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

[KOSAKI Motohiro]

On Wed, Apr 10, 2013 at 12:17 PM, Naoya Horiguchi
<n-horiguchi@xxxxxxxxxxxxx> 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@xxxxxxxxxxxxx>
> Cc: stable@xxxxxxxxxxxxxxx

Acked-by: KOSAKI Motohiro <kosaki.motohiro@xxxxxxxxxxxxxx>
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