linux-kernel - Re: [TOME] Re: [PATCH 3/3] mm/readahead: Move the check for ra_pages after VM

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Date:	Sun, 10 Jul 2011 18:29:09 +0530
From:	Raghavendra D Prabhu <rprabhu@...hang.net>
To:	Wu Fengguang <fengguang.wu@...el.com>
Cc:	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>
Subject: Re: [TOME] Re: [PATCH 3/3] mm/readahead: Move the check for ra_pages
 after VM_SequentialReadHint()

* On Sat, Jul 09, 2011 at 01:53:08PM -0700, Wu Fengguang <fengguang.wu@...el.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.

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

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