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

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

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@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 :)

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.