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. -- 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>
