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SubjectRe: [PATCH mmotm] mempolicy: mbind_range() set_policy() after vma_merge()
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* 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?

>
> > > Reviewed-by: Liam R. Howlett <Liam.Howlett@oracle.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
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