From 292924b260247483a58916f6d3550d8c92f32f55 Mon Sep 17 00:00:00 2001 From: Peter Xu Date: Mon, 6 Apr 2020 20:05:49 -0700 Subject: userfaultfd: wp: apply _PAGE_UFFD_WP bit Firstly, introduce two new flags MM_CP_UFFD_WP[_RESOLVE] for change_protection() when used with uffd-wp and make sure the two new flags are exclusively used. Then, - For MM_CP_UFFD_WP: apply the _PAGE_UFFD_WP bit and remove _PAGE_RW when a range of memory is write protected by uffd - For MM_CP_UFFD_WP_RESOLVE: remove the _PAGE_UFFD_WP bit and recover _PAGE_RW when write protection is resolved from userspace And use this new interface in mwriteprotect_range() to replace the old MM_CP_DIRTY_ACCT. Do this change for both PTEs and huge PMDs. Then we can start to identify which PTE/PMD is write protected by general (e.g., COW or soft dirty tracking), and which is for userfaultfd-wp. Since we should keep the _PAGE_UFFD_WP when doing pte_modify(), add it into _PAGE_CHG_MASK as well. Meanwhile, since we have this new bit, we can be even more strict when detecting uffd-wp page faults in either do_wp_page() or wp_huge_pmd(). After we're with _PAGE_UFFD_WP, a special case is when a page is both protected by the general COW logic and also userfault-wp. Here the userfault-wp will have higher priority and will be handled first. Only after the uffd-wp bit is cleared on the PTE/PMD will we continue to handle the general COW. These are the steps on what will happen with such a page: 1. CPU accesses write protected shared page (so both protected by general COW and uffd-wp), blocked by uffd-wp first because in do_wp_page we'll handle uffd-wp first, so it has higher priority than general COW. 2. Uffd service thread receives the request, do UFFDIO_WRITEPROTECT to remove the uffd-wp bit upon the PTE/PMD. However here we still keep the write bit cleared. Notify the blocked CPU. 3. The blocked CPU resumes the page fault process with a fault retry, during retry it'll notice it was not with the uffd-wp bit this time but it is still write protected by general COW, then it'll go though the COW path in the fault handler, copy the page, apply write bit where necessary, and retry again. 4. The CPU will be able to access this page with write bit set. Suggested-by: Andrea Arcangeli Signed-off-by: Peter Xu Signed-off-by: Andrew Morton Cc: Brian Geffon Cc: Pavel Emelyanov Cc: Mike Kravetz Cc: David Hildenbrand Cc: Martin Cracauer Cc: Mel Gorman Cc: Bobby Powers Cc: Mike Rapoport Cc: "Kirill A . Shutemov" Cc: Maya Gokhale Cc: Johannes Weiner Cc: Marty McFadden Cc: Denis Plotnikov Cc: Hugh Dickins Cc: "Dr . David Alan Gilbert" Cc: Jerome Glisse Cc: Rik van Riel Cc: Shaohua Li Link: http://lkml.kernel.org/r/20200220163112.11409-8-peterx@redhat.com Signed-off-by: Linus Torvalds --- include/linux/mm.h | 5 +++++ 1 file changed, 5 insertions(+) (limited to 'include') diff --git a/include/linux/mm.h b/include/linux/mm.h index c7d87ff5027b..e2f938c5a9d8 100644 --- a/include/linux/mm.h +++ b/include/linux/mm.h @@ -1782,6 +1782,11 @@ extern unsigned long move_page_tables(struct vm_area_struct *vma, #define MM_CP_DIRTY_ACCT (1UL << 0) /* Whether this protection change is for NUMA hints */ #define MM_CP_PROT_NUMA (1UL << 1) +/* Whether this change is for write protecting */ +#define MM_CP_UFFD_WP (1UL << 2) /* do wp */ +#define MM_CP_UFFD_WP_RESOLVE (1UL << 3) /* Resolve wp */ +#define MM_CP_UFFD_WP_ALL (MM_CP_UFFD_WP | \ + MM_CP_UFFD_WP_RESOLVE) extern unsigned long change_protection(struct vm_area_struct *vma, unsigned long start, unsigned long end, pgprot_t newprot, -- cgit v1.2.3