Re: [PATCH] mm: migration: fix migration of huge PMD shared pages

From: Mike Kravetz
Date: Tue Aug 14 2018 - 09:26:58 EST

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
- 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 primary reason for asking the question was with respect to the code
added to try_to_unmap_one. In my testing, the changes I added appeared
to be required. Just wanted to make sure.

I need to fix a build issue and will send another version.
Mike Kravetz