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

[Andrew Morton]

On Mon, 13 Mar 2023 10:08:38 -0700 Mike Kravetz <mike.kravetz@xxxxxxxxxx> wrote:

> 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?

Presumably the lack of lockdep reports about this tells us something?