linux-kernel - Re: [RFC PATCH v2 3/3] mm: mlock: update mlock_pte

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Date:   Tue, 18 Jul 2023 18:32:12 -0700
From:   Yosry Ahmed <yosryahmed@...gle.com>
To:     Yu Zhao <yuzhao@...gle.com>
Cc:     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,
        Yin Fengwei <fengwei.yin@...el.com>,
        Hugh Dickins <hughd@...gle.com>
Subject: Re: [RFC PATCH v2 3/3] mm: mlock: update mlock_pte_range to handle
 large folio

On Tue, Jul 18, 2023 at 4:47 PM Yin Fengwei <fengwei.yin@...el.com> wrote:
>
>
>
> On 7/19/23 06:48, Yosry Ahmed wrote:
> > On Sun, Jul 16, 2023 at 6:58 PM Yin Fengwei <fengwei.yin@...el.com> wrote:
> >>
> >>
> >>
> >> On 7/17/23 08:35, Yu Zhao wrote:
> >>> On Sun, Jul 16, 2023 at 6:00 PM Yin, Fengwei <fengwei.yin@...el.com> wrote:
> >>>>
> >>>> On 7/15/2023 2:06 PM, Yu Zhao wrote:
> >>>>> There is a problem here that I didn't have the time to elaborate: we
> >>>>> can't mlock() a folio that is within the range but not fully mapped
> >>>>> because this folio can be on the deferred split queue. When the split
> >>>>> happens, those unmapped folios (not mapped by this vma but are mapped
> >>>>> into other vmas) will be stranded on the unevictable lru.
> >>>>
> >>>> This should be fine unless I missed something. During large folio split,
> >>>> the unmap_folio() will be migrate(anon)/unmap(file) folio. Folio will be
> >>>> munlocked in unmap_folio(). So the head/tail pages will be evictable always.
> >>>
> >>> It's close but not entirely accurate: munlock can fail on isolated folios.
> >> Yes. The munlock just clear PG_mlocked bit but with PG_unevictable left.
> >>
> >> 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()
> Yes. Yu showed this sequence to me in another email. I thought the putback_lru()
> could correct the none-mlocked but unevictable folio. But it doesn't because
> of this race.

(+Hugh Dickins for vis)

Yu, I am not familiar with the split_folio() case, so I am not sure it
is the same exact race I stated above.

Can you confirm whether or not doing folio_test_clear_mlocked() before
folio_test_clear_lru() would fix the race you are referring to? IIUC,
in this case, we make sure we clear PG_mlocked before we try to to
clear PG_lru. If we fail to clear it, then someone else have the folio
isolated after we clear PG_mlocked, so we can be sure that when they
put the folio back it will be correctly made evictable.

Is my understanding correct?

If yes, I can add this fix to my next version of the RFC series to
rework mlock_count. It would be a lot more complicated with the
current implementation (as I stated in a previous email).

>
> >
> >
> > The page would be stranded on the unevictable list in this case, no?
> > Maybe we should only try to isolate the page (clear PG_lru) after we
> > possibly clear PG_mlocked? In this case if we fail to isolate we know
> > for sure that whoever has the page isolated will observe that
> > PG_mlocked is clear and correctly make the page evictable.
> >
> > This probably would be complicated with the current implementation, as
> > we first need to decrement mlock_count to determine if we want to
> > clear PG_mlocked, and to do so we need to isolate the page as
> > mlock_count overlays page->lru. With the proposal in [1] to rework
> > mlock_count, it might be much simpler as far as I can tell. I intend
> > to refresh this proposal soon-ish.
> >
> > [1]https://lore.kernel.org/lkml/20230618065719.1363271-1-yosryahmed@google.com/
> >
> >>
> >>
> >> Regards
> >> Yin, Fengwei
> >>