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

[Vishal Moola (Oracle)]

On Tue, Apr 01, 2025 at 04:59:46PM +0200, Oscar Salvador wrote:
> 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?

Yup. There's a world in which that folio could be freed and reallocated
as part of another large order page as well (where it would return the
order of that folio).

> > 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@xxxxxxxxxx>
> > Cc: Oscar Salvador <osalvador@xxxxxxx>
> > Signed-off-by: Vishal Moola (Oracle) <vishal.moola@xxxxxxxxx>
> 
> Acked-by: Oscar Salvador <osalvador@xxxxxxx>
> 
> 
> -- 
> Oscar Salvador
> SUSE Labs