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

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Date:   Tue, 18 Jul 2023 15:48:13 -0700
From:   Yosry Ahmed <yosryahmed@...gle.com>
To:     Yin Fengwei <fengwei.yin@...el.com>
Cc:     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 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()


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
>