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

[David Hildenbrand <david@redhat.com>]

On 24.11.22 04:33, Matthew Wilcox wrote:
> On Thu, Nov 24, 2022 at 12:06:56PM +1100, Alistair Popple wrote:
>>
>> David Hildenbrand <david@redhat.com> writes:
>>
>>> 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+
>>>> 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.
>>
>> I think you're right. Without a page reference I don't think it is even
>> safe to look at struct page, at least not without synchronisation
>> against memory hot unplug which could remove the struct page. From a
>> quick glance I didn't see anything here that obviously did that though.
> 
> Memory hotplug is the offending party here.  It has to make sure that
> everything else is definitely quiescent before removing the struct pages.
> Otherwise you can't even try_get a refcount.

At least alloc_contig_range() and memory offlining are mutually 
exclusive due to MIGRATE_ISOLTAE. I recall that ordinary memory 
compaction similarly deals with isolated pageblocks (or some other 
mechanism I forgot) to not race with memory offlining. Wouldn't worry 
about that for now.

-- 
Thanks,

David / dhildenb