linux-kernel - Re: [PATCH mm-unstable] mm/khugepaged: fix collapse_pte_mapped

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Message-ID: <d05a6922-8dbc-e6fb-f2c5-48f331652f20@redhat.com>
Date:   Tue, 22 Aug 2023 17:33:52 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Jann Horn <jannh@...gle.com>, Matthew Wilcox <willy@...radead.org>
Cc:     Hugh Dickins <hughd@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Mike Rapoport <rppt@...nel.org>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Qi Zheng <zhengqi.arch@...edance.com>,
        Yang Shi <shy828301@...il.com>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Peter Xu <peterx@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Will Deacon <will@...nel.org>, Yu Zhao <yuzhao@...gle.com>,
        Alistair Popple <apopple@...dia.com>,
        Ralph Campbell <rcampbell@...dia.com>,
        Ira Weiny <ira.weiny@...el.com>,
        Steven Price <steven.price@....com>,
        SeongJae Park <sj@...nel.org>,
        Lorenzo Stoakes <lstoakes@...il.com>,
        Huang Ying <ying.huang@...el.com>,
        Naoya Horiguchi <naoya.horiguchi@....com>,
        Christophe Leroy <christophe.leroy@...roup.eu>,
        Zack Rusin <zackr@...are.com>, Jason Gunthorpe <jgg@...pe.ca>,
        Axel Rasmussen <axelrasmussen@...gle.com>,
        Anshuman Khandual <anshuman.khandual@....com>,
        Pasha Tatashin <pasha.tatashin@...een.com>,
        Miaohe Lin <linmiaohe@...wei.com>,
        Minchan Kim <minchan@...nel.org>,
        Christoph Hellwig <hch@...radead.org>,
        Song Liu <song@...nel.org>,
        Thomas Hellstrom <thomas.hellstrom@...ux.intel.com>,
        Russell King <linux@...linux.org.uk>,
        "David S. Miller" <davem@...emloft.net>,
        Michael Ellerman <mpe@...erman.id.au>,
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        sparclinux@...r.kernel.org,
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        kernel list <linux-kernel@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>
Subject: Re: [PATCH mm-unstable] mm/khugepaged: fix collapse_pte_mapped_thp()
 versus uffd

On 22.08.23 17:30, Jann Horn wrote:
> On Tue, Aug 22, 2023 at 5:23 PM Matthew Wilcox <willy@...radead.org> wrote:
>> On Tue, Aug 22, 2023 at 04:39:43PM +0200, Jann Horn wrote:
>>>> Perhaps something else will want that same behaviour in future (it's
>>>> tempting, but difficult to guarantee correctness); for now, it is just
>>>> userfaultfd (but by saying "_armed" rather than "_missing", I'm half-
>>>> expecting uffd to add more such exceptional modes in future).
>>>
>>> Hm, yeah, sounds okay. (I guess we'd also run into this if we ever
>>> wanted to make it possible to reliably install PTE markers with
>>> madvise() or something like that, which might be nice for allowing
>>> userspace to create guard pages without unnecessary extra VMAs...)
>>
>> I don't know what a userspace API for this would look like, but I have
>> a dream of creating guard VMAs which only live in the maple tree and
>> don't require the allocation of a struct VMA.  Use some magic reserved
>> pointer value like XA_ZERO_ENTRY to represent them ... seems more
>> robust than putting a PTE marker in the page tables?
> 
> Chrome currently uses a lot of VMAs for its heap, which I think are
> basically alternating PROT_NONE and PROT_READ|PROT_WRITE anonymous
> VMAs. Like this:
> 
> [...]
> 3a10002cf000-3a10002d0000 ---p 00000000 00:00 0
> 3a10002d0000-3a10002e6000 rw-p 00000000 00:00 0
> 3a10002e6000-3a10002e8000 ---p 00000000 00:00 0
> 3a10002e8000-3a10002f2000 rw-p 00000000 00:00 0
> 3a10002f2000-3a10002f4000 ---p 00000000 00:00 0
> 3a10002f4000-3a10002fb000 rw-p 00000000 00:00 0
> 3a10002fb000-3a10002fc000 ---p 00000000 00:00 0
> 3a10002fc000-3a1000303000 rw-p 00000000 00:00 0
> 3a1000303000-3a1000304000 ---p 00000000 00:00 0
> 3a1000304000-3a100031b000 rw-p 00000000 00:00 0
> 3a100031b000-3a100031c000 ---p 00000000 00:00 0
> 3a100031c000-3a1000326000 rw-p 00000000 00:00 0
> 3a1000326000-3a1000328000 ---p 00000000 00:00 0
> 3a1000328000-3a100033a000 rw-p 00000000 00:00 0
> 3a100033a000-3a100033c000 ---p 00000000 00:00 0
> 3a100033c000-3a100038b000 rw-p 00000000 00:00 0
> 3a100038b000-3a100038c000 ---p 00000000 00:00 0
> 3a100038c000-3a100039b000 rw-p 00000000 00:00 0
> 3a100039b000-3a100039c000 ---p 00000000 00:00 0
> 3a100039c000-3a10003af000 rw-p 00000000 00:00 0
> 3a10003af000-3a10003b0000 ---p 00000000 00:00 0
> 3a10003b0000-3a10003e8000 rw-p 00000000 00:00 0
> 3a10003e8000-3a1000401000 ---p 00000000 00:00 0
> 3a1000401000-3a1000402000 rw-p 00000000 00:00 0
> 3a1000402000-3a100040c000 ---p 00000000 00:00 0
> 3a100040c000-3a100046f000 rw-p 00000000 00:00 0
> 3a100046f000-3a1000470000 ---p 00000000 00:00 0
> 3a1000470000-3a100047a000 rw-p 00000000 00:00 0
> 3a100047a000-3a100047c000 ---p 00000000 00:00 0
> 3a100047c000-3a1000492000 rw-p 00000000 00:00 0
> 3a1000492000-3a1000494000 ---p 00000000 00:00 0
> 3a1000494000-3a10004a2000 rw-p 00000000 00:00 0
> 3a10004a2000-3a10004a4000 ---p 00000000 00:00 0
> 3a10004a4000-3a10004b6000 rw-p 00000000 00:00 0
> 3a10004b6000-3a10004b8000 ---p 00000000 00:00 0
> 3a10004b8000-3a10004ea000 rw-p 00000000 00:00 0
> 3a10004ea000-3a10004ec000 ---p 00000000 00:00 0
> 3a10004ec000-3a10005f4000 rw-p 00000000 00:00 0
> 3a10005f4000-3a1000601000 ---p 00000000 00:00 0
> 3a1000601000-3a1000602000 rw-p 00000000 00:00 0
> 3a1000602000-3a1000604000 ---p 00000000 00:00 0
> 3a1000604000-3a100062b000 rw-p 00000000 00:00 0
> 3a100062b000-3a1000801000 ---p 00000000 00:00 0
> [...]
> 
> I was thinking if you used PTE markers as guards, you could maybe turn
> all that into more or less a single VMA?

I proposed the topic "A proper API for sparse memory mappings" for the 
bi-weekly MM meeting on September 20, that would also cover exactly that 
use case. :)

-- 
Cheers,

David / dhildenb