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

[Qi Zheng]

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@xxxxxxxxxxxxx> 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