linux-kernel - Re: [PATCH v1 1/5] mm, hwpoison, hugetlb: introduce SUBPAGE_INDEX

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Date:   Wed, 8 Jun 2022 11:19:29 +0200
From:   David Hildenbrand <david@...hat.com>
To:     HORIGUCHI NAOYA(堀口 直也) 
        <naoya.horiguchi@....com>
Cc:     Naoya Horiguchi <naoya.horiguchi@...ux.dev>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Miaohe Lin <linmiaohe@...wei.com>,
        Liu Shixin <liushixin2@...wei.com>,
        Yang Shi <shy828301@...il.com>,
        Oscar Salvador <osalvador@...e.de>,
        Muchun Song <songmuchun@...edance.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v1 1/5] mm, hwpoison, hugetlb: introduce
 SUBPAGE_INDEX_HWPOISON to save raw error page

On 08.06.22 03:31, HORIGUCHI NAOYA(堀口 直也) wrote:
> On Tue, Jun 07, 2022 at 03:04:15PM +0200, David Hildenbrand wrote:
>> On 02.06.22 07:06, Naoya Horiguchi wrote:
>>> From: Naoya Horiguchi <naoya.horiguchi@....com>
>>>
>>> When handling memory error on a hugetlb page, the error handler tries to
>>> dissolve and turn it into 4kB pages.  If it's successfully dissolved,
>>> PageHWPoison flag is moved to the raw error page, so that's all right.
>>> However, dissolve sometimes fails, then the error page is left as
>>> hwpoisoned hugepage. It's useful if we can retry to dissolve it to save
>>> healthy pages, but that's not possible now because the information about
>>> where the raw error page is lost.
>>>
>>> Use the private field of a tail page to keep that information.  The code
>>> path of shrinking hugepage pool used this info to try delayed dissolve.
>>> This only keeps one hwpoison page for now, which might be OK because it's
>>> simple and multiple hwpoison pages in a hugepage can be rare. But it can
>>> be extended in the future.
>>>
>>>
>>
>> But what would happen now if you have multiple successive MCE events on
>> such a page now?
> 
> The 2nd and later events are ignored due to "already hwpoisoned hugepage",
> this might not be good when the hwpoisoned hugepage is freed/dissolved later.
> So a temporal workaround is to remember "hugepage has multiple hwpoison pages"
> and disable free/dissolve for such hugepages.

Right. We might want to indicate exactly one vs. multiple, and then in
__update_and_free_page(), move the hwpoison flag to one subpage,
eventually exposing other corrupted subpages to the system.

-- 
Thanks,

David / dhildenb