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Wed, 6 May 2026 04:18:00 +0000 (UTC) Received: from dovecot-director2.suse.de ([2a07:de40:b281:106:10:150:64:167]) by imap1.dmz-prg2.suse.org with ESMTPSA id +Z09N/jA+mmlIgAAD6G6ig (envelope-from ); Wed, 06 May 2026 04:18:00 +0000 Date: Wed, 6 May 2026 06:17:59 +0200 From: Oscar Salvador To: "David Hildenbrand (Arm)" Cc: Andrew Morton , Lorenzo Stoakes , "Liam R. Howlett" , Vlastimil Babka , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , Hugh Dickins , Lance Yang , linux-mm@kvack.org, linux-kernel@vger.kernel.org, Bibo Mao , stable@vger.kernel.org Subject: Re: [PATCH] mm: fix __vm_normal_page() to handle missing support for pmd_special()/pud_special() Message-ID: References: <20260430-pmd_special-v1-1-dbcbcfd72c20@kernel.org> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260430-pmd_special-v1-1-dbcbcfd72c20@kernel.org> X-Spam-Flag: NO X-Spam-Score: -4.30 X-Spamd-Result: default: False [-4.30 / 50.00]; BAYES_HAM(-3.00)[100.00%]; NEURAL_HAM_LONG(-1.00)[-1.000]; NEURAL_HAM_SHORT(-0.20)[-1.000]; MIME_GOOD(-0.10)[text/plain]; RCVD_VIA_SMTP_AUTH(0.00)[]; ARC_NA(0.00)[]; MIME_TRACE(0.00)[0:+]; MISSING_XM_UA(0.00)[]; RCPT_COUNT_TWELVE(0.00)[14]; FUZZY_RATELIMITED(0.00)[rspamd.com]; RCVD_TLS_ALL(0.00)[]; DKIM_SIGNED(0.00)[suse.de:s=susede2_rsa,suse.de:s=susede2_ed25519]; FROM_HAS_DN(0.00)[]; TO_DN_SOME(0.00)[]; FROM_EQ_ENVFROM(0.00)[]; TO_MATCH_ENVRCPT_ALL(0.00)[]; RCVD_COUNT_TWO(0.00)[2]; DBL_BLOCKED_OPENRESOLVER(0.00)[imap1.dmz-prg2.suse.org:helo,suse.de:email,linux.dev:email] X-Spam-Level: On Thu, Apr 30, 2026 at 01:31:22PM +0200, David Hildenbrand (Arm) wrote: > On x86 32-bit with THP enabled, zap_huge_pmd() is seen to generate a > "WARNING: mm/memory.c:735 at __vm_normal_page+0x6a/0x7d", from the > VM_WARN_ON_ONCE(is_zero_pfn(pfn) || is_huge_zero_pfn(pfn)); followed > by "BUG: Bad rss-counter state"s, then later "BUG: Bad page state"s > when reclaim gets to call shrink_huge_zero_folio_scan(). > > It's as if the _PAGE_SPECIAL bit never got set in the huge_zero pmd: > and indeed, whereas pte_special() and pte_mkspecial() are subject to a > dedicated CONFIG_ARCH_HAS_PTE_SPECIAL, pmd_special() and pmd_mkspecial() > are subject to CONFIG_ARCH_SUPPORTS_PMD_PFNMAP, which is never enabled > on any 32-bit architecture. > > While the problem was exposed through commit d80a9cb1a64a ("mm/huge_memory: > add and use normal_or_softleaf_folio_pmd()"), it was an oversight in commit > af38538801c6 ("mm/memory: factor out common code from vm_normal_page_*()") > and would result in other problems: > * huge zero folio accounted in smaps, pagemap (PAGE_IS_FILE) and > numamaps as file-backed THP > * folio_walk_start() returning the folio even without FW_ZEROPAGE set. > Callers seem to tolerate that, though. > > ... and triggering the VM_WARN_ON_ONE(), although never reported so far. > > To fix it, teach vm_normal_page_pmd()/vm_normal_page_pud() to consider > whether pmd_special/pud_special is actually implemented. > > Fixes: af38538801c6 ("mm/memory: factor out common code from vm_normal_page_*()") > Reported-by: Hugh Dickins > Closes: https://lore.kernel.org/r/74a75b59-2e13-3985-ee99-d5521f39df2a@google.com > Reported-by: Bibo Mao > Closes: https://lore.kernel.org/r/20260430041121.2839350-1-maobibo@loongson.cn > Debugged-by: Hugh Dickins > Reviewed-by: Lance Yang > Tested-by: Bibo Mao > Cc: stable@vger.kernel.org > Signed-off-by: David Hildenbrand (Arm) Reviewed-by: Oscar Salvador -- Oscar Salvador SUSE Labs