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

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Date:   Mon, 25 Sep 2023 08:48:40 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Yang Shi <yang@...amperecomputing.com>
Cc:     hughd@...gle.com, surenb@...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 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?

> 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.
> 
> Fixed the behavior.
>