From: Wu Fengguang <fengguang.wu@intel.com>
To: Raghavendra D Prabhu <rprabhu@wnohang.net>
Cc: "linux-mm@kvack.org" <linux-mm@kvack.org>,
"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
"akpm@linux-foundation.org" <akpm@linux-foundation.org>
Subject: Re: [TOME] Re: [PATCH 3/3] mm/readahead: Move the check for ra_pages after VM_SequentialReadHint()
Date: Sun, 10 Jul 2011 08:59:06 -0700 [thread overview]
Message-ID: <20110710155906.GB7432@localhost> (raw)
In-Reply-To: <20110710125909.GA4460@Xye>
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@intel.com> 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?
> 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
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next prev parent reply other threads:[~2011-07-10 15:59 UTC|newest]
Thread overview: 11+ messages / expand[flat|nested] mbox.gz Atom feed top
2011-07-09 19:41 [PATCH 0/3] Readahead fixes Raghavendra D Prabhu
2011-07-09 19:41 ` [PATCH 1/3] mm/readahead: Change the check for PageReadahead into an else-if Raghavendra D Prabhu
2011-07-09 20:47 ` Wu Fengguang
2011-07-09 19:41 ` [PATCH 2/3] mm/readahead: Remove file_ra_state from arguments of count_history_pages Raghavendra D Prabhu
2011-07-09 20:47 ` Wu Fengguang
2011-07-09 19:41 ` [PATCH 3/3] mm/readahead: Move the check for ra_pages after VM_SequentialReadHint() Raghavendra D Prabhu
2011-07-09 20:53 ` Wu Fengguang
2011-07-10 12:59 ` [TOME] " Raghavendra D Prabhu
2011-07-10 15:59 ` Wu Fengguang [this message]
2011-07-11 23:02 ` Re. Revised [PATCH 3/3] mm/readahead: Remove the check for ra->ra_pages Raghavendra D Prabhu
2011-07-12 18:09 ` Wu Fengguang
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