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Date:   Fri, 19 Jan 2018 11:33:42 +0100
From:   Michal Hocko <mhocko@kernel.org>
To:     "Kirill A. Shutemov" <kirill@shutemov.name>
Cc:     Dave Hansen <dave.hansen@linux.intel.com>,
        Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>,
        torvalds@linux-foundation.org, kirill.shutemov@linux.intel.com,
        akpm@linux-foundation.org, hannes@cmpxchg.org,
        iamjoonsoo.kim@lge.com, mgorman@techsingularity.net,
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        hillf.zj@alibaba-inc.com, hughd@google.com, oleg@redhat.com,
        peterz@infradead.org, riel@redhat.com, srikar@linux.vnet.ibm.com,
        vdavydov.dev@gmail.com, mingo@kernel.org,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org, x86@kernel.org
Subject: Re: [mm 4.15-rc8] Random oopses under memory pressure.
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On Fri 19-01-18 13:02:59, Kirill A. Shutemov wrote:
> On Thu, Jan 18, 2018 at 06:22:13PM +0100, Michal Hocko wrote:
> > On Thu 18-01-18 18:40:26, Kirill A. Shutemov wrote:
> > [...]
> > > +	/*
> > > +	 * Make sure that pages are in the same section before doing pointer
> > > +	 * arithmetics.
> > > +	 */
> > > +	if (page_to_section(pvmw->page) != page_to_section(page))
> > > +		return false;
> > 
> > OK, THPs shouldn't cross memory sections AFAIK. My brain is meltdown
> > these days so this might be a completely stupid question. But why don't
> > you simply compare pfns? This would be just simpler, no?
> 
> In original code, we already had pvmw->page around and I thought it would
> be easier to get page for the pte intead of looking for pfn for both
> sides.
> 
> We these changes it's no longer the case.
> 
> Do you care enough to send a patch? :)

Well, memory sections are sparsemem concept IIRC. Unless I've missed
something page_to_section is quarded by SECTION_IN_PAGE_FLAGS and that
is conditional to CONFIG_SPARSEMEM. THP is a generic code so using it
there is wrong unless I miss some subtle detail here.

Comparing pfn should be generic enough.
-- 
Michal Hocko
SUSE Labs