[PATCH 2/2] readahead: pass real splice size

[Fengguang Wu]

Pass real splice size to page_cache_readahead_ondemand().

The splice code works in chunks of 16 pages internally.
The readahead code should be told of the overall splice size, instead of
the internal chunk size. Otherwize bad things may happen. Imagine some
17-page random splice reads. The code before this patch will result in
two readahead calls: readahead(16); readahead(1); That leads to one
16-page I/O and one 32-page I/O: one extra I/O and 31 readahead miss pages.

Signed-off-by: Fengguang Wu <wfg@xxxxxxxxxxxxxxxx>
---
 fs/splice.c |   12 +++++-------
 1 file changed, 5 insertions(+), 7 deletions(-)

--- linux-2.6.22-rc3-mm1.orig/fs/splice.c
+++ linux-2.6.22-rc3-mm1/fs/splice.c
@@ -267,7 +267,7 @@ __generic_file_splice_read(struct file *
 			   unsigned int flags)
 {
 	struct address_space *mapping = in->f_mapping;
-	unsigned int loff, nr_pages;
+	unsigned int loff, nr_pages, req_pages;
 	struct page *pages[PIPE_BUFFERS];
 	struct partial_page partial[PIPE_BUFFERS];
 	struct page *page;
@@ -284,10 +284,8 @@ __generic_file_splice_read(struct file *
 
 	index = *ppos >> PAGE_CACHE_SHIFT;
 	loff = *ppos & ~PAGE_CACHE_MASK;
-	nr_pages = (len + loff + PAGE_CACHE_SIZE - 1) >> PAGE_CACHE_SHIFT;
-
-	if (nr_pages > PIPE_BUFFERS)
-		nr_pages = PIPE_BUFFERS;
+	req_pages = (len + loff + PAGE_CACHE_SIZE - 1) >> PAGE_CACHE_SHIFT;
+	nr_pages = min(req_pages, (unsigned)PIPE_BUFFERS);
 
 	/*
 	 * Now fill in the holes:
@@ -307,7 +305,7 @@ __generic_file_splice_read(struct file *
 	 */
 	if (spd.nr_pages < nr_pages)
 		page_cache_readahead_ondemand(mapping, &in->f_ra, in,
-				NULL, index, nr_pages - spd.nr_pages);
+				NULL, index, req_pages - spd.nr_pages);
 
 	while (spd.nr_pages < nr_pages) {
 		/*
@@ -363,7 +361,7 @@ __generic_file_splice_read(struct file *
 
 		if (PageReadahead(page))
 			page_cache_readahead_ondemand(mapping, &in->f_ra, in,
-					page, index, nr_pages - page_nr);
+					page, index, req_pages - page_nr);
 
 		/*
 		 * If the page isn't uptodate, we may need to start io on it

--
-
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