* Dave Hansen <dave@xxxxxxxx> wrote: > On 09/25/2015 11:20 PM, Ingo Molnar wrote: > > * Dave Hansen <dave@xxxxxxxx> wrote: > ... > >> Since follow_pte() fails for all huge > >> pages, it just falls back to pulling the protection key out of the VMA, > >> which _does_ work for huge pages. > > > > That might be true for explicit hugetlb vmas, but what about transparent hugepages > > that can show up in regular vmas? > > All PTEs (large or small) established under a given VMA have the same > protection key. [...] So a 'pte' is only small. The 'large' thing is called a pmd. So follow_pte() is not adequate. But with that removed everything should be fine as the vma (protection) flags are size independent. > So I think it's safe to rely on the VMA entirely. Well, as least as safe as the > PTE. It's definitely a wee bit racy, which I'll elaborate on when I repost the > patches. So the race I can see is wrt. mprotect(), and we should fix that, because the existing method of recovering the 'page fault reason', error_code, is not racy - so the extension of it (the protection key) should not be racy either. By the time user-space processes the signal we might race with other threads, but at least the fault-address/error-reason information itself should be coherent. This can be solved by getting the protection key while still under the down_read() of the vma - instead of your current solution of a second find_vma(). Thanks, Ingo -- 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>