Re: [PATCH v1 6/6] mm/hwpoison: fix unpoison_memory()

From: Ding Hui <dinghui@sangfor.com.cn>
To: Naoya Horiguchi <nao.horiguchi@gmail.com>, linux-mm@kvack.org
Cc: Andrew Morton <akpm@linux-foundation.org>,
	David Hildenbrand <david@redhat.com>,
	Oscar Salvador <osalvador@suse.de>,
	Michal Hocko <mhocko@suse.com>, Tony Luck <tony.luck@intel.com>,
	"Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com>,
	Naoya Horiguchi <naoya.horiguchi@nec.com>,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH v1 6/6] mm/hwpoison: fix unpoison_memory()
Date: Thu, 17 Jun 2021 18:00:21 +0800	[thread overview]
Message-ID: <94984984-f123-85ae-20bc-b40e90d536f6@sangfor.com.cn> (raw)
In-Reply-To: <20210614021212.223326-7-nao.horiguchi@gmail.com>

On 2021/6/14 10:12, Naoya Horiguchi wrote:
> From: Naoya Horiguchi <naoya.horiguchi@nec.com>
> 
> After recent soft-offline rework, error pages can be taken off from
> buddy allocator, but the existing unpoison_memory() does not properly
> undo the operation.  Moreover, due to the recent change on
> __get_hwpoison_page(), get_page_unless_zero() is hardly called for
> hwpoisoned pages.  So __get_hwpoison_page() mostly returns zero (meaning
> to fail to grab page refcount) and unpoison just clears PG_hwpoison
> without releasing a refcount.  That does not lead to a critical issue
> like kernel panic, but unpoisoned pages never get back to buddy (leaked
> permanently), which is not good.

As I mention in [1], I'm not sure about the exactly meaning of "broken" 
in unpoison_memory().

Maybe the misunderstanding is:

I think __get_hwpoison_page() mostly returns one for hwpoisoned page.
In 06be6ff3d2ec ("mm,hwpoison: rework soft offline for free pages"), 
page_handle_poison() is introduced, it will add refcount for all 
soft-offlineed hwpoison page.
In memory_failure() for hard-offline，page_ref_inc() called on free page 
too, and for used page, we do not call put_page() after 
get_hwpoison_page() != 0.
So all hwpoisoned page refcount must be great than zero when 
unpoison_memory() if regardless of racy.

Recently I tested loop soft-offline random pages and unpoison them for 
days, it works fine to me. (with bac9c6fa1f92 patched)

[1]: 
https://lore.kernel.org/lkml/6af291a0-41fa-8112-5297-6a4cdf2337b6@sangfor.com.cn/

> 
> To fix this, we need to identify "taken off" pages from other types of
> hwpoisoned pages.  We can't use refcount or page flags for this purpose,
> so a pseudo flag is defined by hacking ->private field.
> 
> Sometimes hwpoisoned pages can be still in-use, where the refcount should
> be more than 1, so we can't unpoison them immediately and need to wait
> until the all users release their refcount.
> 
> Signed-off-by: Naoya Horiguchi <naoya.horiguchi@nec.com>
> ---

-- 
Thanks,
- Ding Hui