Re: Re: Re: [PATCH 2/5] mm/readahead: Change the condition for SetPageReadahead

[Raghavendra D Prabhu <raghu.prabhu13@gmail.com>]

[-- Attachment #1: Type: text/plain, Size: 2127 bytes --]

Hi,


* On Fri, Sep 28, 2012 at 07:56:23PM +0800, Fengguang Wu <fengguang.wu@intel.com> wrote:
>On Wed, Sep 26, 2012 at 06:59:00AM +0530, Raghavendra D Prabhu wrote:
>> Hi,
>>
>>
>> * On Sat, Sep 22, 2012 at 08:49:20PM +0800, Fengguang Wu <fengguang.wu@intel.com> wrote:
>> >On Sat, Sep 22, 2012 at 04:03:11PM +0530, raghu.prabhu13@gmail.com wrote:
>> >>From: Raghavendra D Prabhu <rprabhu@wnohang.net>
>> >>
>> >>If page lookup from radix_tree_lookup is successful and its index page_idx ==
>> >>nr_to_read - lookahead_size, then SetPageReadahead never gets called, so this
>> >>fixes that.
>> >
>> >NAK. Sorry. It's actually an intentional behavior, so that for the
>> >common cases of many cached files that are accessed frequently, no
>> >PG_readahead will be set at all to pointlessly trap into the readahead
>> >routines once and again.
>>
>> ACK, thanks for explaining that. However, regarding this, I would
>> like to know if the implications of the patch
>> 51daa88ebd8e0d437289f589af29d4b39379ea76 will still apply if
>> PG_readahead is not set.
>
>Would you elaborate the implication and the possible problematic case?

Certainly.


An implication of 51daa88ebd8e0d437289f589af29d4b39379ea76 is 
that, a redundant check for PageReadahead(page) is avoided since  
async is piggy-backed into the synchronous readahead itself.


So, in case of 

     page = find_get_page()
     if(!page)
         page_cache_sync_readahead()
     else if (PageReadahead(page))
         page_cache_async_readahead();

isnt' there a possibility that PG_readahead won't be set at all 
if page is not in cache (causing page_cache_sync_readahead) but 
page at index  (nr_to_read - lookahead_size) is already in the 
cache? (due to if (page) continue; in the code)?


Hence, I changed the condition from equality to >= for setting 
SetPageReadahead(page) (and added a 
variable so that it is done only once). 


>
>Thanks,
>Fengguang
>




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

[-- Attachment #2: Type: application/pgp-signature, Size: 490 bytes --]