readahead: move synchronous readahead call out of splice loop

Move synchronous page_cache_readahead_ondemand() call out of splice loop.

This avoids one pointless page allocation/insertion in case of non-zero
ra_pages, or many pointless readahead calls in case of zero ra_pages.

Note that if a user sets ra_pages to less than PIPE_BUFFERS=16 pages, he will
not get expected readahead behavior anyway.  The splice code works in batches
of 16 pages, which can be taken as another form of synchronous readahead.

Signed-off-by: Fengguang Wu <wfg@mail.ustc.edu.cn>
Cc: Jens Axboe <jens.axboe@oracle.com>
Cc: Rusty Russell <rusty@rustcorp.com.au>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 6 insertions, 5 deletions

diff --git a/fs/splice.c b/fs/splice.c
index 722449f7d5d6..421b3b821152 100644
--- a/fs/splice.c
+++ b/fs/splice.c
@@ -290,13 +290,17 @@ __generic_file_splice_read(struct file *in, loff_t *ppos,
          * Lookup the (hopefully) full range of pages we need.
          */
         spd.nr_pages = find_get_pages_contig(mapping, index, nr_pages, pages);
+        index += spd.nr_pages;
 
         /*
          * If find_get_pages_contig() returned fewer pages than we needed,
-         * allocate the rest and fill in the holes.
+         * readahead/allocate the rest and fill in the holes.
          */
+        if (spd.nr_pages < nr_pages)
+                page_cache_readahead_ondemand(mapping, &in->f_ra, in,
+                                NULL, index, nr_pages - spd.nr_pages);
+
         error = 0;
-        index += spd.nr_pages;
         while (spd.nr_pages < nr_pages) {
                 /*
                  * Page could be there, find_get_pages_contig() breaks on
@@ -304,9 +308,6 @@ __generic_file_splice_read(struct file *in, loff_t *ppos,
                  */
                 page = find_get_page(mapping, index);
                 if (!page) {
-                        page_cache_readahead_ondemand(mapping, &in->f_ra, in,
-                                        NULL, index, nr_pages - spd.nr_pages);
-
                         /*
                          * page didn't exist, allocate one.
                          */