Date:   Mon, 6 Jun 2022 11:12:08 +0800
From:   Miaohe Lin <linmiaohe@...wei.com>
To:     HORIGUCHI NAOYA(堀口 直也) 
        <naoya.horiguchi@....com>
CC:     Mike Kravetz <mike.kravetz@...cle.com>,
        Naoya Horiguchi <naoya.horiguchi@...ux.dev>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        zhenwei pi <pizhenwei@...edance.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v1] mm,hwpoison: set PG_hwpoison for busy hugetlb pages

On 2022/6/2 14:12, HORIGUCHI NAOYA(堀口 直也) wrote:
> On Thu, May 12, 2022 at 07:18:51PM +0800, Miaohe Lin wrote:
>> On 2022/5/12 12:32, HORIGUCHI NAOYA(堀口 直也) wrote:
>>> On Wed, May 11, 2022 at 11:35:55AM -0700, Mike Kravetz wrote:
>>>> On 5/11/22 08:19, Naoya Horiguchi wrote:
>>>>> From: Naoya Horiguchi <naoya.horiguchi@....com>
>>>>>
>>>>> If memory_failure() fails to grab page refcount on a hugetlb page
>>>>> because it's busy, it returns without setting PG_hwpoison on it.
>>>>> This not only loses a chance of error containment, but breaks the rule
>>>>> that action_result() should be called only when memory_failure() do
>>>>> any of handling work (even if that's just setting PG_hwpoison).
>>>>> This inconsistency could harm code maintainability.
>>>>>
>>>>> So set PG_hwpoison and call hugetlb_set_page_hwpoison() for such a case.
>>>>>
>>>>> Fixes: 405ce051236c ("mm/hwpoison: fix race between hugetlb free/demotion and memory_failure_hugetlb()")
>>>>> Signed-off-by: Naoya Horiguchi <naoya.horiguchi@....com>
>>>>> ---
>>>>>  include/linux/mm.h  | 1 +
>>>>>  mm/memory-failure.c | 8 ++++----
>>>>>  2 files changed, 5 insertions(+), 4 deletions(-)
>>>>>
>>>>> diff --git a/include/linux/mm.h b/include/linux/mm.h
>>>>> index d446e834a3e5..04de0c3e4f9f 100644
>>>>> --- a/include/linux/mm.h
>>>>> +++ b/include/linux/mm.h
>>>>> @@ -3187,6 +3187,7 @@ enum mf_flags {
>>>>>  	MF_MUST_KILL = 1 << 2,
>>>>>  	MF_SOFT_OFFLINE = 1 << 3,
>>>>>  	MF_UNPOISON = 1 << 4,
>>>>> +	MF_NO_RETRY = 1 << 5,
>>>>>  };
>>>>>  extern int memory_failure(unsigned long pfn, int flags);
>>>>>  extern void memory_failure_queue(unsigned long pfn, int flags);
>>>>> diff --git a/mm/memory-failure.c b/mm/memory-failure.c
>>>>> index 6a28d020a4da..e3269b991016 100644
>>>>> --- a/mm/memory-failure.c
>>>>> +++ b/mm/memory-failure.c
>>>>> @@ -1526,7 +1526,8 @@ int __get_huge_page_for_hwpoison(unsigned long pfn, int flags)
>>>>>  			count_increased = true;
>>>>>  	} else {
>>>>>  		ret = -EBUSY;
>>>>> -		goto out;
>>>>> +		if (!(flags & MF_NO_RETRY))
>>>>> +			goto out;
>>>>>  	}
>>>>
>>>> Hi Naoya,
>>>>
>>>> We are in the else block because !HPageFreed() and !HPageMigratable().
>>>> IIUC, this likely means the page is isolated.  One common reason for isolation
>>>> is migration.  So, the page could be isolated and on a list for migration.
>>>
>>> Yes, and I also detected this issue by testing race between hugepage allocation
>>> and memory_failure(). 
>>>
>>>>
>>>> I took a quick look at the hugetlb migration code and did not see any checks
>>>> for PageHWPoison after a hugetlb page is isolated.  I could have missed
>>>> something?  If there are no checks, we will read the PageHWPoison page
>>>> in kernel mode while copying to the migration target.
>>>
>>> Yes, that could happen.  This patch does not affect ongoing hugepage migration.
>>> But after the migration source hugepage is freed, the PG_hwpoison should work
>>> to prevent reusing.
>>>
>>>>
>>>> Is this an issue?  Is is something we need to be concerned with?  Memory
>>>> errors can happen at any time, and gracefully handling them is best effort.
>>>
>>> Right, so doing nothing for this case could be OK if doing something causes
>>> some issues or makes code too complicated.  The motivation of this patch is
>>> that now I think memory_failure() should do something (at least setting
>>> PG_hwpoison) unless the page is already hwpoisoned or rejected by
>>> hwpoison_filter(), because of the effect after free as mentioned above.
>>>
>>> This is also expected in other case too. For example, slab is a unhandlable
>>> type of page, but we do set PG_hwpoison.  This flag should not affect any of
>>> ongoing slab-related process, but that's OK because it becomes effective
>>> after the slab page is freed.
>>>
>>> So this patch is intended to align to the behavior.  Allowing hugepage
>>> migration to do something good using PG_hwpoison seems to me an unsolved
>>> separate issue.
>>
>> I tend to agree with Naoya. And could we try to do it better? IMHO, we could do a
>> get_page_unless_zero here to ensure that hugetlb page migration will fail due to
>> this extra page reference and thus preventing the page content from being accessed.
>> Does this work? Or am I miss something?
> 
> Sorry for my missing to answering the question,

Never mind. I almost forget it too. ;)

> 
> Taking page refcount to prevent page migration could work.  One concern is
> how we can distinguish hugepages under migration and those under allocation
> from buddy.  Maybe this was also mentioned in discussion over 
> https://github.com/torvalds/linux/commit/405ce051236cc65b30bbfe490b28ce60ae6aed85
> , there's a small window where an allocating compound page is refcounted and
> hugepage (having deconstructor COMPOUND_PAGE_DTOR), and not protected by
> hugetlb_lock, so simply get_page_unless_zero() might not work (might break
> allocation code).
> If we have more reliable indicator to ensure that a hugepage is under migration,
> that would be helpful.

Yes, I agree with you. If we have more reliable indicator to ensure that a hugepage
is under migration, we could try to do this then. :)

> 
> Thanks,
> Naoya Horiguchi

Many thanks for reply!

> 

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