linux-kernel - Re: [PATCH v2 5/5] mm/hwpoison: replace most of put_page in memory error handling by put_hwpoison

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Date:	Wed, 12 Aug 2015 17:13:39 +0800
From:	Wanpeng Li <wanpeng.li@...mail.com>
To:	Naoya Horiguchi <n-horiguchi@...jp.nec.com>
CC:	Andrew Morton <akpm@...ux-foundation.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 5/5] mm/hwpoison: replace most of put_page in memory
 error handling by put_hwpoison_page

On 8/12/15 4:55 PM, Naoya Horiguchi wrote:
> 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@...mail.com>
> # 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.)

Indeed. :-)

>
> So I think you can apply put_hwpoison_page() for 3 more callsites in
> mm/memory-failure.c.
>  - MF_MSG_POISONED_HUGE case

I have already done this in my patch.

>  - "soft offline: %#lx page already poisoned" case (you mentioned above)
>  - "soft offline: %#lx: failed to split THP" case (you mentioned above)

You are right, I will send a patch rebased on this one since they are
merged.

Regards,
Wanpeng Li

>
> Thanks,
> Naoya Horiguchi

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