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[70.49.81.60]) by smtp.gmail.com with ESMTPSA id 6a1803df08f44-8ffd56c4cd1sm173212066d6.16.2026.07.14.10.57.01 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 14 Jul 2026 10:57:01 -0700 (PDT) Message-ID: <24432756-29fb-46e8-8d7b-de6e724da958@redhat.com> Date: Tue, 14 Jul 2026 13:57:01 -0400 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2] mm/util: don't read __page_2 for order-1 folios in snapshot_page() To: Matthew Wilcox , Aboorva Devarajan Cc: Andrew Morton , David Hildenbrand , Lorenzo Stoakes , "Liam R . Howlett" , Vlastimil Babka , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , Sourabh Jain , Ritesh Harjani , linux-mm@kvack.org, linux-kernel@vger.kernel.org, stable@vger.kernel.org References: <20260708201954.686111-1-aboorvad@linux.ibm.com> Content-Language: en-US, en-CA From: Luiz Capitulino In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 2026-07-08 16:31, Matthew Wilcox wrote: > On Thu, Jul 09, 2026 at 01:49:54AM +0530, Aboorva Devarajan wrote: >> snapshot_page() currently reads __page_2 after checking nr_pages > 1, >> but it should only do so when nr_pages > 2. >> >> 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). > > I appreciate you're absolutely swimming in it, but there's absolutely > no need to inflict IBM terminology on the rest of us ;-) > That second paragraph could simply be: > > 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. I think having a brief description on how the issue was originally reproduced can be useful for determining impact and validating backports. > > Reviewed-by: Matthew Wilcox (Oracle) > >> Fixes: 31a31da8a618 ("mm: move _pincount in folio to page[2] on 32bit") >> Cc: stable@vger.kernel.org # v6.15+ >> Reported-by: Sourabh Jain >> Acked-by: David Hildenbrand (Arm) >> Reviewed-by: Lorenzo Stoakes >> Signed-off-by: Aboorva Devarajan >> --- >> v1 -> v2: >> - Condense the commit message. >> - Drop the code comment. >> >> mm/util.c | 2 +- >> 1 file changed, 1 insertion(+), 1 deletion(-) >> >> diff --git a/mm/util.c b/mm/util.c >> index af2c2103f0d95..34cb43b3eaa4c 100644 >> --- a/mm/util.c >> +++ b/mm/util.c >> @@ -1353,7 +1353,7 @@ void snapshot_page(struct page_snapshot *ps, const struct page *page) >> 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); >> -- >> 2.54.0 >> >> >