Re: [PATCH v6 1/2] mm: migration: fix the FOLL_GET failure on following huge page

From: Michael Ellerman <mpe@ellerman.id.au>
To: Mike Kravetz <mike.kravetz@oracle.com>,
	Andrew Morton <akpm@linux-foundation.org>
Cc: "Wang, Haiyue" <haiyue.wang@intel.com>,
	"linux-mm@kvack.org" <linux-mm@kvack.org>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	"david@redhat.com" <david@redhat.com>,
	"apopple@nvidia.com" <apopple@nvidia.com>,
	"linmiaohe@huawei.com" <linmiaohe@huawei.com>,
	"Huang, Ying" <ying.huang@intel.com>,
	"songmuchun@bytedance.com" <songmuchun@bytedance.com>,
	"naoya.horiguchi@linux.dev" <naoya.horiguchi@linux.dev>,
	"alex.sierra@amd.com" <alex.sierra@amd.com>,
	Heiko Carstens <hca@linux.ibm.com>,
	Vasily Gorbik <gor@linux.ibm.com>,
	Alexander Gordeev <agordeev@linux.ibm.com>,
	Christian Borntraeger <borntraeger@linux.ibm.com>,
	Sven Schnelle <svens@linux.ibm.com>,
	<linuxppc-dev@lists.ozlabs.org>
Subject: Re: [PATCH v6 1/2] mm: migration: fix the FOLL_GET failure on following huge page
Date: Fri, 19 Aug 2022 21:22:32 +1000	[thread overview]
Message-ID: <875yiomq9z.fsf@mpe.ellerman.id.au> (raw)
In-Reply-To: <Yv0ku1mn4LBzg/zG@monkey>

Mike Kravetz <mike.kravetz@oracle.com> writes:
> On 08/16/22 22:43, Andrew Morton wrote:
>> On Wed, 17 Aug 2022 03:31:37 +0000 "Wang, Haiyue" <haiyue.wang@intel.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