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

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Message-ID: <20220308160552.d3dlcaclkqnlkzzj@revolver>
Date:   Tue, 8 Mar 2022 16:05:58 +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 21:29]:
> On Sat, 5 Mar 2022, Liam Howlett wrote:
> > * 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?
> 
> Sorry, I'm reduced to the unhelpful reply of "Yes. So?"
> 
> If vma_merge() finds that vma's new_pol allows it to be merged with prev,
> that still requires mbind_range() (or its call to vma_replace_policy())
> to set_policy() on prev (now assigned to vma), to apply that new_pol to
> the extension of prev - vma_merge() would have checked mpol_equal(),
> but would not have done the set_policy().

I must be missing something.  If mpol_equal() isn't sufficient to ensure
we don't need to set_policy(), then why are the other vma_merge() cases
okay - such as madvise_update_vma() and mlock_fixup()?  Won't the mem
policy change in the same way in these cases?

Thanks,
Liam