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

[Mike Kravetz]

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@xxxxxxxxxxxxxxxxx>
> ---
>  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.

Did you actually see this deadlock in practice?
-- 
Mike Kravetz

> @@ -893,6 +893,11 @@ isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn,
>  		}
>  
>  		if (PageHuge(page) && cc->alloc_contig) {
> +			if (locked) {
> +				unlock_page_lruvec_irqrestore(locked, flags);
> +				locked = NULL;
> +			}
> +
>  			ret = isolate_or_dissolve_huge_page(page, &cc->migratepages);
>  
>  			/*
> -- 
> 2.27.0
> 
>