE_CACHE_SIZE - 1);
         int status;
 
+        /*
+         * Zero any uninitialised parts of the page, and then mark the page
+         * as up to date if it turns out that we're extending the file.
+         */
+        if (!PageUptodate(page)) {
+                unsigned pglen = nfs_page_length(page);
+                unsigned end = offset + len;
+
+                if (pglen == 0) {
+                        zero_user_segments(page, 0, offset,
+                                        end, PAGE_CACHE_SIZE);
+                        SetPageUptodate(page);
+                } else if (end >= pglen) {
+                        zero_user_segment(page, end, PAGE_CACHE_SIZE);
+                        if (offset == 0)
+                                SetPageUptodate(page);
+                } else
+                        zero_user_segment(page, pglen, PAGE_CACHE_SIZE);
+        }
+
         lock_kernel();
         status = nfs_updatepage(file, page, offset, copied);
         unlock_kernel();