Re: [PATCH] mm: migrate: Fix THP's mapcount on isolation

[Gavin Shan <gshan@redhat.com>]

On 11/24/22 4:46 PM, David Hildenbrand wrote:
> On 24.11.22 01:14, Gavin Shan wrote:
>> On 11/23/22 4:56 PM, David Hildenbrand wrote:
>>> On 23.11.22 06:14, Hugh Dickins wrote:
>>>> On Wed, 23 Nov 2022, Gavin Shan wrote:
>>>>
>>>>> The issue is reported when removing memory through virtio_mem device.
>>>>> The transparent huge page, experienced copy-on-write fault, is wrongly
>>>>> regarded as pinned. The transparent huge page is escaped from being
>>>>> isolated in isolate_migratepages_block(). The transparent huge page
>>>>> can't be migrated and the corresponding memory block can't be put
>>>>> into offline state.
>>>>>
>>>>> Fix it by replacing page_mapcount() with total_mapcount(). With this,
>>>>> the transparent huge page can be isolated and migrated, and the memory
>>>>> block can be put into offline state.
>>>>>
>>>>> Fixes: 3917c80280c9 ("thp: change CoW semantics for anon-THP")
>>>>> Cc: stable@vger.kernel.org   # v5.8+
>>>>> Reported-by: Zhenyu Zhang <zhenyzha@redhat.com>
>>>>> Suggested-by: David Hildenbrand <david@redhat.com>
>>>>> Signed-off-by: Gavin Shan <gshan@redhat.com>
>>>>
>>>> Interesting, good catch, looked right to me: except for the Fixes line
>>>> and mention of v5.8.  That CoW change may have added a case which easily
>>>> demonstrates the problem, but it would have been the wrong test on a THP
>>>> for long before then - but only in v5.7 were compound pages allowed
>>>> through at all to reach that test, so I think it should be
>>>>
>>>> Fixes: 1da2f328fa64 ("mm,thp,compaction,cma: allow THP migration for CMA allocations")
>>>> Cc: stable@vger.kernel.org   # v5.7+
>>>>
>>
>> Right, commit 1da2f328fa64 looks more accurate in this particular
>> case, I will fix it up in next revision.
>>
>>>> Oh, no, stop: this is not so easy, even in the latest tree.
>>>>
>>>> Because at the time of that "admittedly racy check", we have no hold
>>>> at all on the page in question: and if it's PageLRU or PageCompound
>>>> at one instant, it may be different the next instant.  Which leaves it
>>>> vulnerable to whatever BUG_ON()s there may be in the total_mapcount()
>>>> path - needs research.  *Perhaps* there are no more BUG_ON()s in the
>>>> total_mapcount() path than in the existing page_mapcount() path.
>>>>
>>>> I suspect that for this to be safe (before your patch and more so after),
>>>> it will be necessary to shift the "admittedly racy check" down after the
>>>> get_page_unless_zero() (and check the sequence of operations when a
>>>> compound page is initialized).
>>>
>>> Grabbing a reference first sounds like the right approach to me.
>>>
>>
>> Yeah, it sounds reasonable to me to grab a page->__refcount in the
>> first place. Looking at isolate_migratepages_block(), the page's refcount
>> is increased by get_page_unless_zero(), but it's too late. To increase
>> the page's refcount at the first place in the function will be conflicting
>> with hugetlb page and non-LRU page. I mean there will be a series to refactor
>> the code so that the page's refcount can be grabbed in the first place.
>>
>> So I plan to post a followup series to refactor the code and grab
>> the page's refcount in the first place. In this way, the fix can be
>> merged as soon as possible. David and Hugh, please let me know if
>> it's reasonable plan? :)
> 
> 
> Can't you just temporarily grab the refcount and drop it again? I mean, it's all racy either way and the code has to be able to cope with such races.
> 

Well, we can do this by moving the hunk of code, which increases page's
refcount, ahead of the check.


   if (unlikely(!get_page_unless_zero(page)))
       goto isolate_fail;

   if (!mapping && (page_count(page) - 1) > total_mapcount(page))
       goto isolate_fail_put;

Thanks,
Gavin