Re: [RESEND][PATCH v5 3/3] hugetlbfs: add swap entry check in follow_hugetlb_page()

[Michal Hocko <mhocko@suse.cz>]

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-horiguchi@ah.jp.nec.com>
> Cc: stable@vger.kernel.org

Acked-by: Michal Hocko <mhocko@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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