On Wed, Dec 13, 2017 at 4:10 PM, Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote: > On Wed, Dec 13, 2017 at 11:12:33PM +0100, Peter Zijlstra wrote: >> On Wed, Dec 13, 2017 at 01:50:22PM -0800, Matthew Wilcox wrote: >> > On Tue, Dec 12, 2017 at 06:32:26PM +0100, Thomas Gleixner wrote: >> > > From: Peter Zijstra <peterz@xxxxxxxxxxxxx> >> > > In order to create VMAs that are not accessible to userspace create a new >> > > VM_NOUSER flag. This can be used in conjunction with >> > > install_special_mapping() to inject 'kernel' data into the userspace map. >> > >> > Maybe I misunderstand the intent behind this, but I was recently looking >> > at something kind of similar. I was calling it VM_NOTLB and it wouldn't >> > put TLB entries into the userspace map at all. The idea was to be able >> > to use the user address purely as a handle for specific kernel pages, >> > which were guaranteed to never be mapped into userspace, so we didn't >> > need to send TLB invalidations when we took those pages away from the user >> > process again. But we'd be able to pass the address to read() or write(). >> >> Since the LDT is strictly per process, the idea was to actually inject >> it into the userspace map. Except of course, userspace must not actually >> be able to access it. So by mapping it !_PAGE_USER its 'invisible'. >> >> But the CPU very much needs the mapping, it will load the LDT entries >> through them. > > So can I use your VM_NOUSER VMAs for my purpose? That is, can I change > the page table without flushing the TLB? The only access to these PTEs > will be through the kernel mapping, so I don't see why I'd need to. I doubt it, since if it's in the kernel pagetables at all, then the mapping can be cached for kernel purposes. But I still think this discussion is off in the weeds. x86 does not actually need any of this stuff. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>
