Re: [Patch v2] mm/huge_memory: add pmd folio to ds_queue in do_huge_zero_wp_pmd()

[Wei Yang <richard.weiyang@gmail.com>]

On Fri, Oct 03, 2025 at 10:08:37AM -0400, Zi Yan wrote:
>On 3 Oct 2025, at 9:49, Lance Yang wrote:
>
>> Hey Wei,
>>
>> On 2025/10/2 09:38, Wei Yang wrote:
>>> We add pmd folio into ds_queue on the first page fault in
>>> __do_huge_pmd_anonymous_page(), so that we can split it in case of
>>> memory pressure. This should be the same for a pmd folio during wp
>>> page fault.
>>>
>>> Commit 1ced09e0331f ("mm: allocate THP on hugezeropage wp-fault") miss
>>> to add it to ds_queue, which means system may not reclaim enough memory
>>
>> IIRC, it was commit dafff3f4c850 ("mm: split underused THPs") that
>> started unconditionally adding all new anon THPs to _deferred_list :)
>>
>>> in case of memory pressure even the pmd folio is under used.
>>>
>>> Move deferred_split_folio() into map_anon_folio_pmd() to make the pmd
>>> folio installation consistent.
>>>
>>> Fixes: 1ced09e0331f ("mm: allocate THP on hugezeropage wp-fault")
>>
>> Shouldn't this rather be the following?
>>
>> Fixes: dafff3f4c850 ("mm: split underused THPs")
>
>Yes, I agree. In this case, this patch looks more like an optimization
>for split underused THPs.
>
>One observation on this change is that right after zero pmd wp, the
>deferred split queue could be scanned, the newly added pmd folio will
>split since it is all zero except one subpage. This means we probably
>should allocate a base folio for zero pmd wp and map the rest to zero
>page at the beginning if split underused THP is enabled to avoid
>this long trip. The downside is that user app cannot get a pmd folio
>if it is intended to write data into the entire folio.

Thanks for raising this.

IMHO, we could face the similar situation in __do_huge_pmd_anonymous_page().
If my understanding is correct, the allocated folio is zeroed and we don't
have idea how user would write data to it.

Since shrinker is active when memory is low, maybe vma_alloc_anon_folio_pmd()
has told use current status of the memory. If it does get a pmd folio, we are
probably having enough memory in the system.

>
>Usama might be able to give some insight here.
>

-- 
Wei Yang
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