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
>
>
