Re: [PATCH v6 1/2] mm: migration: fix migration of huge PMD shared pages

From: Michal Hocko
Date: Fri Aug 24 2018 - 04:42:02 EST

On Thu 23-08-18 13:59:16, 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.
> 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.
> Fixes: 39dde65c9940 ("shared page table for hugetlb page")
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Mike Kravetz <mike.kravetz@xxxxxxxxxx>

Acked-by: Michal Hocko <mhocko@xxxxxxxx>

One nit below.

> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index 3103099f64fd..a73c5728e961 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -4548,6 +4548,9 @@ static unsigned long page_table_shareable(struct vm_area_struct *svma,
> return saddr;
> }
> +#define _range_in_vma(vma, start, end) \
> + ((vma)->vm_start <= (start) && (end) <= (vma)->vm_end)
> +

static inline please. Macros and potential side effects on given
arguments are just not worth the risk. I also think this is something
for more general use. We have that pattern at many places. So I would
stick that to linux/mm.h

Michal Hocko