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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Rspamd-Server: rspam08 X-Rspamd-Queue-Id: 9ABFD40006 X-Stat-Signature: w6bfdwbc4noxsosaxq17d74mwsq7zqqd X-Rspam-User: X-HE-Tag: 1710434545-742652 X-HE-Meta: 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 rAZtAayX 4UAEL48B2SbAeYHjJ/VT6/eonQozMxHuxWB6+j8AnK6uE9Bi36Ock70v+s9miLvpOivHmeL1Ac7UK/SXwOZSsnxw+6i/bTudhbtMYMkRciAtNq7bqCgwlupWSfp/KCx30NoiU3KRga9mmGq8PH5rmodAm0Q6ZoakkRMyuHpZJbK/YTDx38rxMnXgS7EvwTi91mLiuT73L7n9PqHyYR70axlgdsQ== 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 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 ... As far as I can see, makedumpfile always mmap's memory to be dumped (/dev/mem, /proc/vmcore) using PROT_READ+MAP_PRIVATE, resulting in a COW mapping. In my opinion, we should use this fairly simple fix to keep it working for now and look into disabling any MAP_PRIVATE of VM_PFNMAP separately, for all architectures. But I'll leave the decision to x86 maintainers. -- Cheers, David / dhildenb