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

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Message-ID: <Znwwrnk77J0xfNxu@casper.infradead.org>
Date: Wed, 26 Jun 2024 16:15:58 +0100
From: Matthew Wilcox <willy@...radead.org>
To: Ryan Roberts <ryan.roberts@....com>
Cc: Zi Yan <ziy@...dia.com>, ran xiaokai <ranxiaokai627@....com>,
	akpm@...ux-foundation.org, 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 Wed, Jun 26, 2024 at 12:07:04PM +0100, 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@....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.

I told you you'd run into trouble calling them "mTHP" ...