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

[Andrew Morton <akpm@linux-foundation.org>]

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,