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

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Date:   Wed, 24 Aug 2022 11:38:58 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Haiyue Wang <haiyue.wang@...el.com>
Cc:     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, mike.kravetz@...cle.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 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?

> This is an temporary solution to mitigate the side effect of the race
> condition fix by calling follow_page() with FOLL_GET set for huge pages.
> 
> After supporting follow huge page by FOLL_GET is done, this fix can be
> reverted safely.