On Thu, Oct 18, 2018 at 10:28:12AM -0700, Mike Kravetz wrote:
> On 10/10/18 11:04 PM, gregkh@xxxxxxxxxxxxxxxxxxx wrote:
> >
> > The patch below does not apply to the 4.9-stable tree.
> > If someone wants it applied there, or to any other stable or longterm
> > tree, then please email the backport, including the original git commit
> > id to <stable@xxxxxxxxxxxxxxx>.
>
> From: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
>
> mm: migration: fix migration of huge PMD shared pages
>
> commit 017b1660df89f5fb4bfe66c34e35f7d2031100c7 upstream
>
> 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.
>
> mmu notifiers are called before locking page tables, but we can not
> be sure of PMD sharing until page tables are locked. Therefore,
> check for the possibility of PMD sharing before locking so that
> notifiers can prepare for the worst possible case. The mmu notifier
> calls in this commit are different than upstream. That is because
> upstream went to a different model here. Instead of moving to the
> new model, we leave existing model unchanged and only use the
> mmu_*range* calls in this special case.
>
> Fixes: 39dde65c9940 ("shared page table for hugetlb page")
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
> ---
> include/linux/hugetlb.h | 14 +++++++++++
> include/linux/mm.h | 6 +++++
> mm/hugetlb.c | 37 +++++++++++++++++++++++++--
> mm/rmap.c | 56 +++++++++++++++++++++++++++++++++++++++++
> 4 files changed, 111 insertions(+), 2 deletions(-)
Now queued up, thanks.
greg k-h
