Re: [PATCH mmotm] mempolicy: mbind_range() set_policy() after vma_merge()

[Hugh Dickins]

On Sat, 5 Mar 2022, Liam Howlett wrote:
> * Hugh Dickins <hughd@xxxxxxxxxx> [220304 17:48]:
> > On Fri, 4 Mar 2022, Liam Howlett wrote:
> > > * Liam R. Howlett <Liam.Howlett@xxxxxxxxxx> [220304 13:49]:
> > > > * Hugh Dickins <hughd@xxxxxxxxxx> [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