Message-ID: <0BBB65F0-7ED4-40DE-9949-761326993D7D@nvidia.com>
Date: Wed, 04 Feb 2026 15:45:51 -0500
From: Zi Yan <ziy@...dia.com>
To: "David Hildenbrand (arm)" <david@...nel.org>
Cc: 是参差 <shicenci@...il.com>, linux-mm@...ck.org,
 linmiaohe@...wei.com, akpm@...ux-foundation.org,
 linux-kernel@...r.kernel.org, Matthew Wilcox <willy@...radead.org>
Subject: Re: WARNING in memory_failure() at include/linux/huge_mm.h:635
 triggered

On 4 Feb 2026, at 15:31, David Hildenbrand (arm) wrote:

> On 2/4/26 21:13, Zi Yan wrote:
>> On 4 Feb 2026, at 14:55, David Hildenbrand (arm) wrote:
>>
>>> On 2/4/26 20:48, Zi Yan wrote:
>>>>
>>>>
>>>> What do you mean? Changing memory failure code to only handle large_rmappable?
>>>> large_rmappable is a folio flag, memory failure code should see such
>>>
>>> Did you mean "should not" ? :)
>>
>> Yes.
>>
>>>
>>>> non-folio but compound things to begin with, IMHO.
>>>
>>> I would say that we could right now reject in memory failure code any compound pages that do not have PG_large_rmappable set.
>>>
>>> I have the faint recollection that we don't set PG_large_rmappable on hugetlb folios yet, so they have to identified as well.
>>
>> Right. My patchset[1] is trying to add it, since hugetlb is used as a folio
>> in most places and large_rmappable is a folio flag.
>>
>>
>> [1] https://lore.kernel.org/all/20260130034818.472804-1-ziy@nvidia.com/
>
> Still on my todo list :)

Sure. Wait for your input there. :)

>
>>>>
>>>> I think we need to be able to tell between raw page (compound or not),
>>>> mappable page (compound or not, especially for those used with vm_insert_*),
>>>> and folio.
>>>
>>> We can't identify (small) folios just yet. We'd need another page flag for that (just like PG_large_rmappable), and we all know how that ends ;)
>>
>> Yes, I am thinking about removing mapcount in struct page to achieve that.
>
> On my todo list :) Stupid CONFIG_PAGE_MAPCOUNT that is still around and stupid partial-mapping handling.
>
> I worked on this after LPC but was distracted by PTO :D
>
>> And only pages used for vm_insert_*() and folios need mapcount.
>
> vm_insert_*() won't need it for non-folio things. Only folios. We just have to teach the zap code to also leave the mapcount of these non-folio things alone. IOW, identify them when we map/unmap as "not folio" and not touch the mapcount.

Oh, that sounds great. I thought I would need to convert all drive code
that does vm_insert_*() to use folio. Basically, I hit
__folio_large_mapcount_sanity_checks() on _mm_id_mapcount when I moved
_mm_id_mapcount and friends from prep_compound_page() to page_rmappable_folio().
IIUC, __folio_add_file_rmap() can just return if a non-folio compound page
is encountered. Of course, remove part needs to do the same.


>
>> Code
>> uses vm_insert_*() on pages would probably have a struct mappable_page
>> with mapcount.
>
> I don't think we'll need a mapcount for them. Only for folios.
>
>>
>>>
>>> With Willy's work we'll be able to identify folios reliably.
>>>
>>> How to deal with that vm_insert_* crap, especially for non-folio pages, is also future work based on that.
>>
>> I think it might the other way around. memdesc does not have mapcount,
>> if we do not have a separate struct for these mappable pages now,
>> what do we use at memdesc time? folio?
>
> Folios will have mapcount related information, yes. Long term, memdescs will for sure not have any.
>
> Real fun begins once we start messing with refcounts. vm_insert_*() will be "fun" on non-folio things.

Yeah, maybe we will refcounts for every used memdescs. But who knows.

Best Regards,
Yan, Zi

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