linux-kernel - Re: [PATCH 1/1] mm: prevent poison consumption when splitting THP

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Message-ID: <c980f6ae-ba44-425b-b7bf-511cb6957c2d@redhat.com>
Date: Tue, 30 Sep 2025 10:53:58 +0200
From: David Hildenbrand <david@...hat.com>
To: Lance Yang <lance.yang@...ux.dev>, "Zhuo, Qiuxu" <qiuxu.zhuo@...el.com>
Cc: "Luck, Tony" <tony.luck@...el.com>, Jiaqi Yan <jiaqiyan@...gle.com>,
 "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
 "lorenzo.stoakes@...cle.com" <lorenzo.stoakes@...cle.com>,
 "linmiaohe@...wei.com" <linmiaohe@...wei.com>,
 "ziy@...dia.com" <ziy@...dia.com>,
 "baolin.wang@...ux.alibaba.com" <baolin.wang@...ux.alibaba.com>,
 "Liam.Howlett@...cle.com" <Liam.Howlett@...cle.com>,
 "npache@...hat.com" <npache@...hat.com>,
 "ryan.roberts@....com" <ryan.roberts@....com>,
 "dev.jain@....com" <dev.jain@....com>, "baohua@...nel.org"
 <baohua@...nel.org>, "nao.horiguchi@...il.com" <nao.horiguchi@...il.com>,
 "Chen, Farrah" <farrah.chen@...el.com>,
 "linux-mm@...ck.org" <linux-mm@...ck.org>,
 "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
 Andrew Zaborowski <andrew.zaborowski@...el.com>
Subject: Re: [PATCH 1/1] mm: prevent poison consumption when splitting THP

On 30.09.25 03:48, Lance Yang wrote:
> On Tue, Sep 30, 2025 at 3:07 AM David Hildenbrand <david@...hat.com> wrote:
>>
>> On 29.09.25 18:30, Zhuo, Qiuxu wrote:
>>> Hi Tony,
>>>
>>>> From: Luck, Tony <tony.luck@...el.com>
>>>> [...]
>>>> Subject: RE: [PATCH 1/1] mm: prevent poison consumption when splitting THP
>>>>
>>>>> Miaohe mentioned in another e-mail that there was an HWPoisoned flag
>>>> for the raw error 4K page.
>>>>> We could use that flag just to skip that raw error page and still use
>>>>> the zeropage for other healthy sub-pages. I'll try that.
>>>>
>>>> That HWPoisoned flag is only set for raw pages where an error has been
>>>> detected. Maybe Linux could implement an
>>>> "is_this_page_all_zero_mc_safe()"[1] that would catch undetected poison
>>>
>>> This sounds like a great suggestion to me.
>>> Let's see what others think about this and the name (though the name already LGTM 😊).
>>
>> The function name is just ... special. Not the good type of special IMHO. :)
>>
>> Note that we'll be moving to pages_identical() in [1]. Maybe we would
>> want a pages_identical_mc() or sth. like that as a follow up later.
>>
>>
>> So in any case, make that a follow-up work on top of a simple fix.
> 
> Yeah. IIRC, as David suggested earlier, we can just check if a page is
> poisoned using PageHWPoison().
> 
> Perhaps we should move this check into pages_identical()? This would make
> it a central place to determine if pages are safe to access and merge ;)

I would have to go into memcmp_pages(). Would be an option, but not sure 
if we should rather let callers deal with that.

For example, in some cases it might be sufficient to just check if the 
large folio has any poisoned page and give up early.

-- 
Cheers

David / dhildenb