linux-kernel - Re: [PATCH v2 1/4] mm/huge_memory: change folio_split_supported() to folio_check

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Message-Id: <20251125094446.c78c5139f00ad8e00e9bd5d9@linux-foundation.org>
Date: Tue, 25 Nov 2025 09:44:46 -0800
From: Andrew Morton <akpm@...ux-foundation.org>
To: "David Hildenbrand (Red Hat)" <david@...nel.org>
Cc: Zi Yan <ziy@...dia.com>, Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
 Baolin Wang <baolin.wang@...ux.alibaba.com>, "Liam R. Howlett"
 <Liam.Howlett@...cle.com>, Nico Pache <npache@...hat.com>, Ryan Roberts
 <ryan.roberts@....com>, Dev Jain <dev.jain@....com>, Barry Song
 <baohua@...nel.org>, Lance Yang <lance.yang@...ux.dev>, Miaohe Lin
 <linmiaohe@...wei.com>, Naoya Horiguchi <nao.horiguchi@...il.com>, Wei Yang
 <richard.weiyang@...il.com>, Balbir Singh <balbirs@...dia.com>,
 linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/4] mm/huge_memory: change folio_split_supported()
 to folio_check_splittable()

On Tue, 25 Nov 2025 09:58:03 +0100 "David Hildenbrand (Red Hat)" <david@...nel.org> wrote:

> > + * Return: 0 - @folio can be split to @new_order, otherwise an error number is
> > + * returned.
> > + */
> > +int folio_check_splittable(struct folio *folio, unsigned int new_order,
> > +			   enum split_type split_type, bool warns)
> >   {
> > +	VM_WARN_ON_FOLIO(!folio_test_locked(folio), folio);
> > +	/*
> > +	 * Folios that just got truncated cannot get split. Signal to the
> > +	 * caller that there was a race.
> > +	 *
> > +	 * TODO: this will also currently refuse shmem folios that are in the
> > +	 * swapcache.
> > +	 */
> 
> Per the other discussion, should this even be:
> 
> "this will also currently refuse folios without a mapping in the 
> swapcache (shmem or to-be-anon folios)"
> 
> IOW, to spell out that anon folios that were read into the swapcache but 
> not mapped yet into page tables (where we set folio->mapping).

This?

--- a/mm/huge_memory.c~mm-huge_memory-change-folio_split_supported-to-folio_check_splittable-fix
+++ a/mm/huge_memory.c
@@ -3714,7 +3714,8 @@ int folio_check_splittable(struct folio
 	 * caller that there was a race.
 	 *
 	 * TODO: this will also currently refuse shmem folios that are in the
-	 * swapcache.
+	 * swapcache.  Currently it will also refuse folios without a mapping
+	 * in the swapcache (shmem or to-be-anon folios).
 	 */
 	if (!folio_test_anon(folio) && !folio->mapping)
 		return -EBUSY;
_