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

[Qi Zheng]

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.

Thanks,
Qi

>