+ mm-util-dont-read-__page_2-for-order-1-folios-in-snapshot_page.patch added to mm-hotfixes-unstable branch

+ mm-util-dont-read-__page_2-for-order-1-folios-in-snapshot_page.patch added to mm-hotfixes-unstable branch

[Andrew Morton]

The patch titled
     Subject: mm/util: don't read __page_2 for order-1 folios in snapshot_page()
has been added to the -mm mm-hotfixes-unstable branch.  Its filename is
     mm-util-dont-read-__page_2-for-order-1-folios-in-snapshot_page.patch

This patch will shortly appear at
     https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/mm-util-dont-read-__page_2-for-order-1-folios-in-snapshot_page.patch

This patch will later appear in the mm-hotfixes-unstable branch at
    git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

Before you just go and hit "reply", please:
   a) Consider who else should be cc'ed
   b) Prefer to cc a suitable mailing list as well
   c) Ideally: find the original patch on the mailing list and do a
      reply-to-all to that, adding suitable additional cc's

*** Remember to use Documentation/process/submit-checklist.rst when testing your code ***

The -mm tree is included into linux-next via various
branches at git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm
and is updated there most days

------------------------------------------------------
From: Aboorva Devarajan <aboorvad@linux.ibm.com>
Subject: mm/util: don't read __page_2 for order-1 folios in snapshot_page()
Date: Thu, 9 Jul 2026 01:49:54 +0530

snapshot_page() currently reads __page_2 after checking nr_pages > 1, but
it should only do so when nr_pages > 2.

If an order-1 folio is allocated at the end of a vmemmap section,
__page_2 will not exist and reading it will cause a fault.

During DLPAR memory remove on a 22 TB ppc64le LPAR, snapshot_page() oopsed
on the page isolation path while reading an order-1 folio's __page_2 from
an adjacent absent section (unmapped vmemmap).

Fix this to avoid reading memmap that doesn't exist (e.g., a vmemmap
hole).

Link: https://lore.kernel.org/20260708201954.686111-1-aboorvad@linux.ibm.com
Fixes: 31a31da8a618 ("mm: move _pincount in folio to page[2] on 32bit")
Signed-off-by: Aboorva Devarajan <aboorvad@linux.ibm.com>
Reported-by: Sourabh Jain <sourabhjain@linux.ibm.com>
Acked-by: David Hildenbrand (Arm) <david@kernel.org>
Reviewed-by: Lorenzo Stoakes <ljs@kernel.org>
Reviewed-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Liam R. Howlett <liam@infradead.org>
Cc: Luiz Capitulino <luizcap@redhat.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: "Ritesh Harjani (IBM)" <ritesh.list@gmail.com>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Vlastimil Babka <vbabka@kernel.org>
Cc: <stable@vger.kernel.org> # v6.15+
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 mm/util.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- a/mm/util.c~mm-util-dont-read-__page_2-for-order-1-folios-in-snapshot_page
+++ a/mm/util.c
@@ -1353,7 +1353,7 @@ again:
 	if (ps->idx < MAX_FOLIO_NR_PAGES) {
 		memcpy(&ps->folio_snapshot, foliop, 2 * sizeof(struct page));
 		nr_pages = folio_nr_pages(&ps->folio_snapshot);
-		if (nr_pages > 1)
+		if (nr_pages > 2)
 			memcpy(&ps->folio_snapshot.__page_2, &foliop->__page_2,
 			       sizeof(struct page));
 		set_ps_flags(ps, foliop, page);
_

Patches currently in -mm which might be from aboorvad@linux.ibm.com are

mm-util-dont-read-__page_2-for-order-1-folios-in-snapshot_page.patch

Re: + mm-util-dont-read-__page_2-for-order-1-folios-in-snapshot_page.patch added to mm-hotfixes-unstable branch

[Matthew Wilcox]

On Wed, Jul 08, 2026 at 01:56:52PM -0700, Andrew Morton wrote:
> From: Aboorva Devarajan <aboorvad@linux.ibm.com>
> Subject: mm/util: don't read __page_2 for order-1 folios in snapshot_page()
> Date: Thu, 9 Jul 2026 01:49:54 +0530
> 
> snapshot_page() currently reads __page_2 after checking nr_pages > 1, but
> it should only do so when nr_pages > 2.
> 
> If an order-1 folio is allocated at the end of a vmemmap section,
> __page_2 will not exist and reading it will cause a fault.
> 
> During DLPAR memory remove on a 22 TB ppc64le LPAR, snapshot_page() oopsed
> on the page isolation path while reading an order-1 folio's __page_2 from
> an adjacent absent section (unmapped vmemmap).
> 
> Fix this to avoid reading memmap that doesn't exist (e.g., a vmemmap
> hole).
> 
> Link: https://lore.kernel.org/20260708201954.686111-1-aboorvad@linux.ibm.com
> Fixes: 31a31da8a618 ("mm: move _pincount in folio to page[2] on 32bit")
> Signed-off-by: Aboorva Devarajan <aboorvad@linux.ibm.com>
> Reported-by: Sourabh Jain <sourabhjain@linux.ibm.com>
> Acked-by: David Hildenbrand (Arm) <david@kernel.org>
> Reviewed-by: Lorenzo Stoakes <ljs@kernel.org>
> Reviewed-by: Matthew Wilcox (Oracle) <willy@infradead.org>

You're taking my R-b without taking my rewording of the second
paragraph?

Re: + mm-util-dont-read-__page_2-for-order-1-folios-in-snapshot_page.patch added to mm-hotfixes-unstable branch

[Andrew Morton]

On Wed, 8 Jul 2026 23:06:42 +0100 Matthew Wilcox <willy@infradead.org> wrote:

> On Wed, Jul 08, 2026 at 01:56:52PM -0700, Andrew Morton wrote:
> > From: Aboorva Devarajan <aboorvad@linux.ibm.com>
> > Subject: mm/util: don't read __page_2 for order-1 folios in snapshot_page()
> > Date: Thu, 9 Jul 2026 01:49:54 +0530
> > 
> > snapshot_page() currently reads __page_2 after checking nr_pages > 1, but
> > it should only do so when nr_pages > 2.
> > 
> > If an order-1 folio is allocated at the end of a vmemmap section,
> > __page_2 will not exist and reading it will cause a fault.
> > 
> > During DLPAR memory remove on a 22 TB ppc64le LPAR, snapshot_page() oopsed
> > on the page isolation path while reading an order-1 folio's __page_2 from
> > an adjacent absent section (unmapped vmemmap).
> > 
> > Fix this to avoid reading memmap that doesn't exist (e.g., a vmemmap
> > hole).
> > 
> > Link: https://lore.kernel.org/20260708201954.686111-1-aboorvad@linux.ibm.com
> > Fixes: 31a31da8a618 ("mm: move _pincount in folio to page[2] on 32bit")
> > Signed-off-by: Aboorva Devarajan <aboorvad@linux.ibm.com>
> > Reported-by: Sourabh Jain <sourabhjain@linux.ibm.com>
> > Acked-by: David Hildenbrand (Arm) <david@kernel.org>
> > Reviewed-by: Lorenzo Stoakes <ljs@kernel.org>
> > Reviewed-by: Matthew Wilcox (Oracle) <willy@infradead.org>
> 
> You're taking my R-b without taking my rewording of the second
> paragraph?

I added your sentence.  Aboorva's info was potentially useful, absent a
Link: to a report,