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

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Message-ID: <1907a8c0-9860-4ca0-be59-bec0e772332b@arm.com>
Date: Wed, 26 Jun 2024 12:07:04 +0100
From: Ryan Roberts <ryan.roberts@....com>
To: Zi Yan <ziy@...dia.com>, ran xiaokai <ranxiaokai627@....com>,
 akpm@...ux-foundation.org, willy@...radead.org
Cc: vbabka@...e.cz, svetly.todorov@...verge.com, ran.xiaokai@....com.cn,
 baohua@...nel.org, peterx@...hat.com, linux-kernel@...r.kernel.org,
 linux-mm@...ck.org, linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH 2/2] kpageflags: fix wrong KPF_THP on non-pmd-mappable
 compound pages

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@....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.

Thanks,
Ryan