Re: [PATCH 2/2] kpageflags: fix wrong KPF_THP on non-pmd-mappable compound pages

[Ryan Roberts <ryan.roberts@arm.com>]

On 26/06/2024 15:40, Zi Yan wrote:
> On Wed Jun 26, 2024 at 7:07 AM EDT, Ryan Roberts wrote:
>> On 26/06/2024 04:06, Zi Yan wrote:
>>> On Tue Jun 25, 2024 at 10:49 PM EDT, ran xiaokai wrote:
>>>> From: Ran Xiaokai <ran.xiaokai@zte.com.cn>
>>>>
>>>> KPF_COMPOUND_HEAD and KPF_COMPOUND_TAIL are set on "common" compound
>>>> pages, which means of any order, but KPF_THP should only be set
>>>> when the folio is a 2M pmd mappable THP. 
>>
>> Why should KPF_THP only be set on 2M THP? What problem does it cause as it is
>> currently configured?
>>
>> I would argue that mTHP is still THP so should still have the flag. And since
>> these smaller mTHP sizes are disabled by default, only mTHP-aware user space
>> will be enabling them, so I'll naively state that it should not cause compat
>> issues as is.
>>
>> Also, the script at tools/mm/thpmaps relies on KPF_THP being set for all mTHP
>> sizes to function correctly. So that would need to be reworked if making this
>> change.
> 
> + more folks working on mTHP
> 
> I agree that mTHP is still THP, but we might want different
> stats/counters for it, since people might want to keep the old THP counters
> consistent. See recent commits on adding mTHP counters:
> ec33687c6749 ("mm: add per-order mTHP anon_fault_alloc and anon_fault_fallback
> counters"), 1f97fd042f38 ("mm: shmem: add mTHP counters for anonymous shmem")
> 
> and changes to make THP counter to only count PMD THP:
> 835c3a25aa37 ("mm: huge_memory: add the missing folio_test_pmd_mappable() for
> THP split statistics")
> 
> In this case, I wonder if we want a new KPF_MTHP bit for mTHP and some
> adjustment on tools/mm/thpmaps.

That would work for me, assuming we have KPF bits to spare?