On 2024/3/15 0:42, 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 ... > > 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. Hi, x86 maintainers: kindle ping. >
