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

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Date:   Wed, 8 Jun 2022 01:31:30 +0000
From:   HORIGUCHI NAOYA(堀口　直也) 
        <naoya.horiguchi@....com>
To:     David Hildenbrand <david@...hat.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 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.

Thanks,
Naoya Horiguchi