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V" , Muchun Song , linux-mm@kvack.org, linuxppc-dev@lists.ozlabs.org, linux-kernel@vger.kernel.org, stable@vger.kernel.org, Muchun Song Subject: Re: [PATCH] powerpc/mm: fix uninitialized ZONE_DEVICE struct pages with PMDs In-Reply-To: <20261029063601.1961415-1-songmuchun@bytedance.com> Date: Thu, 29 Oct 2026 21:15:10 +0530 Message-ID: References: <20261029063601.1961415-1-songmuchun@bytedance.com> X-Rspamd-Server: rspam05 X-Rspamd-Queue-Id: CB9F9A0006 X-Stat-Signature: cgxjhqu37ohhryfgpibwye4jsguhbb7z X-Rspam-User: X-Spam-Status: No, score=-12.8 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,FREEMAIL_FORGED_FROMDOMAIN,FREEMAIL_FROM, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER,INCLUDES_PATCH, MAILING_LIST_MULTI,SPF_HELO_PASS,SPF_PASS autolearn=ham autolearn_force=no version=4.0.1 X-HE-Tag: 1793289558-679070 X-HE-Meta: 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 bEb9do6s 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 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: X-Spam-Checker-Version: SpamAssassin 4.0.1 (2024-03-26) on aws-us-west-2-korg-listserv-alma10-1 X-Archived-By: Muchun Song writes: Thanks Muchun, > PowerPC radix vmemmap population can reuse an existing PMD mapping while > populating a device DAX range whose section is marked as HVO-enabled. The > reused mapping is an ordinary vmemmap mapping: the redundant tail struct > pages are not remapped to the shared tail page. > > Leaving the section marked as HVO-enabled makes later ZONE_DEVICE > initialization treat the range as optimized and initialize only the struct > pages that would be backed by a real HVO mapping. The remaining tail struct > pages can then be left uninitialized even though they are still used as > metadata for the device DAX compound page. > > This first triggers a zone mismatch warning in __init_single_page(), then > faults while initializing the pageblock flags: > > WARNING: mm/mm_init.c:600 at __init_single_page+0xd0/0xf4 > Call Trace: > memmap_init_zone_device+0x164/0x430 > memremap_pages+0x43c/0x93c > devm_memremap_pages+0x3c/0xa4 > dev_dax_probe+0x264/0x3d0 > dax_bus_probe+0x140/0x1c8 > > BUG: Kernel NULL pointer dereference on read at 0x00001400 > __set_pfnblock_flags_mask.constprop.0+0x34/0x160 > memmap_init_zone_device+0x358/0x430 > > On kernels without these initialization-time diagnostics, the > uninitialized metadata remains latent. When userspace later maps and > accesses the affected device DAX range, the kernel operates on those struct > pages and panics at that point instead of during device initialization. > > The Fixes target predates the section-based vmemmap optimization > infrastructure, so older kernels do not have section state that can be > marked non-HVO. Backports to such kernels need to handle the PMD-reuse case > in powerpc radix code directly, for example by initializing the tail struct > pages that core ZONE_DEVICE initialization would otherwise skip. > Actually that would be totally new code right? It won't be a straight forward backport, correct? ... > Mark the section as non-HVO when reusing an existing PMD mapping, so the > section's HVO state matches the actual vmemmap layout and all required > struct pages are initialized. > > Fixes: f2b79c0d7968 ("powerpc/book3s64/radix: add support for vmemmap optimization for radix") > Cc: stable@vger.kernel.org ... then should we mark Cc: stable here with the above mentioned fixes tag? Because this patch cannot be backported as is to older kernels - for the reasons you mentioned above. > Reported-by: Pavithra > Closes: https://lore.kernel.org/r/97f85592bb78a273be8519e6897e7f9b9bd31469.1793070429.git.ritesh.list@gmail.com > Signed-off-by: Muchun Song > Reviewed-by: Ritesh Harjani (IBM) > --- > arch/powerpc/mm/book3s64/radix_pgtable.c | 11 ++++++++++- > 1 file changed, 10 insertions(+), 1 deletion(-) > I think, this patch should go via mm tree, due to all the dependencies coming from there, right? -ritesh