linux-kernel - Re: [PATCH 21/46] hugetlb: use struct hugetlb_pte for walk_hugetlb

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Message-ID: <06423461-c543-56fe-cc63-cabda6871104@redhat.com>
Date:   Mon, 16 Jan 2023 11:17:52 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Peter Xu <peterx@...hat.com>,
        James Houghton <jthoughton@...gle.com>
Cc:     Mike Kravetz <mike.kravetz@...cle.com>,
        Muchun Song <songmuchun@...edance.com>,
        David Rientjes <rientjes@...gle.com>,
        Axel Rasmussen <axelrasmussen@...gle.com>,
        Mina Almasry <almasrymina@...gle.com>,
        Zach O'Keefe <zokeefe@...gle.com>,
        Manish Mishra <manish.mishra@...anix.com>,
        Naoya Horiguchi <naoya.horiguchi@....com>,
        "Dr . David Alan Gilbert" <dgilbert@...hat.com>,
        "Matthew Wilcox (Oracle)" <willy@...radead.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Baolin Wang <baolin.wang@...ux.alibaba.com>,
        Miaohe Lin <linmiaohe@...wei.com>,
        Yang Shi <shy828301@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 21/46] hugetlb: use struct hugetlb_pte for
 walk_hugetlb_range

On 12.01.23 22:33, Peter Xu wrote:
> On Thu, Jan 12, 2023 at 04:17:52PM -0500, James Houghton wrote:
>> I'll look into it, but doing it this way will use _mapcount, so we
>> won't be able to use the vmemmap optimization. I think even if we do
>> use Hugh's approach, refcount is still being kept on the head page, so
>> there's still an overflow risk there (but maybe I am
>> misunderstanding).
> 
> Could you remind me what's the issue if using refcount on the small pages
> rather than the head (assuming vmemmap still can be disabled)?

The THP-way of doing things is refcounting on the head page. All folios 
use a single refcount on the head.

There has to be a pretty good reason to do it differently.

-- 
Thanks,

David / dhildenb