Re: [PATCH v2 5/5] mm/hwpoison: replace most of put_page in memory error handling by put_hwpoison_page

[Naoya Horiguchi]

On Mon, Aug 10, 2015 at 07:28:23PM +0800, Wanpeng Li wrote:
> Replace most of put_page in memory error handling by put_hwpoison_page,
> except the ones at the front of soft_offline_page since the page maybe
> THP page and the get refcount in madvise_hwpoison is against the single
> 4KB page instead of the logic in get_hwpoison_page.
>
> Signed-off-by: Wanpeng Li <wanpeng.li@xxxxxxxxxxx>

# Sorry for my late response.

If I read correctly, get_user_pages_fast() (called by madvise_hwpoison)
for a THP tail page takes a refcount from each of head and tail page.
gup_huge_pmd() does this in the fast path, and get_page_foll() does this
in the slow path (maybe via the following code path)

  get_user_pages_unlocked
    __get_user_pages_unlocked
      __get_user_pages_locked
        __get_user_pages
          follow_page_mask
            follow_trans_huge_pmd (with FOLL_GET set)
              get_page_foll

So this should be equivalent to what get_hwpoison_page() does for thp pages
with regard to refcounting.

And I'm expecting that a refcount taken by get_hwpoison_page() is released
by put_hwpoison_page() even if the page's status is changed during error
handling (the typical (or only?) case is successful thp split.)

So I think you can apply put_hwpoison_page() for 3 more callsites in
mm/memory-failure.c.
 - MF_MSG_POISONED_HUGE case
 - "soft offline: %#lx page already poisoned" case (you mentioned above)
 - "soft offline: %#lx: failed to split THP" case (you mentioned above)

Thanks,
Naoya Horiguchi--
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