Re: [PATCH] mm/compaction: Fix bug in hugetlb handling pathway

Oscar Salvador
2025-04-01T14:59:49Z

On Mon, Mar 31, 2025 at 07:10:24PM -0700, Vishal Moola (Oracle) wrote:
> The compaction code doesn't take references on pages until we're certain
> we should attempt to handle it.
>
> In the hugetlb case, isolate_or_dissolve_huge_page() may return -EBUSY
> without taking a reference to the folio associated with our pfn. If our
> folio's refcount drops to 0, compound_nr() becomes unpredictable, making
> low_pfn and nr_scanned unreliable.
> The user-visible effect is minimal - this should rarely happen (if ever).

So, with compound_order() we either return the real order of the
compound page or '0', right?

> Fix this by storing the folio statistics earlier on the stack (just like
> the THP and Buddy cases).
>
> Also revert commit 66fe1cf7f581 ("mm: compaction: use helper compound_nr
> in isolate_migratepages_block")
> to make backporting easier.
>
> Fixes: 369fa227c219 ("mm: make alloc_contig_range handle free hugetlb pages")
> Cc: Miaohe Lin <linmiaohe@huawei.com>
> Cc: Oscar Salvador <osalvador@suse.de>
> Signed-off-by: Vishal Moola (Oracle) <vishal.moola@gmail.com>

Acked-by: Oscar Salvador <osalvador@suse.de>

--
Oscar Salvador
SUSE Labs