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SubjectRe: [PATCH v6 1/2] mm: migration: fix migration of huge PMD shared pages
On Fri 24-08-18 11:08:24, Mike Kravetz wrote:
[...]
> From: Mike Kravetz <mike.kravetz@oracle.com>
> Date: Fri, 24 Aug 2018 10:58:20 -0700
> Subject: [PATCH v7 1/2] mm: migration: fix migration of huge PMD shared pages
>
> 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@vger.kernel.org
> Signed-off-by: Mike Kravetz <mike.kravetz@oracle.com>

With the locking expectations for mmu_notifier_invalidate_range
exlained I do not see any other issues.

Acked-by: Michal Hocko <mhocko@suse.com>

> ---
> include/linux/hugetlb.h | 14 ++++++++++++++
> include/linux/mm.h | 6 ++++++
> mm/hugetlb.c | 37 ++++++++++++++++++++++++++++++++++--
> mm/rmap.c | 42 ++++++++++++++++++++++++++++++++++++++---
> 4 files changed, 94 insertions(+), 5 deletions(-)
>
> diff --git a/include/linux/hugetlb.h b/include/linux/hugetlb.h
> index 36fa6a2a82e3..4ee95d8c8413 100644
> --- a/include/linux/hugetlb.h
> +++ b/include/linux/hugetlb.h
> @@ -140,6 +140,8 @@ pte_t *huge_pte_alloc(struct mm_struct *mm,
> pte_t *huge_pte_offset(struct mm_struct *mm,
> unsigned long addr, unsigned long sz);
> int huge_pmd_unshare(struct mm_struct *mm, unsigned long *addr, pte_t *ptep);
> +void adjust_range_if_pmd_sharing_possible(struct vm_area_struct *vma,
> + unsigned long *start, unsigned long *end);
> struct page *follow_huge_addr(struct mm_struct *mm, unsigned long address,
> int write);
> struct page *follow_huge_pd(struct vm_area_struct *vma,
> @@ -170,6 +172,18 @@ static inline unsigned long hugetlb_total_pages(void)
> return 0;
> }
>
> +static inline int huge_pmd_unshare(struct mm_struct *mm, unsigned long *addr,
> + pte_t *ptep)
> +{
> + return 0;
> +}
> +
> +static inline void adjust_range_if_pmd_sharing_possible(
> + struct vm_area_struct *vma,
> + unsigned long *start, unsigned long *end)
> +{
> +}
> +
> #define follow_hugetlb_page(m,v,p,vs,a,b,i,w,n) ({ BUG(); 0; })
> #define follow_huge_addr(mm, addr, write) ERR_PTR(-EINVAL)
> #define copy_hugetlb_page_range(src, dst, vma) ({ BUG(); 0; })
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 68a5121694ef..40ad93bc9548 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -2463,6 +2463,12 @@ static inline struct vm_area_struct *find_exact_vma(struct mm_struct *mm,
> return vma;
> }
>
> +static inline bool range_in_vma(struct vm_area_struct *vma,
> + unsigned long start, unsigned long end)
> +{
> + return (vma && vma->vm_start <= start && end <= vma->vm_end);
> +}
> +
> #ifdef CONFIG_MMU
> pgprot_t vm_get_page_prot(unsigned long vm_flags);
> void vma_set_page_prot(struct vm_area_struct *vma);
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index 3103099f64fd..f469315a6a0f 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -4556,12 +4556,40 @@ static bool vma_shareable(struct vm_area_struct *vma, unsigned long addr)
> /*
> * check on proper vm_flags and page table alignment
> */
> - if (vma->vm_flags & VM_MAYSHARE &&
> - vma->vm_start <= base && end <= vma->vm_end)
> + if (vma->vm_flags & VM_MAYSHARE && range_in_vma(vma, base, end))
> return true;
> return false;
> }
>
> +/*
> + * Determine if start,end range within vma could be mapped by shared pmd.
> + * If yes, adjust start and end to cover range associated with possible
> + * shared pmd mappings.
> + */
> +void adjust_range_if_pmd_sharing_possible(struct vm_area_struct *vma,
> + unsigned long *start, unsigned long *end)
> +{
> + unsigned long check_addr = *start;
> +
> + if (!(vma->vm_flags & VM_MAYSHARE))
> + return;
> +
> + for (check_addr = *start; check_addr < *end; check_addr += PUD_SIZE) {
> + unsigned long a_start = check_addr & PUD_MASK;
> + unsigned long a_end = a_start + PUD_SIZE;
> +
> + /*
> + * If sharing is possible, adjust start/end if necessary.
> + */
> + if (range_in_vma(vma, a_start, a_end)) {
> + if (a_start < *start)
> + *start = a_start;
> + if (a_end > *end)
> + *end = a_end;
> + }
> + }
> +}
> +
> /*
> * Search for a shareable pmd page for hugetlb. In any case calls pmd_alloc()
> * and returns the corresponding pte. While this is not necessary for the
> @@ -4659,6 +4687,11 @@ int huge_pmd_unshare(struct mm_struct *mm, unsigned long *addr, pte_t *ptep)
> {
> return 0;
> }
> +
> +void adjust_range_if_pmd_sharing_possible(struct vm_area_struct *vma,
> + unsigned long *start, unsigned long *end)
> +{
> +}
> #define want_pmd_share() (0)
> #endif /* CONFIG_ARCH_WANT_HUGE_PMD_SHARE */
>
> diff --git a/mm/rmap.c b/mm/rmap.c
> index eb477809a5c0..1e79fac3186b 100644
> --- a/mm/rmap.c
> +++ b/mm/rmap.c
> @@ -1362,11 +1362,21 @@ static bool try_to_unmap_one(struct page *page, struct vm_area_struct *vma,
> }
>
> /*
> - * We have to assume the worse case ie pmd for invalidation. Note that
> - * the page can not be free in this function as call of try_to_unmap()
> - * must hold a reference on the page.
> + * For THP, we have to assume the worse case ie pmd for invalidation.
> + * For hugetlb, it could be much worse if we need to do pud
> + * invalidation in the case of pmd sharing.
> + *
> + * Note that the page can not be free in this function as call of
> + * try_to_unmap() must hold a reference on the page.
> */
> end = min(vma->vm_end, start + (PAGE_SIZE << compound_order(page)));
> + if (PageHuge(page)) {
> + /*
> + * If sharing is possible, start and end will be adjusted
> + * accordingly.
> + */
> + adjust_range_if_pmd_sharing_possible(vma, &start, &end);
> + }
> mmu_notifier_invalidate_range_start(vma->vm_mm, start, end);
>
> while (page_vma_mapped_walk(&pvmw)) {
> @@ -1409,6 +1419,32 @@ static bool try_to_unmap_one(struct page *page, struct vm_area_struct *vma,
> subpage = page - page_to_pfn(page) + pte_pfn(*pvmw.pte);
> address = pvmw.address;
>
> + if (PageHuge(page)) {
> + if (huge_pmd_unshare(mm, &address, pvmw.pte)) {
> + /*
> + * huge_pmd_unshare unmapped an entire PMD
> + * page. There is no way of knowing exactly
> + * which PMDs may be cached for this mm, so
> + * we must flush them all. start/end were
> + * already adjusted above to cover this range.
> + */
> + flush_cache_range(vma, start, end);
> + flush_tlb_range(vma, start, end);
> + mmu_notifier_invalidate_range(mm, start, end);
> +
> + /*
> + * The ref count of the PMD page was dropped
> + * which is part of the way map counting
> + * is done for shared PMDs. Return 'true'
> + * here. When there is no other sharing,
> + * huge_pmd_unshare returns false and we will
> + * unmap the actual page and drop map count
> + * to zero.
> + */
> + page_vma_mapped_walk_done(&pvmw);
> + break;
> + }
> + }
>
> if (IS_ENABLED(CONFIG_MIGRATION) &&
> (flags & TTU_MIGRATION) &&
> --
> 2.17.1

--
Michal Hocko
SUSE Labs

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