Re: [PATCH 2/2] mm: compaction: fix the possible deadlock when isolating hugetlb pages

[Baolin Wang <baolin.wang@linux.alibaba.com>]

On 3/14/2023 1:08 AM, Mike Kravetz wrote:
> On 03/13/23 18:37, Baolin Wang wrote:
>> When trying to isolate a migratable pageblock, it can contain several
>> normal pages or several hugetlb pages (e.g. CONT-PTE 64K hugetlb on arm64)
>> in a pageblock. That means we may hold the lru lock of a normal page to
>> continue to isolate the next hugetlb page by isolate_or_dissolve_huge_page()
>> in the same migratable pageblock.
>>
>> However in the isolate_or_dissolve_huge_page(), it may allocate a new hugetlb
>> page and dissolve the old one by alloc_and_dissolve_hugetlb_folio() if the
>> hugetlb's refcount is zero. That means we can still enter the direct compaction
>> path to allocate a new hugetlb page under the current lru lock, which
>> may cause possible deadlock.
>>
>> To avoid this possible deadlock, we should release the lru lock when trying
>> to isolate a hugetbl page. Moreover it does not make sense to take the lru
>> lock to isolate a hugetlb, which is not in the lru list.
>>
>> Fixes: 369fa227c219 ("mm: make alloc_contig_range handle free hugetlb pages")
>> Signed-off-by: Baolin Wang <baolin.wang@linux.alibaba.com>
>> ---
>>   mm/compaction.c | 5 +++++
>>   1 file changed, 5 insertions(+)
>>
>> diff --git a/mm/compaction.c b/mm/compaction.c
>> index c9d9ad958e2a..ac8ff152421a 100644
>> --- a/mm/compaction.c
>> +++ b/mm/compaction.c
> 
> Thanks!
> 
> I suspect holding the lru lock when calling isolate_or_dissolve_huge_page was
> not considered.  However, I wonder if this can really happen in practice?
> 
> Before the code below, there is this:
> 
> 		/*
> 		 * Periodically drop the lock (if held) regardless of its
> 		 * contention, to give chance to IRQs. Abort completely if
> 		 * a fatal signal is pending.
> 		 */
> 		if (!(low_pfn % COMPACT_CLUSTER_MAX)) {
> 			if (locked) {
> 				unlock_page_lruvec_irqrestore(locked, flags);
> 				locked = NULL;
> 			}
> 			...
> 		}
> 
> It would seem that the pfn of a hugetlb page would always be a multiple of
> COMPACT_CLUSTER_MAX so we would drop the lock.  However, I am not sure if
> that is ALWAYS true and would prefer something like the code you suggested.

Well, this is not always true, suppose the CONT-PTE hugetlb on ARM arch, 
which contains 16 contiguous normal pages.

> Did you actually see this deadlock in practice?

I did not see this issue in practice until now, but I think it can be 
triggered from code inspection if trying to isolate a CONT-PTE hugetlb.