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Subject: Re: [PATCH 2/2] mm/memory-failure.c: not necessary to recalculate
 hpage
To:     Wei Yang <richard.weiyang@gmail.com>
Cc:     Wei Yang <richardw.yang@linux.intel.com>,
        n-horiguchi@ah.jp.nec.com, akpm@linux-foundation.org,
        linux-mm@kvack.org, linux-kernel@vger.kernel.org
References: <20191118082003.26240-1-richardw.yang@linux.intel.com>
 <20191118082003.26240-2-richardw.yang@linux.intel.com>
 <fdba31c8-d0c0-83a8-62d1-c04c1e894218@redhat.com>
 <20191202222827.isaelnqmuyn7zrns@master>
From:   David Hildenbrand <david@redhat.com>
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Organization: Red Hat GmbH
Message-ID: <37eedde2-05ab-e42e-7bcd-09090b090366@redhat.com>
Date:   Thu, 5 Dec 2019 16:06:20 +0100
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On 02.12.19 23:28, Wei Yang wrote:
> On Wed, Nov 20, 2019 at 04:07:38PM +0100, David Hildenbrand wrote:
>> On 18.11.19 09:20, Wei Yang wrote:
>>> hpage is not changed.
>>>
>>> Signed-off-by: Wei Yang <richardw.yang@linux.intel.com>
>>> ---
>>>   mm/memory-failure.c | 1 -
>>>   1 file changed, 1 deletion(-)
>>>
>>> diff --git a/mm/memory-failure.c b/mm/memory-failure.c
>>> index 392ac277b17d..9784f4339ae7 100644
>>> --- a/mm/memory-failure.c
>>> +++ b/mm/memory-failure.c
>>> @@ -1319,7 +1319,6 @@ int memory_failure(unsigned long pfn, int flags)
>>>   =09=09}
>>>   =09=09unlock_page(p);
>>>   =09=09VM_BUG_ON_PAGE(!page_count(p), p);
>>> -=09=09hpage =3D compound_head(p);
>>>   =09}
>>>   =09/*
>>>
>>
>> I am *absolutely* no transparent huge page expert (sorry :) ), but won't=
 the
>> split_huge_page(p) eventually split the compound page, such that
>> compound_head(p) will return something else after that call?
>>
>=20
> Hi, David
>=20
> Took sometime to look into the code and re-think about it. Found maybe we=
 can
> simplify this in another way.
>=20
> First, code touches here means split_huge_page() succeeds and "p" is now =
a PTE
> page. So compound_head(p) =3D=3D p.

While this would also be my intuition, I can't state that this is
guaranteed to be the case (IOW, I did not check the code/documentation) :)

>=20
> Then let's look at who will use hpage in the following function. There ar=
e two
> uses in current upstream:
>=20
>     * page_flags calculation
>     * hwpoison_user_mappings()
>=20
> The first one would be removed in next patch since PageHuge is handled at=
 the
> beginning.
>=20
> And in the second place, comment says if split succeeds, hpage points to =
page
> "p".
>=20
> After all, we don't need to re-calculate hpage after split, and just repl=
ace
> hpage in hwpoison_user_mappings() with p is enough.

That assumption would only be true in case all compound pages at this
point are transparent huge pages, no? AFAIK that is not necessarily
true. Or am I missing something?


--=20
Thanks,

David / dhildenb