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

[David Hildenbrand <david@redhat.com>]

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.

-- 
Thanks,

David / dhildenb