linux-kernel - Re: [PATCH mmotm] mempolicy: mbind_range() set_policy() after vma

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Date:   Sat, 5 Mar 2022 02:00:39 +0000
From:   Liam Howlett <liam.howlett@...cle.com>
To:     Hugh Dickins <hughd@...gle.com>
CC:     Andrew Morton <akpm@...ux-foundation.org>,
        Oleg Nesterov <oleg@...hat.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>
Subject: Re: [PATCH mmotm] mempolicy: mbind_range() set_policy() after
 vma_merge()

* Hugh Dickins <hughd@...gle.com> [220304 17:48]:
> On Fri, 4 Mar 2022, Liam Howlett wrote:
> > * Liam R. Howlett <Liam.Howlett@...cle.com> [220304 13:49]:
> > > * Hugh Dickins <hughd@...gle.com> [220303 23:36]:
> > 
> > I just thought of something after my initial email
> > 
> > How does the ->set_policy() requirement on tmpfs play out for the
> > mpol_equal() check earlier in that for loop?
> 
> It took me a while to page all this back in (and remind myself of
> what is case 8) to answer that question!
> 
> The answer is that the mpol_equal() check at the top of the loop is on
> an existing, unmodified vma; so it's right to assume that any necessary
> set_policy() has already been done.
> 
> Whereas the mpol_equal() check being removed in this patch, is being
> done on a vma which may have just been extended to cover a greater range:
> so although the relevant set_policy() may have already been done on a part
> of its range, there is now another part which needs the policy applied.

Doesn't the policy get checked during vma_merge()?  Specifically the
mpol_equal(policy, vma_policy(next)) check?

> 
> > > Reviewed-by: Liam R. Howlett <Liam.Howlett@...cle.com>
> 
> Thank you, your review is very welcome (but mainly I Cc'ed to alert
> you to how I'm probably stepping on your toes a little here - sorry).

Yes, I figured as much and I really appreciate it.  I'm quite confident
I translated this bug to my patch set.

Thanks,
Liam