[PATCH 3/3 with new changelog] readahead: trigger mmap sequential readahead on PG_readahead

[Wu Fengguang <fengguang.wu@intel.com>]

Previously the mmap sequential readahead is triggered by updating
ra->prev_pos on each page fault and compare it with current page offset.

In the mosbench exim benchmark which does multi-threaded page faults on
shared struct file, the ra->mmap_miss and ra->prev_pos updates are found
to cause excessive cache line bouncing on tmpfs, which actually disabled
readahead totally (shmem_backing_dev_info.ra_pages == 0).

So remove the ra->prev_pos recording, and instead tag PG_readahead to
trigger the possible sequential readahead. It's not only more simple,
but also will work more reliably on concurrent reads on shared struct file.

Tested-by: Tim Chen <tim.c.chen@intel.com>
Reported-by: Andi Kleen <ak@linux.intel.com>
Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
---
 mm/filemap.c |    6 ++----
 1 file changed, 2 insertions(+), 4 deletions(-)

--- linux-next.orig/mm/filemap.c	2011-04-23 16:52:21.000000000 +0800
+++ linux-next/mm/filemap.c	2011-04-24 09:59:08.000000000 +0800
@@ -1531,8 +1531,7 @@ static void do_sync_mmap_readahead(struc
 	if (!ra->ra_pages)
 		return;
 
-	if (VM_SequentialReadHint(vma) ||
-			offset - 1 == (ra->prev_pos >> PAGE_CACHE_SHIFT)) {
+	if (VM_SequentialReadHint(vma)) {
 		page_cache_sync_readahead(mapping, ra, file, offset,
 					  ra->ra_pages);
 		return;
@@ -1555,7 +1554,7 @@ static void do_sync_mmap_readahead(struc
 	ra_pages = max_sane_readahead(ra->ra_pages);
 	ra->start = max_t(long, 0, offset - ra_pages / 2);
 	ra->size = ra_pages;
-	ra->async_size = 0;
+	ra->async_size = ra_pages / 4;
 	ra_submit(ra, mapping, file);
 }
 
@@ -1661,7 +1660,6 @@ retry_find:
 		return VM_FAULT_SIGBUS;
 	}
 
-	ra->prev_pos = (loff_t)offset << PAGE_CACHE_SHIFT;
 	vmf->page = page;
 	return ret | VM_FAULT_LOCKED;
 

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