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

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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@xxxxxxxxxx>
>>
>> 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.

Thanks,
Ryan





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