linux-kernel - Re: [PATCH v6 1/2] mm: migration: fix the FOLL

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Date:   Fri, 19 Aug 2022 21:22:32 +1000
From:   Michael Ellerman <mpe@...erman.id.au>
To:     Mike Kravetz <mike.kravetz@...cle.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     "Wang, Haiyue" <haiyue.wang@...el.com>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "david@...hat.com" <david@...hat.com>,
        "apopple@...dia.com" <apopple@...dia.com>,
        "linmiaohe@...wei.com" <linmiaohe@...wei.com>,
        "Huang, Ying" <ying.huang@...el.com>,
        "songmuchun@...edance.com" <songmuchun@...edance.com>,
        "naoya.horiguchi@...ux.dev" <naoya.horiguchi@...ux.dev>,
        "alex.sierra@....com" <alex.sierra@....com>,
        Heiko Carstens <hca@...ux.ibm.com>,
        Vasily Gorbik <gor@...ux.ibm.com>,
        Alexander Gordeev <agordeev@...ux.ibm.com>,
        Christian Borntraeger <borntraeger@...ux.ibm.com>,
        Sven Schnelle <svens@...ux.ibm.com>,
        <linuxppc-dev@...ts.ozlabs.org>
Subject: Re: [PATCH v6 1/2] mm: migration: fix the FOLL_GET failure on
 following huge page

Mike Kravetz <mike.kravetz@...cle.com> writes:
> On 08/16/22 22:43, Andrew Morton wrote:
>> On Wed, 17 Aug 2022 03:31:37 +0000 "Wang, Haiyue" <haiyue.wang@...el.com> wrote:
>>
>> > > >  		}
>> > >
>> > > I would be better to fix this for real at those three client code sites?
>> >
>> > Then 5.19 will break for a while to wait for the final BIG patch ?
>>
>> If that's the proposal then your [1/2] should have had a cc:stable and
>> changelog words describing the plan for 6.0.
>>
>> But before we do that I'd like to see at least a prototype of the final
>> fixes to s390 and hugetlb, so we can assess those as preferable for
>> backporting.  I don't think they'll be terribly intrusive or risky?
>
> I will start on adding follow_huge_pgd() support.  Although, I may need
> some help with verification from the powerpc folks, as that is the only
> architecture which supports hugetlb pages at that level.
>
> mpe any suggestions?

I'm happy to test.

I have a system where I can allocate 1GB huge pages.

I'm not sure how to actually test this path though. I hacked up the
vm/migration.c test to allocate 1GB hugepages, but I can't see it going
through follow_huge_pgd() (using ftrace).

Maybe I hacked it up badly, I'll have a closer look on Monday. But if
you have any tips on how to trigger that path let me know :)

cheers