linux-kernel - Re: [PATCH 2/2] mm/hugetlb: refactor subpage recording

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Message-ID: <20210127021023.GC4605@ziepe.ca>
Date:   Tue, 26 Jan 2021 22:10:23 -0400
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Mike Kravetz <mike.kravetz@...cle.com>
Cc:     "Matthew Wilcox (Oracle)" <willy@...radead.org>,
        Joao Martins <joao.m.martins@...cle.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        John Hubbard <jhubbard@...dia.com>,
        Dave Hansen <dave.hansen@...el.com>
Subject: Re: [PATCH 2/2] mm/hugetlb: refactor subpage recording

On Tue, Jan 26, 2021 at 05:58:53PM -0800, Mike Kravetz wrote:

> As pointed out by Joao, you can also see the differences in pfn_to_page
> for CONFIG_SPARSE_VMEMMAP and CONFIG_SPARSEMEM.  The only time we might
> have issues is with CONFIG_SPARSEMEM.  I would bet CONFIG_SPARSE_VMEMMAP
> is far more common.

I think it is fine to have a different pfn_to_page, it should just be
illegal to combine pages into a compound if their tail pages are not
linear in the map.

Matt's folio work might present an option to audit the whole mm for
this pattern and provide some folio_next_tail_page() accessor that
does the fast thing - but I question the value of such a project for a
2008 era PPC platform with 16GB pages (seriously?) that may be using
VMEMMAP today anyhow??

Maybe others know of more modern use cases

Jason