linux-kernel - Re: [PATCH] mm: replace is_zero_pfn with is_huge_zero

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Message-Id: <20190826170934.7c2f4340@thinkpad>
Date:   Mon, 26 Aug 2019 17:09:34 +0200
From:   Gerald Schaefer <gerald.schaefer@...ibm.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Yu Zhao <yuzhao@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Ralph Campbell <rcampbell@...dia.com>,
        Jérôme Glisse <jglisse@...hat.com>,
        Will Deacon <will@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        "Aneesh Kumar K . V" <aneesh.kumar@...ux.ibm.com>,
        Dave Airlie <airlied@...hat.com>,
        Thomas Hellstrom <thellstrom@...are.com>,
        Souptick Joarder <jrdr.linux@...il.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm: replace is_zero_pfn with is_huge_zero_pmd for thp

On Mon, 26 Aug 2019 06:18:58 -0700
Matthew Wilcox <willy@...radead.org> wrote:

> Why did you not cc Gerald who wrote the patch?  You can't just
> run get_maintainers.pl and call it good.
> 
> On Sun, Aug 25, 2019 at 02:06:21PM -0600, Yu Zhao wrote:
> > For hugely mapped thp, we use is_huge_zero_pmd() to check if it's
> > zero page or not.
> > 
> > We do fill ptes with my_zero_pfn() when we split zero thp pmd, but
> >  this is not what we have in vm_normal_page_pmd().
> > pmd_trans_huge_lock() makes sure of it.
> > 
> > This is a trivial fix for /proc/pid/numa_maps, and AFAIK nobody
> > complains about it.
> > 
> > Signed-off-by: Yu Zhao <yuzhao@...gle.com>
> > ---
> >  mm/memory.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/mm/memory.c b/mm/memory.c
> > index e2bb51b6242e..ea3c74855b23 100644
> > --- a/mm/memory.c
> > +++ b/mm/memory.c
> > @@ -654,7 +654,7 @@ struct page *vm_normal_page_pmd(struct vm_area_struct *vma, unsigned long addr,
> >  
> >  	if (pmd_devmap(pmd))
> >  		return NULL;
> > -	if (is_zero_pfn(pfn))
> > +	if (is_huge_zero_pmd(pmd))
> >  		return NULL;
> >  	if (unlikely(pfn > highest_memmap_pfn))
> >  		return NULL;
> > -- 
> > 2.23.0.187.g17f5b7556c-goog
> >   

Looks good to me. The "_pmd" versions for can_gather_numa_stats() and
vm_normal_page() were introduced to avoid using pte_present/dirty() on
pmds, which is not affected by this patch.

In fact, for vm_normal_page_pmd() I basically copied most of the code
from vm_normal_page(), including the is_zero_pfn(pfn) check, which does
look wrong to me now. Using is_huge_zero_pmd() should be correct.

Maybe the description could also mention the symptom of this bug?
I would assume that it affects anon/dirty accounting in gather_pte_stats(),
for huge mappings, if zero page mappings are not correctly recognized.

Regards,
Gerald