Re: [PATCH v2] mm: migration: fix migration of huge PMD shared pages

From: Greg KH
Date: Tue Aug 14 2018 - 09:27:21 EST

On Mon, Aug 13, 2018 at 05:30:58PM -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
> no 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 due to ECC memory errors.
> 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 while continually writing to the huge pages being migrated.
> 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 the reference on the PMD page. After this, flush
> caches and TLB.
> Fixes: 39dde65c9940 ("shared page table for hugetlb page")
> Signed-off-by: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
> ---
> v2: Fixed build issue for !CONFIG_HUGETLB_PAGE and typos in comment


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