linux-kernel - Re: [PATCH 2/2] mm/hugetlb: Use a folio in hugetlb

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Date:   Fri, 2 Jun 2023 21:17:08 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Peng Zhang <zhangpeng362@...wei.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        akpm@...ux-foundation.org, mike.kravetz@...cle.com,
        muchun.song@...ux.dev, sidhartha.kumar@...cle.com,
        vishal.moola@...il.com, wangkefeng.wang@...wei.com,
        sunnanyong@...wei.com
Subject: Re: [PATCH 2/2] mm/hugetlb: Use a folio in hugetlb_wp()

On Fri, Jun 02, 2023 at 09:54:08AM +0800, Peng Zhang wrote:
> From: ZhangPeng <zhangpeng362@...wei.com>
> 
> We can replace nine implict calls to compound_head() with one by using
> old_folio. However, we still need to keep old_page because we need to
> know which page in the folio we are copying.

Do we?  It's my understanding (and I am far from an expert here ...)
that the 'pte_t *' we are passed *inside hugetlbfs* is not in fact a pte
pointer at all but actually a pmd or pud pointer.  See how we do this:

        pte_t pte = huge_ptep_get(ptep);

and so the page we get back is always a head page, and we can go
directly to a folio.  ie this is different from the THP cases.