linux-kernel - Re: [PATCH] mm: huge_memory: convert __do_huge_pmd_anonymous

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Message-ID: <Zia2f2EI8LWFE2XA@casper.infradead.org>
Date: Mon, 22 Apr 2024 20:11:59 +0100
From: Matthew Wilcox <willy@...radead.org>
To: Jianfeng Wang <jianfeng.w.wang@...cle.com>
Cc: linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	akpm@...ux-foundation.org
Subject: Re: [PATCH] mm: huge_memory: convert __do_huge_pmd_anonymous_page()
 to use folios

On Mon, Apr 22, 2024 at 11:12:16AM -0700, Jianfeng Wang wrote:
> Change __do_huge_pmd_anonymous_page() to take folio as input, as its
> caller has used folio. Save one unnecessary call to compound_head().

I don't like this patch.  It makes the assumption that folios will never
be larger than PMD size, and I don't think that's an assumption that's
going to last another five years.  Look where you had to
do &folio->page:

> +	clear_huge_page(&folio->page, vmf->address, HPAGE_PMD_NR);

> +		entry = mk_huge_pmd(&folio->page, vma->vm_page_prot);

For mk_huge_pmd() in particular, you need to know the precise page, and
not just use the first page of the folio.