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[2003:d8:2f30:ec00:8f7e:58a4:ebf0:6a36]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-3a4eac7db15sm2057441f8f.23.2025.05.28.11.00.35 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 28 May 2025 11:00:36 -0700 (PDT) Message-ID: <84a19161-765d-4aba-b42f-5fdd8c986bdf@redhat.com> Date: Wed, 28 May 2025 20:00:34 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] mm: fix COW mapping handing in generic_access_phys To: Jason Gunthorpe Cc: Peter Xu , Jinjiang Tu , akpm@linux-foundation.org, lorenzo.stoakes@oracle.com, Liam.Howlett@oracle.com, vbabka@suse.cz, rppt@kernel.org, surenb@google.com, mhocko@suse.com, linux-mm@kvack.org, wangkefeng.wang@huawei.com References: <20250528015617.302681-1-tujinjiang@huawei.com> <0d4f0180-52e6-47c9-b141-54e7e7c86880@redhat.com> <5b9f5952-9979-426f-857a-dffa9b7963af@redhat.com> <20250528162915.GR61950@nvidia.com> <8b307f83-6a16-46c1-b71b-64af9ca0f592@redhat.com> <20250528174754.GA61950@nvidia.com> From: David Hildenbrand Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Rspamd-Server: rspam01 X-Rspamd-Queue-Id: 9FD0E10001B X-Stat-Signature: 7rami5dq8g6x1rmc7d4upwr1kdpgetmm X-Rspam-User: X-HE-Tag: 1748455241-618173 X-HE-Meta: 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 0JKExawK DW4f9Ze94ICtKi0OXr6mtDqavC3xjkXwBN6BbR9dX3K1TvaPqfeIeyCsd4x1YXivBnmIh07/PdNxiSyfPVpRRLn/4pIej/CtiMP11nAWR2UVtH9dMm+AKAHKo+NJhV4gynPQ2QoVg1u0O1nxTmdG4a1pR26GvfnupUKOqvoMvLhNhuZ2JmxpuXNWu01Z1ji6it8qs4ZNf+J7IYxW6zS1jkmnMu3GsywU0sCPJIX+Z33yZ8beb0GhRxtDFSRHvgknxH212hJ8aczW9/5YgWJQMGVFHzXEn1j3tjM1jyQGQUthyfcTTB79sCdM9nQggkylxId/i7DtFE0DukZltpNkBsz6TDhJy/IjL+VfV7AbmAej/lAoVBIqRe5ABXSybQd9JwrRYMiyYNcgtFYzhZNerftvr0ePydhoxvc1Iopok3irxoaHyUFufqlGflryn8tnQ5QFHGRbACyXgNF8= X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 28.05.25 19:47, Jason Gunthorpe wrote: > On Wed, May 28, 2025 at 07:32:59PM +0200, David Hildenbrand wrote: >> On 28.05.25 18:29, Jason Gunthorpe wrote: >>> On Wed, May 28, 2025 at 12:06:07PM -0400, Peter Xu wrote: >>>> #define VM_PFNMAP 0x00000400 /* Page-ranges managed without "struct page", just pure PFN */ >>>> >>>> I'm not confident to blame any driver yet to have those special cases for >>>> VM_PFNMAP, because it only says "managed without struct page", it didn't >>>> say "it must not contain struct page".. Hence it hints the core mm "please >>>> do not manage these mappings with struct page at all". Still sounds fair >>>> contract, even if not ideal. >>> >>> I think it is pretty clear, if a VMA has VM_PFNMAP then nothing must >>> ever try to obtain a struct page from any PTEs in it, for any reason, >>> even if things in it might have a struct page. In practice it means >>> nothing can call vm_normal_page() on a VM_PFNMAP. >> >> No, not until we remove any COW mappings of VM_PFNMAP. > > I stand by the statement, the COW mapping thing is broken. Just > because it is wrong doesn't mean it is allowed to call > vm_normal_page() on VM_PFNMAP. > > It makes no sense at all to have VM_PFNMAP where someone has COW'd the > pages and effectively turned it into a MIXEDMAP. > Well, not quite. MIXEDMAP is clearly documented that whatever has a refcount (struct page) is refcounted. That's not the case for PFNMAP, obviously, where nothing is refcounted, except ... anon pages in COW mappings. Having refcounted anon page in a VM_PFNMAP doesn't suddenly turn the whole thing into a MIXEDMAP where other things with a "struct page" are suddenly refcounted as well. > It should just be a MIXEDMAP at that point!! > > IMHO we should have the core code complain with a dmesg for VM_PFNMAP> without VM_SHARED and get driver authors to fix it. Either add the > missed VM_SHARED or changed to MIXEDMAP. Make it WARN_ON in a few > years. > > If they can't follow the rules required to use MIXEDMAP then they > certainly do not have working COW!! > >>> In either case GUP doesn't really work on the VMA. PFNMAP is totally>>> blocked, and for MIXEDMAP userspace has no way to discover which >>> subset of the VMA is GUPable. I think that GUP is supported on >>> MIXEDMAP at all is a bit of a weirdo thing. >> >> IIRC (after recent discussions with Lorenzo) there are use cases for that. > > Really? Somehow userspace can know which sub slice of the VMA is > GUP'able? Gross :P Well, the primary use case is mapping kernel allocations into user space and GUP'ing them. So the pages are refcounted ... MIXEDMAP is likely just abused for that ... so I take back that that's a requirement, rather that MIXEDMAP is automatically used when using the interfaces -- vm_insert_pages -- to insert refcounted pages into page tables :) ... and that should definitely be changed, such that MIXEDMAP is really only used when actually having non-refcounted things in there. -- Cheers, David / dhildenb