On Mon, Aug 13, 2018 at 11:21:41PM +0000, Mike Kravetz wrote: > On 08/13/2018 03:58 AM, Kirill A. Shutemov wrote: > > On Sun, Aug 12, 2018 at 08:41:08PM -0700, Mike Kravetz wrote: > >> The page migration code employs try_to_unmap() to try and unmap the > >> source page. This is accomplished by using rmap_walk to find all > >> vmas where the page is mapped. This search stops when page mapcount > >> is zero. For shared PMD huge pages, the page map count is always 1 > >> not matter the number of mappings. Shared mappings are tracked via > >> the reference count of the PMD page. Therefore, try_to_unmap stops > >> prematurely and does not completely unmap all mappings of the source > >> page. > >> > >> This problem can result is data corruption as writes to the original > >> source page can happen after contents of the page are copied to the > >> target page. Hence, data is lost. > >> > >> This problem was originally seen as DB corruption of shared global > >> areas after a huge page was soft offlined. DB developers noticed > >> they could reproduce the issue by (hotplug) offlining memory used > >> to back huge pages. A simple testcase can reproduce the problem by > >> creating a shared PMD mapping (note that this must be at least > >> PUD_SIZE in size and PUD_SIZE aligned (1GB on x86)), and using > >> migrate_pages() to migrate process pages between nodes. > >> > >> To fix, have the try_to_unmap_one routine check for huge PMD sharing > >> by calling huge_pmd_unshare for hugetlbfs huge pages. If it is a > >> shared mapping it will be 'unshared' which removes the page table > >> entry and drops reference on PMD page. After this, flush caches and > >> TLB. > >> > >> Signed-off-by: Mike Kravetz <mike.kravetz@xxxxxxxxxx> > >> --- > >> I am not %100 sure on the required flushing, so suggestions would be > >> appreciated. This also should go to stable. It has been around for > >> a long time so still looking for an appropriate 'fixes:'. > > > > I believe we need flushing. And huge_pmd_unshare() usage in > > __unmap_hugepage_range() looks suspicious: I don't see how we flush TLB in > > that case. > > Thanks Kirill, > > __unmap_hugepage_range() has two callers: > 1) unmap_hugepage_range, which wraps the call with tlb_gather_mmu and > tlb_finish_mmu on the range. IIUC, this should cause an appropriate > TLB flush. > 2) __unmap_hugepage_range_final via unmap_single_vma. unmap_single_vma > has three callers: > - unmap_vmas which assumes the caller will flush the whole range after > return. > - zap_page_range wraps the call with tlb_gather_mmu/tlb_finish_mmu > - zap_page_range_single wraps the call with tlb_gather_mmu/tlb_finish_mmu > > So, it appears we are covered. But, I could be missing something. My problem here is that the mapping that moved by huge_pmd_unshare() in not accounted into mmu_gather and can be missed on tlb_finish_mmu(). -- Kirill A. Shutemov
