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

[Lance Yang <lance.yang@linux.dev>]

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@redhat.com> 
>> wrote:
>>>
>>> On 29.09.25 18:30, Zhuo, Qiuxu wrote:
>>>> Hi Tony,
>>>>
>>>>> From: Luck, Tony <tony.luck@intel.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 :)