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Date:   Thu, 17 Jun 2021 12:15:38 +0100
From:   Mel Gorman <mgorman@techsingularity.net>
To:     Naoya Horiguchi <nao.horiguchi@gmail.com>
Cc:     linux-mm@kvack.org, Andrew Morton <akpm@linux-foundation.org>,
        Mike Kravetz <mike.kravetz@oracle.com>,
        David Hildenbrand <david@redhat.com>,
        Oscar Salvador <osalvador@suse.de>,
        Michal Hocko <mhocko@suse.com>,
        Naoya Horiguchi <naoya.horiguchi@nec.com>,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH mmotm v1] mm/hwpoison: disable pcp for
 page_handle_poison()
Message-ID: <20210617111538.GO30378@techsingularity.net>
References: <20210617092626.291006-1-nao.horiguchi@gmail.com>
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On Thu, Jun 17, 2021 at 06:26:26PM +0900, Naoya Horiguchi wrote:
> From: Naoya Horiguchi <naoya.horiguchi@nec.com>
> 
> Recent changes by patch "mm/page_alloc: allow high-order pages to be
> stored on the per-cpu lists" makes kernels determine whether to use pcp
> by pcp_allowed_order(), which breaks soft-offline for hugetlb pages.
> 
> Soft-offline dissolves a migration source page, then removes it from
> buddy free list, so it's assumed that any subpage of the soft-offlined
> hugepage are recognized as a buddy page just after returning from
> dissolve_free_huge_page().  pcp_allowed_order() returns true for
> hugetlb, so this assumption is no longer true.
> 
> So disable pcp during dissolve_free_huge_page() and
> take_page_off_buddy() to prevent soft-offlined hugepages from linking to
> pcp lists.  Soft-offline should not be common events so the impact on
> performance should be minimal.  And I think that the optimization of
> Mel's patch could benefit to hugetlb so zone_pcp_disable() is called
> only in hwpoison context.
> 
> Signed-off-by: Naoya Horiguchi <naoya.horiguchi@nec.com>

I think this is ok, it'll remove a page that is to be poisoned from the
PCP lists and put them back in the page allocator. It's a heavy but rare
operation and identifying what PCP list a free page is on would be
tricky so

Acked-by: Mel Gorman <mgorman@techsingularity.net>

The alternative I guess would be specical casing update_and_free_page to
bypass the PCP but it'd be clumsy from an API point of view and I don't
think it's worth the effort.

-- 
Mel Gorman
SUSE Labs