linux-kernel - Re: [PATCH] mm: mempolicy: keep VMA walk if both MPOL_MF_STRICT and MPOL_MF

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Message-ID: <CAJuCfpExMWXHfZjgZ=UKf4k=zxrNOLx2R-a_wQdZ3O_+JTOq4w@mail.gmail.com>
Date:   Wed, 27 Sep 2023 14:39:21 -0700
From:   Suren Baghdasaryan <surenb@...gle.com>
To:     Yang Shi <yang@...amperecomputing.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>, hughd@...gle.com,
        willy@...radead.org, mhocko@...e.com, vbabka@...e.cz,
        osalvador@...e.de, aquini@...hat.com, kirill@...temov.name,
        rientjes@...gle.com, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, stable@...r.kernel.org
Subject: Re: [PATCH] mm: mempolicy: keep VMA walk if both MPOL_MF_STRICT and
 MPOL_MF_MOVE are specified

On Mon, Sep 25, 2023 at 10:16 AM Yang Shi <yang@...amperecomputing.com> wrote:
>
>
>
> On 9/25/23 8:48 AM, Andrew Morton wrote:
> > On Wed, 20 Sep 2023 15:32:42 -0700 Yang Shi <yang@...amperecomputing.com> wrote:
> >
> >> When calling mbind() with MPOL_MF_{MOVE|MOVEALL} | MPOL_MF_STRICT,
> >> kernel should attempt to migrate all existing pages, and return -EIO if
> >> there is misplaced or unmovable page.  Then commit 6f4576e3687b
> >> ("mempolicy: apply page table walker on queue_pages_range()") messed up
> >> the return value and didn't break VMA scan early ianymore when MPOL_MF_STRICT
> >> alone.  The return value problem was fixed by commit a7f40cfe3b7a
> >> ("mm: mempolicy: make mbind() return -EIO when MPOL_MF_STRICT is specified"),
> >> but it broke the VMA walk early if unmovable page is met, it may cause some
> >> pages are not migrated as expected.
> > So I'm thinking that a7f40cfe3b7a is the suitable Fixes: target?
>
> Yes, thanks. My follow-up email also added this.
>
> >
> >> The code should conceptually do:
> >>
> >>   if (MPOL_MF_MOVE|MOVEALL)
> >>       scan all vmas
> >>       try to migrate the existing pages
> >>       return success
> >>   else if (MPOL_MF_MOVE* | MPOL_MF_STRICT)
> >>       scan all vmas
> >>       try to migrate the existing pages
> >>       return -EIO if unmovable or migration failed
> >>   else /* MPOL_MF_STRICT alone */
> >>       break early if meets unmovable and don't call mbind_range() at all
> >>   else /* none of those flags */
> >>       check the ranges in test_walk, EFAULT without mbind_range() if discontig.

With this change I think my temporary fix at
https://lore.kernel.org/all/20230918211608.3580629-1-surenb@google.com/
can be removed because we either scan all vmas (which means we locked
them all) or we break early and do not call mbind_range() at all (in
which case we don't need vmas to be locked).

> >>
> >> Fixed the behavior.
> >>
>