From: Andrea Arcangeli aarcange@redhat.com
[ Upstream commit d7c3393413fe7e7dc54498ea200ea94742d61e18 ]
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 Signed-off-by: Sasha Levin sashal@kernel.org --- mm/migrate.c | 25 ++++++++++++++++++------- 1 file changed, 18 insertions(+), 7 deletions(-)
diff --git a/mm/migrate.c b/mm/migrate.c index 0c48191a90368..4d3588c012034 100644 --- a/mm/migrate.c +++ b/mm/migrate.c @@ -2048,15 +2048,26 @@ 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);
@@ -2070,7 +2081,7 @@ 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);