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

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Message-ID: <a94541b0-7c3c-4551-a03c-e1f83445baa7@linux.dev>
Date: Tue, 30 Sep 2025 18:20:22 +0800
From: Lance Yang <lance.yang@...ux.dev>
To: David Hildenbrand <david@...hat.com>, "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 2025/9/30 18:13, Lance Yang wrote:
> 
> 
> On 2025/9/30 16:53, David Hildenbrand wrote:
>> 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.
> 
> FWIW, one idea I had was to create a unified pre-flight checker, like
> folio_pages_identical_prepare(struct folio *folio). A caller could use
> it before a loop of pages_identical() calls to pre-check a folio :)

Forgot to add:

It would centralize all folio-level checks.

So if we ever need a new check in the future, we'd only modify the
prepare helper, not all the individual callers.