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Date:   Fri, 4 Mar 2022 18:28:39 -0800 (PST)
From:   Hugh Dickins <hughd@google.com>
To:     Liam Howlett <liam.howlett@oracle.com>
cc:     Hugh Dickins <hughd@google.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Oleg Nesterov <oleg@redhat.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "linux-mm@kvack.org" <linux-mm@kvack.org>
Subject: Re: [PATCH mmotm] mempolicy: mbind_range() set_policy() after
 vma_merge()
In-Reply-To: <20220305020021.qmwg5dkham4lyz6v@revolver>
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References: <319e4db9-64ae-4bca-92f0-ade85d342ff@google.com> <20220304184927.vkq6ewn6uqtcesma@revolver> <20220304190531.6giqbnnaka4xhovx@revolver> <6038ebc2-bc88-497d-a3f3-5936726fb023@google.com> <20220305020021.qmwg5dkham4lyz6v@revolver>
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On Sat, 5 Mar 2022, Liam Howlett wrote:
> * Hugh Dickins <hughd@google.com> [220304 17:48]:
> > On Fri, 4 Mar 2022, Liam Howlett wrote:
> > > * Liam R. Howlett <Liam.Howlett@Oracle.com> [220304 13:49]:
> > > > * Hugh Dickins <hughd@google.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().

Hugh