Re: [PATCH 1/2] mm: swap: use folio_batch_reinit() in folio_batch_move_lru()

[Qi Zheng <zhengqi.arch@bytedance.com>]

On 2023/4/2 21:36, Qi Zheng wrote:
> Hi Andrew,
> 
> On 2023/4/1 06:04, Andrew Morton wrote:
>> On Fri, 31 Mar 2023 17:58:57 +0800 Qi Zheng 
>> <zhengqi.arch@bytedance.com> wrote:
>>
>>> In folio_batch_move_lru(), the folio_batch is not freshly
>>> initialised, so it should call folio_batch_reinit() as
>>> pagevec_lru_move_fn() did before.
>>>
>>> ...
>>>
>>> --- a/mm/swap.c
>>> +++ b/mm/swap.c
>>> @@ -222,7 +222,7 @@ static void folio_batch_move_lru(struct 
>>> folio_batch *fbatch, move_fn_t move_fn)
>>>       if (lruvec)
>>>           unlock_page_lruvec_irqrestore(lruvec, flags);
>>>       folios_put(fbatch->folios, folio_batch_count(fbatch));
>>> -    folio_batch_init(fbatch);
>>> +    folio_batch_reinit(fbatch);
>>>   }
>>>   static void folio_batch_add_and_move(struct folio_batch *fbatch,
>>
>> Well...  why?  This could leave the kernel falsely thinking that the
>> folio's pages have been drained from the per-cpu LRU addition
>> magazines.
>>
>> Maybe that's desirable, maybe not, but I think this change needs much
>> much more explanation describing why it is beneficial.
>>
>>
>> folio_batch_reinit() seems to be a custom thing for the mlock code -
>> perhaps it just shouldn't exist, and its operation should instead be
>> open-coded in mlock_folio_batch().
> 
> The folio_batch_reinit() corresponds to pagevec_reinit(),
> the pagevec_reinit() was originally used in pagevec_lru_move_fn()
> and mlock_pagevec(), not a custom thing for the mlock code.
> 
> 
> The commit c2bc16817aa0 ("mm/swap: add folio_batch_move_lru()")
> introduces folio_batch_move_lru() to replace pagevec_lru_move_fn(),
> but calls folio_batch_init() (corresponding to pagevec_init()) instead
> of folio_batch_reinit() (corresponding to pagevec_reinit()). This
> change was not explained in the commit message and seems like an
> oversight.
> 
>>
>>
>> The dynamics and rules around ->percpu_pvec_drained are a bit
>> mysterious.  A code comment which explains all of this would be
>> useful.
> 
> The commit d9ed0d08b6c6 ("mm: only drain per-cpu pagevecs once per
> pagevec usage") originally introduced the ->drained (which was later
> renamed to ->percpu_pvec_drained by commit 7f0b5fb953e7), which is
> intended to drain per-cpu pagevecs only once per pagevec usage.
> 
> Maybe it would be better to add the following code comment:
> 
> diff --git a/mm/swap.c b/mm/swap.c
> index 423199ee8478..107c4a13e476 100644
> --- a/mm/swap.c
> +++ b/mm/swap.c
> @@ -1055,6 +1055,7 @@ EXPORT_SYMBOL(release_pages);
>    */
>   void __pagevec_release(struct pagevec *pvec)
>   {
> +       /* Only drain per-cpu pagevecs once per pagevec usage */
>          if (!pvec->percpu_pvec_drained) {
>                  lru_add_drain();
>                  pvec->percpu_pvec_drained = true;
> 
> Please let me know if I missed something.

Maybe the commit message can be modified as follows:

```
The ->percpu_pvec_drained was originally introduced by commit
d9ed0d08b6c6 ("mm: only drain per-cpu pagevecs once per pagevec usage")
to drain per-cpu pagevecs only once per pagevec usage. But after
commit c2bc16817aa0 ("mm/swap: add folio_batch_move_lru()"), the
->percpu_pvec_drained will be reset to false by calling
folio_batch_init() in folio_batch_move_lru(), which may cause per-cpu
pagevecs to be drained multiple times per pagevec usage. This is not
what we expected, let's use folio_batch_reinit() in
folio_batch_move_lru() to fix it.
```

Also +CC Mel Gorman to confirm this. :)

Thanks,
Qi

> 
> Thanks,
> Qi
> 
>>
> 
> 

-- 
Thanks,
Qi