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authorAndrea Arcangeli <aarcange@redhat.com>2018-10-26 15:10:36 -0700
committerLinus Torvalds <torvalds@linux-foundation.org>2018-10-26 16:38:15 -0700
commitd7c3393413fe7e7dc54498ea200ea94742d61e18 (patch)
treecf017fab48f0e298608438ba4d990213ca268357
parent026d1eaf5ef1a5d6258b46e4e411cd9f5ab8c41d (diff)
downloadop-kernel-dev-d7c3393413fe7e7dc54498ea200ea94742d61e18.zip
op-kernel-dev-d7c3393413fe7e7dc54498ea200ea94742d61e18.tar.gz
mm: thp: fix MADV_DONTNEED vs migrate_misplaced_transhuge_page race condition
Patch series "migrate_misplaced_transhuge_page race conditions". Aaron found a new instance of the THP MADV_DONTNEED race against pmdp_clear_flush* variants, that was apparently left unfixed. While looking into the race found by Aaron, I may have found two more issues in migrate_misplaced_transhuge_page. These race conditions would not cause kernel instability, but they'd corrupt userland data or leave data non zero after MADV_DONTNEED. I did only minor testing, and I don't expect to be able to reproduce this (especially the lack of ->invalidate_range before migrate_page_copy, requires the latest iommu hardware or infiniband to reproduce). The last patch is noop for x86 and it needs further review from maintainers of archs that implement flush_cache_range() (not in CC yet). To avoid confusion, it's not the first patch that introduces the bug fixed in the second patch, even before removing the pmdp_huge_clear_flush_notify, that _notify suffix was called after migrate_page_copy already run. This patch (of 3): This is a corollary of ced108037c2aa ("thp: fix MADV_DONTNEED vs. numa balancing race"), 58ceeb6bec8 ("thp: fix MADV_DONTNEED vs. MADV_FREE race") and 5b7abeae3af8c ("thp: fix MADV_DONTNEED vs clear soft dirty race). When the above three fixes where posted Dave asked https://lkml.kernel.org/r/929b3844-aec2-0111-fef7-8002f9d4e2b9@intel.com but apparently this was missed. The pmdp_clear_flush* in migrate_misplaced_transhuge_page() was introduced in a54a407fbf7 ("mm: Close races between THP migration and PMD numa clearing"). The important part of such commit is only the part where the page lock is not released until the first do_huge_pmd_numa_page() finished disarming the pagenuma/protnone. The addition of pmdp_clear_flush() wasn't beneficial to such commit and there's no commentary about such an addition either. I guess the pmdp_clear_flush() in such commit was added just in case for safety, but it ended up introducing the MADV_DONTNEED race condition found by Aaron. At that point in time nobody thought of such kind of MADV_DONTNEED race conditions yet (they were fixed later) so the code may have looked more robust by adding the pmdp_clear_flush(). This specific race condition won't destabilize the kernel, but it can confuse userland because after MADV_DONTNEED the memory won't be zeroed out. This also optimizes the code and removes a superfluous TLB flush. [akpm@linux-foundation.org: reflow comment to 80 cols, fix grammar and typo (beacuse)] Link: http://lkml.kernel.org/r/20181013002430.698-2-aarcange@redhat.com Signed-off-by: Andrea Arcangeli <aarcange@redhat.com> Reported-by: Aaron Tomlin <atomlin@redhat.com> Acked-by: Mel Gorman <mgorman@suse.de> Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com> Cc: Jerome Glisse <jglisse@redhat.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
-rw-r--r--mm/migrate.c25
1 files changed, 18 insertions, 7 deletions
diff --git a/mm/migrate.c b/mm/migrate.c
index 1ea27b3..93d9a1e 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -2031,15 +2031,26 @@ int migrate_misplaced_transhuge_page(struct mm_struct *mm,
entry = maybe_pmd_mkwrite(pmd_mkdirty(entry), vma);
/*
- * Clear the old entry under pagetable lock and establish the new PTE.
- * Any parallel GUP will either observe the old page blocking on the
- * page lock, block on the page table lock or observe the new page.
- * The SetPageUptodate on the new page and page_add_new_anon_rmap
- * guarantee the copy is visible before the pagetable update.
+ * Overwrite the old entry under pagetable lock and establish
+ * the new PTE. Any parallel GUP will either observe the old
+ * page blocking on the page lock, block on the page table
+ * lock or observe the new page. The SetPageUptodate on the
+ * new page and page_add_new_anon_rmap guarantee the copy is
+ * visible before the pagetable update.
*/
flush_cache_range(vma, mmun_start, mmun_end);
page_add_anon_rmap(new_page, vma, mmun_start, true);
- pmdp_huge_clear_flush_notify(vma, mmun_start, pmd);
+ /*
+ * At this point the pmd is numa/protnone (i.e. non present) and the TLB
+ * has already been flushed globally. So no TLB can be currently
+ * caching this non present pmd mapping. There's no need to clear the
+ * pmd before doing set_pmd_at(), nor to flush the TLB after
+ * set_pmd_at(). Clearing the pmd here would introduce a race
+ * condition against MADV_DONTNEED, because MADV_DONTNEED only holds the
+ * mmap_sem for reading. If the pmd is set to NULL at any given time,
+ * MADV_DONTNEED won't wait on the pmd lock and it'll skip clearing this
+ * pmd.
+ */
set_pmd_at(mm, mmun_start, pmd, entry);
update_mmu_cache_pmd(vma, address, &entry);
@@ -2053,7 +2064,7 @@ int migrate_misplaced_transhuge_page(struct mm_struct *mm,
* No need to double call mmu_notifier->invalidate_range() callback as
* the above pmdp_huge_clear_flush_notify() did already call it.
*/
- mmu_notifier_invalidate_range_only_end(mm, mmun_start, mmun_end);
+ mmu_notifier_invalidate_range_end(mm, mmun_start, mmun_end);
/* Take an "isolate" reference and put new page on the LRU. */
get_page(new_page);
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