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

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Date:	Sat, 9 Jul 2011 13:53:08 -0700
From:	Wu Fengguang <fengguang.wu@...el.com>
To:	Raghavendra D Prabhu <raghu.prabhu13@...il.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>,
	Raghavendra D Prabhu <rprabhu@...hang.net>
Subject: Re: [PATCH 3/3] mm/readahead: Move the check for ra_pages after
 VM_SequentialReadHint()

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;

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

Thanks,
Fengguang
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