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

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Message-ID: <Z-wor4KyJ-var3kp@fedora>
Date: Tue, 1 Apr 2025 10:55:59 -0700
From: "Vishal Moola (Oracle)" <vishal.moola@...il.com>
To: Oscar Salvador <osalvador@...e.de>
Cc: linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	akpm@...ux-foundation.org, muchun.song@...ux.dev,
	Miaohe Lin <linmiaohe@...wei.com>
Subject: Re: [PATCH] mm/compaction: Fix bug in hugetlb handling pathway

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@...wei.com>
> > Cc: Oscar Salvador <osalvador@...e.de>
> > Signed-off-by: Vishal Moola (Oracle) <vishal.moola@...il.com>
> 
> Acked-by: Oscar Salvador <osalvador@...e.de>
> 
> 
> -- 
> Oscar Salvador
> SUSE Labs