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

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Date:   Tue, 18 Oct 2016 11:33:41 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     "Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>
Cc:     Jan Stancek <jstancek@...hat.com>,
        Mike Kravetz <mike.kravetz@...cle.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH] mm/hugetlb: Use the right pte val for compare in
 hugetlb_cow

On Tue, 18 Oct 2016 21:12:45 +0530 "Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com> 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.