Re: [PATCH] mm/hugetlb: Use the right pte val for compare in hugetlb_cow

From: Andrew Morton
Date: Tue Oct 18 2016 - 14:33:53 EST

On Tue, 18 Oct 2016 21:12:45 +0530 "Aneesh Kumar K.V" <aneesh.kumar@xxxxxxxxxxxxxxxxxx> wrote:

> We cannot use the pte value used in set_pte_at for pte_same comparison,
> because archs like ppc64, filter/add new pte flag in set_pte_at. Instead
> fetch the pte value inside hugetlb_cow. We are comparing pte value to
> make sure the pte didn't change since we dropped the page table lock.
> hugetlb_cow get called with page table lock held, and we can take a copy
> of the pte value before we drop the page table lock.
> With hugetlbfs, we optimize the MAP_PRIVATE write fault path with no
> previous mapping (huge_pte_none entries), by forcing a cow in the fault
> path. This avoid take an addition fault to covert a read-only mapping
> to read/write. Here we were comparing a recently instantiated pte (via
> set_pte_at) to the pte values from linux page table. As explained above
> on ppc64 such pte_same check returned wrong result, resulting in us
> taking an additional fault on ppc64.

>From my reading this is a minor performance improvement and a -stable
backport isn't needed. But it is unclear whether the impact warrants a
4.9 merge.

Please be careful about describing end-user visible impacts when fixing
bugs, thanks.