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

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Date:   Fri, 4 Mar 2022 14:48:08 -0800 (PST)
From:   Hugh Dickins <hughd@...gle.com>
To:     Liam Howlett <liam.howlett@...cle.com>
cc:     Hugh Dickins <hughd@...gle.com>,
        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()

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.

> > 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).

Hugh