Re. Revised [PATCH 3/3] mm/readahead: Remove the check forra->ra_pages

[Raghavendra D Prabhu]

* On Sun, Jul 10, 2011 at 08:59:06AM -0700, Wu Fengguang <fengguang.wu@xxxxxxxxx> wrote:
> On Sun, Jul 10, 2011 at 08:59:09PM +0800, Raghavendra D Prabhu wrote:
> > * 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.
>
> Ok, I see.
>
> > 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
>
> Good point. tmpfs is using shmem_fault().. Can you remove the test?
I have removed that test. Patch attached.
> > do_sync_mmap_readahead is called in a major page fault context.
>
> Right. This is irrelevant however, because if pa_pages==0, the
> page faults will normally be major ones.
>
> Thanks,
> Fengguang
>
> > >So the patch adds the call into page_cache_sync_readahead() just to return..

> > >Thanks,
> > >Fengguang

> > --------------------------
> > Raghavendra Prabhu
> > GPG Id : 0xD72BE977
> > Fingerprint: B93F EBCB 8E05 7039 CD3C A4B8 A616 DCA1 D72B E977
> > www: wnohang.net
======================================================================
 The check for ra->ra_pages is not required since fs like tmpfs which have
 ra_pages set to 0 don't use filemap_fault as part of their VMA ops (it uses
 shmem_fault). Also, page_cache_sync_readahead does its own check for ra_pages.

 Signed-off-by: Raghavendra D Prabhu <rprabhu@xxxxxxxxxxx>
 ---
  mm/filemap.c |    2 --
  1 files changed, 0 insertions(+), 2 deletions(-)

 diff --git a/mm/filemap.c b/mm/filemap.c
 index 074c23d..0bcd276 100644
 --- 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,
 --
 1.7.6


--------------------------
Raghavendra Prabhu
GPG Id : 0xD72BE977
Fingerprint: B93F EBCB 8E05 7039 CD3C A4B8 A616 DCA1 D72B E977
www: wnohang.net
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