Re: [TOME] Re: [PATCH 3/3] mm/readahead: Move the check for ra_pagesafter VM_SequentialReadHint()

From: Raghavendra D Prabhu
Date: Sun Jul 10 2011 - 09:10:15 EST

* On Sat, Jul 09, 2011 at 01:53:08PM -0700, Wu Fengguang <fengguang.wu@xxxxxxxxx> wrote:
On Sun, Jul 10, 2011 at 03:41:20AM +0800, Raghavendra D Prabhu wrote:
page_cache_sync_readahead checks for ra->ra_pages again, so moving the check after VM_SequentialReadHint.

NAK. This patch adds nothing but overheads.

--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -1566,8 +1566,6 @@ static void do_sync_mmap_readahead(struct vm_area_struct *vma,
/* If we don't want any read-ahead, don't bother */
if (VM_RandomReadHint(vma))
return;
- if (!ra->ra_pages)
- return;

if (VM_SequentialReadHint(vma)) {
page_cache_sync_readahead(mapping, ra, file, offset,
@@ -1575,6 +1573,9 @@ static void do_sync_mmap_readahead(struct vm_area_struct *vma,
return;
}

+ if (!ra->ra_pages)
+ return;
+

page_cache_sync_readahead() has the same

if (!ra->ra_pages)
return;
1. Yes, I saw that and that is why I moved it after the condition, so that duplicate checks are
not needed -- ie., if VM_SequentialReadHint is true, then
(!ra->ra_pages) is checked twice otherwise.

2. Also, another thought, is the check needed at its original place (if
not it can be removed), reasons being -- filesystems like tmpfs which
have ra_pages set to 0 don't use filemap_fault in their VMA ops and also
do_sync_mmap_readahead is called in a major page fault context.

So the patch adds the call into page_cache_sync_readahead() just to return..

Thanks,
Fengguang

--------------------------
Raghavendra Prabhu
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