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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Rspamd-Queue-Id: 6CA0D20023 X-Rspam-User: X-Stat-Signature: uxtkr8h45kq5mikqumfue455boy68r9o X-Rspamd-Server: rspam01 X-HE-Tag: 1710436364-432196 X-HE-Meta: U2FsdGVkX18l1ZOc/6xiuJbHfDuLxYdpyllz0F8mbNetfW48aHFiBUdn2P3XsfKQMg4By4pIFT5De8s57r1n8Iakeilja6t5tEvPL/X13DVupQDDuwhsCtC0xCnRuByn8W5tLkJ2DeQk0OR0uwbuYelIL9vlbM3NwnEo7TUbbQuNR5VP2lhYt3NouN4Jf22taeiocO6fL0i8ahyQFYkqMhhMGodO2U3S0fXw6RHaEghlscE8rjjlqU6Vm/J14gp+fbXijldNdCkGQdJaA8WjrEqWhgVZCRxbxIbQMcfPIvhqaYGwisJOC2kaBL8agkuQbOQTRKfVnSWbdqw3BLAAnaA0E82GDyJaClZfNMtTQAxMInlQgDscXvC0JNlmsJ8aA0v92EPMR4uGfuXXkgc5aQx/3yTYyBT8lK8gb3fefAFydRaQl7Q9IKbWQ0ow88OXs+bTEzEM2ME0MDzaLifT8sASMeQwJOjfpBk0WhvYjfxyjJd2gARNGy9iakY38jcqp1zWXG/XtCRz7BUYQSQ1vNOA/s5cOXh1cFL5PDbEAB8OA8eaU5pT4cvxnxb2K4HNjYF6yPRITS3SBbUHh8k1C9xEQdEB484HbL7x1eU8lYlRgHvvTA7+8wnDdVwcOjJuqin0mrFfJJbRRzaFDgAboOjqm3ZF1E7Z8QysnEhUHsdYcdWGluaJduJ9ZhWepjwvJTRzAXUHrKFkMWUyIXxX2oBuzgwQJjUJkHOxr5wCbuTl06dE/+fJgOmKNCoCJwr2ab1tiUZVMYFA7VzImv5ENwSXT1NDGWy5HOzZ0LHURb3MHw4rl+KQ3hhnS3lQBdDX8kCjDFBWx8fZi2J7fwzrBqPTs3o/F8RBqQEksIZDAoxdVY7nPGFKDJ/wTMvkvNLSqRsCw92PDV7r5zStNxLj1dySF9n8vpmPS187lUVaEJFwrUx0xePL1qjUTXnRqK5039UuAgldfImSJoywxcQ +kZEdjwa C22Fw5Am3zXsp5//B9eRe0/FENNA1C+J2osfZrbyKS8sfBZJtvKPcrxPgMPdYnVRWHconuV6OhnBX4oC76ve0+/Ncje1e/cQo621pU1YR5FOiVsjaQKyqtLNDMAg1bBX9sYQ8JwlvzDQgEnmr7s5suCjT5uUDeo3pLJSiA8vdMFWDFMJ0MbPg9O/hp4EgvwUITUeszuoqn4GnaJWgC80babAxr1/MMiQw0JvCWbk+8IaARCMv8RDyYeEdpRX7X8sUdL+mSCM7dUwYLcRv+IMsUvgvpUe5P7Ixe0maA5Zai0iHhTQczRR6diLLMOH8hNEWBMIP24FMTVLSNIb7k9PnfIlf52FHD9oHzXdJ8t0/m7QGGYXhW/j/Pl62SWw+ynrgKT+FKGYGDUFrZdBaEcMjOytfyTe2ipzRmT8lsYHY/jBdVPpRlxg/C88QHZWFwK6hZi5G3piuxsK3eqyF8AVrW85LVQ== 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 14.03.24 17:42, David Hildenbrand wrote: > On 12.03.24 20:38, David Hildenbrand wrote: >> On 12.03.24 20:22, Matthew Wilcox wrote: >>> On Tue, Mar 12, 2024 at 07:11:18PM +0100, David Hildenbrand wrote: >>>> PAT handling won't do the right thing in COW mappings: the first PTE >>>> (or, in fact, all PTEs) can be replaced during write faults to point at >>>> anon folios. Reliably recovering the correct PFN and cachemode using >>>> follow_phys() from PTEs will not work in COW mappings. >>> >>> I guess the first question is: Why do we want to support COW mappings >>> of VM_PAT areas? What breaks if we just disallow it? >> >> Well, that was my first approach. Then I decided to be less radical (IOW >> make my life easier by breaking less user space) and "fix it" with >> minimal effort. >> >> Chances of breaking some weird user space is possible, although I >> believe for most such mappings MAP_PRIVATE doesn't make too much sense >> sense. >> >> Nasty COW support for VM_PFNMAP mappings dates back forever. So does PAT >> support. >> >> I can try finding digging through some possible user space users tomorrow. > > As discussed, MAP_PRIVATE doesn't make too much sense for most PFNMAP > mappings. > > However, /dev/mem and /proc/vmcore are still used with MAP_PRIVATE in > some cases. > > Side note: /proc/vmcore is a bit weird: mmap_vmcore() sets VM_MIXEDMAP, > and then we might call remap_pfn_range(), which sets VM_PFNMAP. I'm not > so sure if that's what we want to happen ... Correction: at least mmap_vmcore() ends up clearing VM_MAYWRITE. So no COW mapping. We could do the same to at least keep PROT_READ|MAP_PRIVATE working. If user space specified PROT_WRITE for whatever reason, it's not that easy. -- Cheers, David / dhildenb