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Date:   Thu, 21 May 2020 18:21:42 -0700 (PDT)
From:   Hugh Dickins <hughd@google.com>
To:     Anshuman Khandual <anshuman.khandual@arm.com>
cc:     Hugh Dickins <hughd@google.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Naoya Horiguchi <naoya.horiguchi@nec.com>,
        Zi Yan <ziy@nvidia.com>, John Hubbard <jhubbard@nvidia.com>,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org
Subject: Re: [PATCH mmotm] mm/vmstat: Add events for PMD based THP migration
 without split fix
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References: <alpine.LSU.2.11.2005210643340.482@eggly.anvils> <f1673e4f-64ca-9cf2-861c-7e8a7deeede1@arm.com>
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On Fri, 22 May 2020, Anshuman Khandual wrote:
> On 05/21/2020 07:19 PM, Hugh Dickins wrote:
> > Fix 5.7-rc6-mm1 page migration crash in unmap_and_move(): when the
> > page to be migrated has been freed from under us, that is considered 
> > a MIGRATEPAGE_SUCCESS, but no newpage has been allocated (and I don't
> > think it would ever need to be counted as a successful THP migration).
> > 
> > Signed-off-by: Hugh Dickins <hughd@google.com>
> > ---
> > Fix to mm-vmstat-add-events-for-pmd-based-thp-migration-without-split.patch
> > 
> >  mm/migrate.c |    2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > --- 5.7-rc6-mm1/mm/migrate.c	2020-05-20 12:21:56.117693827 -0700
> > +++ linux/mm/migrate.c	2020-05-20 15:08:12.319476978 -0700
> > @@ -1248,7 +1248,7 @@ out:
> >  	 * we want to retry.
> >  	 */
> >  	if (rc == MIGRATEPAGE_SUCCESS) {
> > -		if (PageTransHuge(newpage))
> > +		if (newpage && PageTransHuge(newpage))
> >  			thp_migration_success(true);
> >  		put_page(page);
> >  		if (reason == MR_MEMORY_FAILURE) {
> > 
> 
> Thanks Hugh. I am preparing to respin the vmstat patch accommodating some
> earlier comments. Wondering if I should also fold these changes here after
> adding your signed-off-by ?

Thanks for asking, but please just fold this one-line fixup into your
respin without my signed-off-by: I'm not heavily invested in these stats,
just want to avoid the crash; and don't know if I would want to sign the
result.  You could add something like [hughd: fixed oops on NULL newpage]
to confirm that the fix is on board, but the fix is what matters.

Hugh