Message-ID: <CAJD7tkYH-9YoLMSc4RLd0P4hmMcV4mzko8oijLXNOA_dquHJqA@mail.gmail.com>
Date:   Thu, 20 Jul 2023 18:35:26 -0700
From:   Yosry Ahmed <yosryahmed@...gle.com>
To:     "Yin, Fengwei" <fengwei.yin@...el.com>
Cc:     Hugh Dickins <hughd@...gle.com>, Yu Zhao <yuzhao@...gle.com>,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        akpm@...ux-foundation.org, willy@...radead.org, david@...hat.com,
        ryan.roberts@....com, shy828301@...il.com
Subject: Re: [RFC PATCH v2 3/3] mm: mlock: update mlock_pte_range to handle
 large folio

On Thu, Jul 20, 2023 at 6:12 PM Yin, Fengwei <fengwei.yin@...el.com> wrote:
>
>
>
> On 7/21/2023 4:51 AM, Yosry Ahmed wrote:
> > On Thu, Jul 20, 2023 at 5:03 AM Yin, Fengwei <fengwei.yin@...el.com> wrote:
> >>
> >>
> >>
> >> On 7/19/2023 11:44 PM, Yosry Ahmed wrote:
> >>> On Wed, Jul 19, 2023 at 7:26 AM Hugh Dickins <hughd@...gle.com> wrote:
> >>>>
> >>>> On Wed, 19 Jul 2023, Yin Fengwei wrote:
> >>>>>>>>>>>>>> Could this also happen against normal 4K page? I mean when user try to munlock
> >>>>>>>>>>>>>> a normal 4K page and this 4K page is isolated. So it become unevictable page?
> >>>>>>>>>>>>> Looks like it can be possible. If cpu 1 is in __munlock_folio() and
> >>>>>>>>>>>>> cpu 2 is isolating the folio for any purpose:
> >>>>>>>>>>>>>
> >>>>>>>>>>>>> cpu1                        cpu2
> >>>>>>>>>>>>>                             isolate folio
> >>>>>>>>>>>>> folio_test_clear_lru() // 0
> >>>>>>>>>>>>>                             putback folio // add to unevictable list
> >>>>>>>>>>>>> folio_test_clear_mlocked()
> >>>>>>>>>>                                folio_set_lru()
> >>>>> Let's wait the response from Huge and Yu. :).
> >>>>
> >>>> I haven't been able to give it enough thought, but I suspect you are right:
> >>>> that the current __munlock_folio() is deficient when folio_test_clear_lru()
> >>>> fails.
> >>>>
> >>>> (Though it has not been reported as a problem in practice: perhaps because
> >>>> so few places try to isolate from the unevictable "list".)
> >>>>
> >>>> I forget what my order of development was, but it's likely that I first
> >>>> wrote the version for our own internal kernel - which used our original
> >>>> lruvec locking, which did not depend on getting PG_lru first (having got
> >>>> lru_lock, it checked memcg, then tried again if that had changed).
> >>>
> >>> Right. Just holding the lruvec lock without clearing PG_lru would not
> >>> protect against memcg movement in this case.
> >>>
> >>>>
> >>>> I was uneasy with the PG_lru aspect of upstream lru_lock implementation,
> >>>> but it turned out to work okay - elsewhere; but it looks as if I missed
> >>>> its implication when adapting __munlock_page() for upstream.
> >>>>
> >>>> If I were trying to fix this __munlock_folio() race myself (sorry, I'm
> >>>> not), I would first look at that aspect: instead of folio_test_clear_lru()
> >>>> behaving always like a trylock, could "folio_wait_clear_lru()" or whatever
> >>>> spin waiting for PG_lru here?
> >>>
> >>> +Matthew Wilcox
> >>>
> >>> It seems to me that before 70dea5346ea3 ("mm/swap: convert lru_add to
> >>> a folio_batch"), __pagevec_lru_add_fn() (aka lru_add_fn()) used to do
> >>> folio_set_lru() before checking folio_evictable(). While this is
> >>> probably extraneous since folio_batch_move_lru() will set it again
> >>> afterwards, it's probably harmless given that the lruvec lock is held
> >>> throughout (so no one can complete the folio isolation anyway), and
> >>> given that there were no problems introduced by this extra
> >>> folio_set_lru() as far as I can tell.
> >> After checking related code, Yes. Looks fine if we move folio_set_lru()
> >> before if (folio_evictable(folio)) in lru_add_fn() because of holding
> >> lru lock.
> >>
> >>>
> >>> If we restore folio_set_lru() to lru_add_fn(), and revert 2262ace60713
> >>> ("mm/munlock:
> >>> delete smp_mb() from __pagevec_lru_add_fn()") to restore the strict
> >>> ordering between manipulating PG_lru and PG_mlocked, I suppose we can
> >>> get away without having to spin. Again, that would only be possible if
> >>> reworking mlock_count [1] is acceptable. Otherwise, we can't clear
> >>> PG_mlocked before PG_lru in __munlock_folio().
> >> What about following change to move mlocked operation before check lru
> >> in __munlock_folio()?
> >
> > It seems correct to me on a high level, but I think there is a subtle problem:
> >
> > We clear PG_mlocked before trying to isolate to make sure that if
> > someone already has the folio isolated they will put it back on an
> > evictable list, then if we are able to isolate the folio ourselves and
> > find that the mlock_count is > 0, we set PG_mlocked again.
> >
> > There is a small window where PG_mlocked might be temporarily cleared
> > but the folio is not actually munlocked (i.e we don't update the
> > NR_MLOCK stat). In that window, a racing reclaimer on a different cpu
> > may find VM_LOCKED from in a different vma, and call mlock_folio(). In
> > mlock_folio(), we will call folio_test_set_mlocked(folio) and see that
> > PG_mlocked is clear, so we will increment the MLOCK stats, even though
> > the folio was already mlocked. This can cause MLOCK stats to be
> > unbalanced (increments more than decrements), no?
> Looks like NR_MLOCK is always connected to PG_mlocked bit. Not possible
> to be unbalanced.
>
> Let's say:
>   mlock_folio()  NR_MLOCK increase and set mlocked
>   mlock_folio()  NR_MLOCK NO change as folio is already mlocked
>
>   __munlock_folio() with isolated folio. NR_MLOCK decrease (0) and
>                                          clear mlocked
>
>   folio_putback_lru()
>   reclaimed mlock_folio()  NR_MLOCK increase and set mlocked
>
>   munlock_folio()  NR_MLOCK decrease (0) and clear mlocked
>   munlock_folio()  NR_MLOCK NO change as folio has no mlocked set

Right. The problem with the diff is that we temporarily clear
PG_mlocked *without* updating NR_MLOCK.

Consider a folio that is mlocked by two vmas. NR_MLOCK = folio_nr_pages.

Assume cpu 1 is doing __munlock_folio from one of the vmas, while cpu
2 is doing reclaim.

cpu 1                                        cpu2
clear PG_mlocked
                                                 folio_referenced()
                                                   mlock_folio()
                                                     set PG_mlocked
                                                       add to NR_MLOCK
mlock_count > 0
set PG_mlocked
goto out

Result: NR_MLOCK = folio_nr_pages * 2.

When the folio is munlock()'d later from the second vma, NR_MLOCK will
be reduced to folio_nr_pages, but there are not mlocked folios.

This is the scenario that I have in mind. Please correct me if I am wrong.

>
>
> Regards
> Yin, Fengwei
>
> >
> >>
> >> diff --git a/mm/mlock.c b/mm/mlock.c
> >> index 0a0c996c5c21..514f0d5bfbfd 100644
> >> --- a/mm/mlock.c
> >> +++ b/mm/mlock.c
> >> @@ -122,7 +122,9 @@ static struct lruvec *__mlock_new_folio(struct folio *folio, struct lruvec *lruv
> >>  static struct lruvec *__munlock_folio(struct folio *folio, struct lruvec *lruvec)
> >>  {
> >>         int nr_pages = folio_nr_pages(folio);
> >> -       bool isolated = false;
> >> +       bool isolated = false, mlocked = true;
> >> +
> >> +       mlocked = folio_test_clear_mlocked(folio);
> >>
> >>         if (!folio_test_clear_lru(folio))
> >>                 goto munlock;
> >> @@ -134,13 +136,17 @@ static struct lruvec *__munlock_folio(struct folio *folio, struct lruvec *lruvec
> >>                 /* Then mlock_count is maintained, but might undercount */
> >>                 if (folio->mlock_count)
> >>                         folio->mlock_count--;
> >> -               if (folio->mlock_count)
> >> +               if (folio->mlock_count) {
> >> +                       if (mlocked)
> >> +                               folio_set_mlocked(folio);
> >>                         goto out;
> >> +               }
> >>         }
> >>         /* else assume that was the last mlock: reclaim will fix it if not */
> >>
> >>  munlock:
> >> -       if (folio_test_clear_mlocked(folio)) {
> >> +       if (mlocked) {
> >>                 __zone_stat_mod_folio(folio, NR_MLOCK, -nr_pages);
> >>                 if (isolated || !folio_test_unevictable(folio))
> >>                         __count_vm_events(UNEVICTABLE_PGMUNLOCKED, nr_pages);
> >>
> >>
> >>>
> >>> I am not saying this is necessarily better than spinning, just a note
> >>> (and perhaps selfishly making [1] more appealing ;)).
> >>>
> >>> [1]https://lore.kernel.org/lkml/20230618065719.1363271-1-yosryahmed@google.com/
> >>>
> >>>>
> >>>> Hugh

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