linux-kernel - Re: [PATCH v7 1/2] mm: migration: fix the FOLL

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Date:   Fri, 26 Aug 2022 09:51:08 -0700
From:   Mike Kravetz <mike.kravetz@...cle.com>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     Haiyue Wang <haiyue.wang@...el.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, david@...hat.com, apopple@...dia.com,
        linmiaohe@...wei.com, ying.huang@...el.com,
        songmuchun@...edance.com, naoya.horiguchi@...ux.dev,
        alex.sierra@....com, gerald.schaefer@...ux.ibm.com,
        Baolin Wang <baolin.wang@...ux.alibaba.com>
Subject: Re: [PATCH v7 1/2] mm: migration: fix the FOLL_GET failure on
 following huge page

On 08/24/22 11:38, Andrew Morton wrote:
> On Tue, 23 Aug 2022 21:58:40 +0800 Haiyue Wang <haiyue.wang@...el.com> wrote:
> 
> > Not all huge page APIs support FOLL_GET option, so move_pages() syscall
> > will fail to get the page node information for some huge pages.
> > 
> > Like x86 on linux 5.19 with 1GB huge page API follow_huge_pud(), it will
> > return NULL page for FOLL_GET when calling move_pages() syscall with the
> > NULL 'nodes' parameter, the 'status' parameter has '-2' error in array.
> > 
> > Note: follow_huge_pud() now supports FOLL_GET in linux 6.0.
> >       Link: https://lore.kernel.org/all/20220714042420.1847125-3-naoya.horiguchi@linux.dev
> > 
> > But these huge page APIs don't support FOLL_GET:
> >   1. follow_huge_pud() in arch/s390/mm/hugetlbpage.c
> >   2. follow_huge_addr() in arch/ia64/mm/hugetlbpage.c
> >      It will cause WARN_ON_ONCE for FOLL_GET.
> >   3. follow_huge_pgd() in mm/hugetlb.c
> 
> What happened to the proposal to fix these three sites so this patch is
> not needed?
> 

Note mpe's comments here:
https://lore.kernel.org/linux-mm/87r113jgqn.fsf@mpe.ellerman.id.au/

It has been a while since powerpc supported hugetlb pages at the PGD
level.  And, this was the only architecture which had pages at this
level.  Unless I am missing something, this means there is no issue with
follow_huge_pgd as there is no way this code could be invoked when
commit 4cd614841c06 was made.
-- 
Mike Kravetz