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SubjectRe: [PATCH v3 3/3] hugetlbfs: add swap entry check in follow_hugetlb_page()
(4/3/13 2:35 PM), Naoya Horiguchi wrote:
> With applying the previous patch "hugetlbfs: stop setting VM_DONTDUMP in
> initializing vma(VM_HUGETLB)" to reenable hugepage coredump, if a memory
> error happens on a hugepage and the affected processes try to access
> the error hugepage, we hit VM_BUG_ON(atomic_read(&page->_count) <= 0)
> in get_page().
>
> The reason for this bug is that coredump-related code doesn't recognise
> "hugepage hwpoison entry" with which a pmd entry is replaced when a memory
> error occurs on a hugepage.
> In other words, physical address information is stored in different bit layout
> between hugepage hwpoison entry and pmd entry, so follow_hugetlb_page()
> which is called in get_dump_page() returns a wrong page from a given address.
>
> We need to filter out only hwpoison hugepages to have data on healthy
> hugepages in coredump. So this patch makes follow_hugetlb_page() avoid
> trying to get page when a pmd is in swap entry like format.
>
> ChangeLog v3:
> - add comment about using is_swap_pte()
>
> Signed-off-by: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
> Reviewed-by: Michal Hocko <mhocko@suse.cz>
> Acked-by: Konstantin Khlebnikov <khlebnikov@openvz.org>
> Cc: stable@vger.kernel.org
> ---
> mm/hugetlb.c | 8 +++++++-
> 1 file changed, 7 insertions(+), 1 deletion(-)
>
> diff --git v3.9-rc3.orig/mm/hugetlb.c v3.9-rc3/mm/hugetlb.c
> index 0d1705b..3bc20bd 100644
> --- v3.9-rc3.orig/mm/hugetlb.c
> +++ v3.9-rc3/mm/hugetlb.c
> @@ -2966,9 +2966,15 @@ long follow_hugetlb_page(struct mm_struct *mm, struct vm_area_struct *vma,
> * Some archs (sparc64, sh*) have multiple pte_ts to
> * each hugepage. We have to make sure we get the
> * first, for the page indexing below to work.
> + *
> + * is_swap_pte test covers both is_hugetlb_entry_hwpoisoned
> + * and hugepages under migration in which case
> + * hugetlb_fault waits for the migration and bails out
> + * properly for HWPosined pages.
> */
> pte = huge_pte_offset(mm, vaddr & huge_page_mask(h));
> - absent = !pte || huge_pte_none(huge_ptep_get(pte));
> + absent = !pte || huge_pte_none(huge_ptep_get(pte)) ||
> + is_swap_pte(huge_ptep_get(pte));

Hmmm...

Now absent has two meanings. 1) skip hugetlb_fault() and return immediately if FOLL_DUMP is used.
2) call hugetlb_fault() if to be need page population or cow.

The description of this patch only explain about (2). and I'm not convinced why we don't need to
dump pages under migraion.














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