Message-ID: <d0454798-b8d6-d47f-9b57-b196d0a6e357@redhat.com>
Date:   Wed, 1 Feb 2023 19:01:32 +0100
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Subject: Re: [PATCH 21/46] hugetlb: use struct hugetlb_pte for
 walk_hugetlb_range
To:     James Houghton <jthoughton@google.com>
Cc:     Peter Xu <peterx@redhat.com>,
        Mike Kravetz <mike.kravetz@oracle.com>,
        Muchun Song <songmuchun@bytedance.com>,
        David Rientjes <rientjes@google.com>,
        Axel Rasmussen <axelrasmussen@google.com>,
        Mina Almasry <almasrymina@google.com>,
        Zach O'Keefe <zokeefe@google.com>,
        Manish Mishra <manish.mishra@nutanix.com>,
        Naoya Horiguchi <naoya.horiguchi@nec.com>,
        "Dr . David Alan Gilbert" <dgilbert@redhat.com>,
        "Matthew Wilcox (Oracle)" <willy@infradead.org>,
        Vlastimil Babka <vbabka@suse.cz>,
        Baolin Wang <baolin.wang@linux.alibaba.com>,
        Miaohe Lin <linmiaohe@huawei.com>,
        Yang Shi <shy828301@gmail.com>,
        Andrew Morton <akpm@linux-foundation.org>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org
References: <CADrL8HUggALQET-09Zw3BhFjZdw_G9+v6CU=qtGtK=KZ_DeAsw@mail.gmail.com>
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From:   David Hildenbrand <david@redhat.com>
Organization: Red Hat
In-Reply-To: <CADrL8HWNitzWTs4N=-CdGaY_GNUi4PORoN9r=qH3xbHmdcMwjA@mail.gmail.com>
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>>> 1. The RFC v2 way, which is head-only, and we increment the compound
>>> mapcount for each PT mapping we have. So a PTE-mapped 2M page,
>>> compound_mapcount=512, subpage->_mapcount=0 (ignoring the -1 bias).
>>> 2. The THP-like way. If we are fully mapping the hugetlb page with the
>>> hstate-level PTE, we increment the compound mapcount, otherwise we
>>> increment subpage->_mapcount.
>>> 3. The RFC v1 way (the way you have suggested above), which is
>>> head-only, and we increment the compound mapcount if the hstate-level
>>> PTE is made present.
>>>
>>> With #1 and #2, there is no concern with folio_mapcount(). But with
>>> #3, folio_mapcount() for a PTE-mapped 2M page mapped in a single VMA
>>> would yield 1 instead of 512 (right?). That's what I mean.
>>
>> My 2 cents:
>>
>> The mapcount is primarily used (in hugetlb context) to
>>
>> (a) Detect if a page might be shared. mapcount > 1 implies that two
>> independent page table hierarchies are mapping the page. We care about
>> mapcount == 1 vs. mapcount != 1.
>>
>> (b) Detect if unmapping was sucessfull. We care about mapcount == 0 vs.
>> mapcount != 0.
>>
>> For hugetlb, I don't see why we should care about the subpage mapcount
>> at all.
> 
> Agreed -- it shouldn't really matter all that much.
> 
>>
>> For (a) it's even good to count "somehow mapped into a single page table
>> structure" as "mapcount == 1" For (b), we don't care as long as "still
>> mapped" implies "mapcount != 0".
> 
> Thanks for your thoughts, David. So it sounds like you're still
> squarely in the #3 camp. :)

Well, yes. As long as we can get it implemented in a clean way ... :)

-- 
Thanks,

David / dhildenb