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

[Ding Hui <dinghui@sangfor.com.cn>]

On 2021/6/18 4:36 下午, HORIGUCHI NAOYA(堀口 直也) wrote:
> On Thu, Jun 17, 2021 at 06:00:21PM +0800, Ding Hui wrote:
>> 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.
> 
> Hi, Ding,
> 
> Thanks for the comment.  I feel that I failed to define the exact issue in
> unpoison.  Maybe I saw and misinterpreted some random error as unpoison's
> issue during developing other hwpoison patches, so please don't take serious
> my previous wrong word "broken", sorry about that.
> 
> Anyway I reconsider how to handle this 6/6, maybe it will be a clear
> description of the problem, and will be simplified.
> 
>>
>> Recently I tested loop soft-offline random pages and unpoison them for days,
>> it works fine to me. (with bac9c6fa1f92 patched)
> 
> Thank you for testing,
> 

Hi Naoya,

I'm afraid of my description about testing is ambiguous for others, let 
me clarify that I ran stress soft-offline test case from mce-test 
project (https://git.kernel.org/pub/scm/utils/cpu/mce/mce-test.git) for 
days to verify my modify about NR_FREE_PAGES (bac9c6fa1f92), without 
your current patchset, the case is loop soft-offline random pages and 
unpoison them, and it works basic fine to me.

-- 
Thanks,
-dinghui