* David Hildenbrand <david@xxxxxxxxxx> 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. I'd much prefer restricting VM_PAT areas than expanding support. Could we try the trivial restriction approach first, and only go with your original patch if that fails? Thanks, Ingo
