Re: [RFC PATCH 5/5] Revert "mm/migrate: __unmap_and_move() push good newpage to LRU"

["Huang, Ying" <ying.huang@intel.com>]

Yosry Ahmed <yosryahmed@google.com> writes:

> On Sun, Jun 18, 2023 at 7:00 PM Huang, Ying <ying.huang@intel.com> wrote:
>>
>> Hi, Yosry,
>>
>> Yosry Ahmed <yosryahmed@google.com> writes:
>>
>> > This reverts commit c3096e6782b733158bf34f6bbb4567808d4e0740.
>> >
>> > That commit made sure we immediately add the new page to the LRU before
>> > remove_migration_ptes() is called in migrate_move_folio() (used to be
>> > __unmap_and_move() back then), such that the rmap walk will rebuild the
>> > correct mlock_count for the page again. This was needed because the
>> > mlock_count was lost when the page is isolated. This is no longer the
>> > case since mlock_count no longer overlays page->lru.
>> >
>> > Revert the commit (the code was foliated afterward the commit, so the
>> > revert is updated as such).
>> >
>> > Signed-off-by: Yosry Ahmed <yosryahmed@google.com>
>> > ---
>> >  mm/migrate.c | 24 +++++++++---------------
>> >  1 file changed, 9 insertions(+), 15 deletions(-)
>> >
>> > diff --git a/mm/migrate.c b/mm/migrate.c
>> > index 01cac26a3127..68f693731865 100644
>> > --- a/mm/migrate.c
>> > +++ b/mm/migrate.c
>> > @@ -1279,19 +1279,6 @@ static int migrate_folio_move(free_page_t put_new_page, unsigned long private,
>> >       if (unlikely(!is_lru))
>> >               goto out_unlock_both;
>>
>> The patch itself looks good to me!  Thanks!
>>
>> Reviewed-by: "Huang, Ying" <ying.huang@intel.com>
>
> Thanks for taking a look!
>
>>
>> And, it seems that we can remove the above 2 lines and "out_unlock_both"
>> label now.  That can make the code simpler a little.  Right?
>
> I am not familiar with this code. If we remove the above condition
> then pages that have is_lru == 0 (i.e __PageMovable(src) is true) and
> page_was_mapped == 1 will call remove_migration_ptes(). This wouldn't
> happen without removing the above 2 lines. If this combination is
> impossible (is_lru == 0 && page_was_mapped == 1), then yeah we can
> remove the above condition.
>
> It looks like __SetPageMovable() is only called by zsmalloc, z3fold,
> and balloon_page_insert(). The former 2 will never have those pages
> mapped into userspace. I am not familiar with balloon_page_insert(),
> but my gut feeling is that those are pages used by the driver and are
> also not mapped into userspace.

You can take a look at migrate_folio_unmap(), where "page_was_mapped"
will not be set to 1 if !is_lru.

Best Regards,
Huang, Ying

> So I guess we can just remove the condition, but a confirmation for
> the above would be reassuring :)
>
>>
>> Best Regards,
>> Huang, Ying
>>
>> > -     /*
>> > -      * When successful, push dst to LRU immediately: so that if it
>> > -      * turns out to be an mlocked page, remove_migration_ptes() will
>> > -      * automatically build up the correct dst->mlock_count for it.
>> > -      *
>> > -      * We would like to do something similar for the old page, when
>> > -      * unsuccessful, and other cases when a page has been temporarily
>> > -      * isolated from the unevictable LRU: but this case is the easiest.
>> > -      */
>> > -     folio_add_lru(dst);
>> > -     if (page_was_mapped)
>> > -             lru_add_drain();
>> > -
>> >       if (page_was_mapped)
>> >               remove_migration_ptes(src, dst, false);
>> >
>> > @@ -1301,9 +1288,16 @@ static int migrate_folio_move(free_page_t put_new_page, unsigned long private,
>> >       /*
>> >        * If migration is successful, decrease refcount of dst,
>> >        * which will not free the page because new page owner increased
>> > -      * refcounter.
>> > +      * refcounter. As well, if it is LRU folio, add the folio to LRU
>> > +      * list in here. Use the old state of the isolated source folio to
>> > +      * determine if we migrated a LRU folio. dst was already unlocked
>> > +      * and possibly modified by its owner - don't rely on the folio
>> > +      * state.
>> >        */
>> > -     folio_put(dst);
>> > +     if (unlikely(!is_lru))
>> > +             folio_put(dst);
>> > +     else
>> > +             folio_putback_lru(dst);
>> >
>> >       /*
>> >        * A folio that has been migrated has all references removed
>>