On Wed 10-04-13 12:17:49, Naoya Horiguchi wrote:
> # I suspended Reviewed and Acked given for the previous version, because
> # it has a non-minor change. If you want to restore it, please let me know.
> -----
> 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.
>
> The expected behavior is like this:
>
> absent is_swap_pte FOLL_DUMP Expected behavior
> -------------------------------------------------------------------
> true false false hugetlb_fault
> false true false hugetlb_fault
> false false false return page
> true false true skip page (to avoid allocation)
> false true true hugetlb_fault
> false false true return page
>
> With this patch, we can call hugetlb_fault() and take proper actions
> (we wait for migration entries, fail with VM_FAULT_HWPOISON_LARGE for
> hwpoisoned entries,) and as the result we can dump all hugepages except
> for hwpoisoned ones.
>
> ChangeLog v5:
> - improve comment and description.
>
> ChangeLog v4:
> - move is_swap_page() to right place.
>
> ChangeLog v3:
> - add comment about using is_swap_pte()
>
> Signed-off-by: Naoya Horiguchi <n-horigu...@ah.jp.nec.com>
> Cc: sta...@vger.kernel.org
Acked-by: Michal Hocko <mho...@suse.cz>
Thanks!
> ---
> mm/hugetlb.c | 12 +++++++++++-
> 1 file changed, 11 insertions(+), 1 deletion(-)
>
> diff --git v3.9-rc3.orig/mm/hugetlb.c v3.9-rc3/mm/hugetlb.c
> index 0d1705b..bf26ee8 100644
> --- v3.9-rc3.orig/mm/hugetlb.c
> +++ v3.9-rc3/mm/hugetlb.c
> @@ -2983,7 +2983,17 @@ long follow_hugetlb_page(struct mm_struct *mm, struct
> vm_area_struct *vma,
> break;
> }
>
> - if (absent ||
> + /*
> + * We need call hugetlb_fault for both hugepages under migration
> + * (in which case hugetlb_fault waits for the migration,) and
> + * hwpoisoned hugepages (in which case we need to prevent the
> + * caller from accessing to them.) In order to do this, we use
> + * here is_swap_pte instead of is_hugetlb_entry_migration and
> + * is_hugetlb_entry_hwpoisoned. This is because it simply covers
> + * both cases, and because we can't follow correct pages
> + * directly from any kind of swap entries.
> + */
> + if (absent || is_swap_pte(huge_ptep_get(pte)) ||
> ((flags & FOLL_WRITE) && !pte_write(huge_ptep_get(pte)))) {
> int ret;
>
> --
> 1.7.11.7
>
> --
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--
Michal Hocko
SUSE Labs
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