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[2003:d8:2f30:ec00:8f7e:58a4:ebf0:6a36]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-3a4eac7d7a7sm2102427f8f.21.2025.05.28.11.22.09 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 28 May 2025 11:22:09 -0700 (PDT) Message-ID: <331096ae-e648-4a6e-8728-5cc0bf319b3b@redhat.com> Date: Wed, 28 May 2025 20:22:08 +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: <5b9f5952-9979-426f-857a-dffa9b7963af@redhat.com> <20250528162915.GR61950@nvidia.com> <8b307f83-6a16-46c1-b71b-64af9ca0f592@redhat.com> <20250528174754.GA61950@nvidia.com> <84a19161-765d-4aba-b42f-5fdd8c986bdf@redhat.com> <20250528181506.GA192531@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-Queue-Id: 288E040014 X-Stat-Signature: cxws658pffstf7hngd4im1u75drjzp66 X-Rspam-User: X-Rspamd-Server: rspam07 X-HE-Tag: 1748456534-321256 X-HE-Meta: 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 skGGSyfi 7iM9VWlPtTiVqaMFyqi3h4bbOZ8YwjOcPCilGMhf+db9otugmmitz228Ef23gxoPsOrvZvRmf3T9Iv8Ou3DeSqYIjlkZ98kP0v+58+7NaMkKGoOliXW2Pn8XOWTqllAx0drMatyopCNly+S478GjLOjVQBqmtHpw9jl+Kj4ysZmtXfHNw+tRz0Nr2M7OVm6NRHz/MvaU2nhvZFAINRmNrtsb5d8PV7xdc/XeqTuAvkKcKCyWRCK+Fjr874UId2k5kLgHD/kJ0zhnZ7CHxq7RbCA/QZRfwplxrtYHLMhC92QnJRdRLfP5sXshWN0nB/Z6ZgiAIO/bQDaQXWKX2xoi+vheqazhSQP4Nxn8SHeJY0GA7g+CvoGoPwYCRuu88uUdObsvYl/++M8MRKcGGxcLLAbkkcKErbSC6u30HXG2od5ULhwkpt7uTTJcdNjZIsdNB1uxb2YjlHkSXmJw= 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 20:15, Jason Gunthorpe wrote: > On Wed, May 28, 2025 at 08:00:34PM +0200, David Hildenbrand wrote: > >> 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. > > The special COW rules for PFNMAP require a single linear mapping so > that vma->vm_pgoff can encode the "special" range. Yes, I have a semi-ugly patch to adjust remove that special casing. (less ugly than current handling: *still* lookup the struct page, and if it references an anonymous folio, we ... have an anonymous folio. Because nobody should ever,ever,ever PFNMAP an anonymous folio. Ever.) So the only way to > get struct pages in a PFNMAP that also satisfy the COW rule is if you > have a single contiguous chunk of them. In remap_pfn_range_internal() we sanity-check that right now. And fail if it is not in place. Any COW VM_PFNMAP that doesn't go through remap_pfn_range() would be ... broken. I don't think we have them. > > Otherwise they turn into MIXEDMAP on non-special arches. > > Which is a little detail I wonder how many DRM drivers trip up on :| > >> ... and that should definitely be changed, such that MIXEDMAP is really only >> used when actually having non-refcounted things in there. > > MIXEDMAP has to be used any time you can't meet the special rules that > remap_pfn_range() sets for PFNMAP. > > Really I think we have three special cases, not just two: > > VM_MIXEDMAP - The ptes can hold struct pages and if they do the refcount is > managed by the core code > > VM_PFNMAP|VM_SHARED - The ptes can never be converted into struct pages, nothing > can ever refcount them and you can use fault/etc to store > things > > VM_PFNMAP|!VM_SHARED - Must only be created by remap_pfn_range() and can store > exactly 1 contiguous range of address that cannot be converted to > struct pages and will never be refcounted. Probably can't > use fault Yes. -- Cheers, David / dhildenb