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From:   Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
To:     Anshuman Khandual <anshuman.khandual@arm.com>
CC:     "linux-mm@kvack.org" <linux-mm@kvack.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Michal Hocko <mhocko@kernel.org>,
        "Andrew Morton" <akpm@linux-foundation.org>,
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        "xishi.qiuxishi@alibaba-inc.com" <xishi.qiuxishi@alibaba-inc.com>,
        "Laurent Dufour" <ldufour@linux.vnet.ibm.com>
Subject: Re: [RFC][PATCH v1 02/11] mm: soft-offline: add missing error check
 of set_hwpoison_free_buddy_page()
Thread-Topic: [RFC][PATCH v1 02/11] mm: soft-offline: add missing error
 check of set_hwpoison_free_buddy_page()
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Date:   Tue, 13 Nov 2018 00:16:53 +0000
Message-ID: <20181113001652.GA5945@hori1.linux.bs1.fc.nec.co.jp>
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Hi Anshuman,

On Fri, Nov 09, 2018 at 03:50:41PM +0530, Anshuman Khandual wrote:
> 
> 
> On 11/09/2018 12:17 PM, Naoya Horiguchi wrote:
> > set_hwpoison_free_buddy_page() could fail, then the target page is
> > finally not isolated, so it's better to report -EBUSY for userspace
> > to know the failure and chance of retry.
> > 
> 
> IIUC set_hwpoison_free_buddy_page() could only fail if the page is not
> free in the buddy. At least for soft_offline_huge_page() that wont be
> the case otherwise dissolve_free_huge_page() would have returned non
> zero -EBUSY. Is there any other reason set_hwpoison_free_buddy_page()
> would not succeed ?

There is a race window between page freeing (after successful soft-offline
-> page migration case) and the containment by set_hwpoison_free_buddy_page().
Or a target page can be allocated just after get_any_page() decided that
the target page is a free page.
So set_hwpoison_free_buddy_page() would safely fail in such cases.

Thanks,
Naoya Horiguchi

> 
> > And for consistency, this patch moves set_hwpoison_free_buddy_page()
> > in unmap_and_move() to __soft_offline_page().
> 
> Yeah this check should be handled in soft offline functions not inside
> migrations they trigger.
>