linux-kernel - Re: [PATCH v3 04/11] readahead: rework loop in page_cache_ra

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Date: Tue, 26 Mar 2024 11:55:06 +0100
From: Hannes Reinecke <hare@...e.de>
To: Pankaj Raghav <kernel@...kajraghav.com>,
 Matthew Wilcox <willy@...radead.org>
Cc: linux-xfs@...r.kernel.org, linux-fsdevel@...r.kernel.org,
 gost.dev@...sung.com, chandan.babu@...cle.com, mcgrof@...nel.org,
 djwong@...nel.org, linux-mm@...ck.org, linux-kernel@...r.kernel.org,
 david@...morbit.com, akpm@...ux-foundation.org,
 Pankaj Raghav <p.raghav@...sung.com>
Subject: Re: [PATCH v3 04/11] readahead: rework loop in
 page_cache_ra_unbounded()

On 3/26/24 11:06, Pankaj Raghav wrote:
> On 26/03/2024 11:00, Hannes Reinecke wrote:
>> On 3/26/24 10:44, Pankaj Raghav wrote:
>>> Hi Hannes,
>>>
>>> On 26/03/2024 10:39, Hannes Reinecke wrote:
>>>> On 3/25/24 19:41, Matthew Wilcox wrote:
>>>>> On Wed, Mar 13, 2024 at 06:02:46PM +0100, Pankaj Raghav (Samsung) wrote:
>>>>>> @@ -239,8 +239,8 @@ void page_cache_ra_unbounded(struct readahead_control *ractl,
>>>>>>                  * not worth getting one just for that.
>>>>>>                  */
>>>>>>                 read_pages(ractl);
>>>>>> -            ractl->_index++;
>>>>>> -            i = ractl->_index + ractl->_nr_pages - index - 1;
>>>>>> +            ractl->_index += folio_nr_pages(folio);
>>>>>> +            i = ractl->_index + ractl->_nr_pages - index;
>>>>>>                 continue;
>>>>>>             }
>>>>>>     @@ -252,13 +252,14 @@ void page_cache_ra_unbounded(struct readahead_control *ractl,
>>>>>>                 folio_put(folio);
>>>>>>                 read_pages(ractl);
>>>>>>                 ractl->_index++;
>>>>>> -            i = ractl->_index + ractl->_nr_pages - index - 1;
>>>>>> +            i = ractl->_index + ractl->_nr_pages - index;
>>>>>>                 continue;
>>>>>>             }
>>>>>
>>>>> You changed index++ in the first hunk, but not the second hunk.  Is that
>>>>> intentional?
>>>>
>>>> Hmm. Looks you are right; it should be modified, too.
>>>> Will be fixing it up.
>>>>
>>> You initially had also in the second hunk:
>>> ractl->index += folio_nr_pages(folio);
>>>
>>> and I changed it to what it is now.
>>>
>>> The reason is in my reply to willy:
>>> https://lore.kernel.org/linux-xfs/s4jn4t4betknd3y4ltfccqxyfktzdljiz7klgbqsrccmv3rwrd@orlwjz77oyxo/
>>>
>>> Let me know if you agree with it.
>>>
>> Bah. That really is overly complicated. When we attempt a conversion that conversion should be
>> stand-alone, not rely on some other patch modifications later on.
>> We definitely need to work on that to make it easier to review, even
>> without having to read the mail thread.
>>
> 
> I don't know understand what you mean by overly complicated. This conversion is standalone and it is
> wrong to use folio_nr_pages after we `put` the folio. This patch just reworks the loop and in the
> next patch I add min order support to readahead.
> 
> This patch doesn't depend on the next patch.
> 

Let me rephrase: what does 'ractl->_index' signify?
 From my understanding it should be the index of the
first folio/page in ractl, right?

If so I find it hard to understand how we _could_ increase it by one; 
_index should _always_ in units of the minimal pagemap size.
And if we don't have it here (as you suggested in the mailthread)
I'd rather move this patch _after_ the minimal pagesize is introduced
to ensure that _index is always incremented by the right amount.

Cheers,

Hannes