From mboxrd@z Thu Jan 1 00:00:00 1970 Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.remote-ip=95.215.58.197 ARC-Seal: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; cv=none; t=1793329414; b=TNpa/3Cix7mtT2jlnJSAjAu9ZxZL+l6Lb5OquWD3Mz4CXJMYV5Kx1fkA28qfAM56RP9e x27X8zFFeifAGTUfK1K22ZJbGWzKp1+Qkg9uGqsLeh1m4ejSvjAHk7uTeUEuz61iDPO3v 0XA/kVWHbaoVpycd8EL83R1uYNpsIkrlXtix2qBSmE72Ts2M2ypM6KgISWC9q+M7iEx/v AxgeVQ/ZEh9/BSLfNW1it+b7vOSTezFrVQrPcEzLdXde+h/90IpXzb47GoeP/U++DTrnu Z8y0D3c9p01goS7oP7ua9zzid+AMnvICfCRZ39PJqOmLJQQKX4OEsFcCHanKKU7VImw== ARC-Message-Signature: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; c=relaxed/relaxed; t=1793329414; h=DMARC-Filter:X-Envelope-To:DKIM-Signature:X-Envelope-To:Received: X-Mizu-Trace-ID:X-Migadu-Flow:Content-Type:Mime-Version:Subject:From: In-Reply-To:Date:Cc:Content-Transfer-Encoding:Message-Id:References: To:X-Mailer; bh=P1HTdK9Us/nXxv6524NGuZeaCWy7CFdEOV2AIIWt1JQ=; b=CAHjPovUA9oiI1QKp55TOfWavVuzDlSqbI8ewGwgMMXESR0zRqwMIUq0pVjv9prm2eKZ IixsxxQJJH7wzQ1/Y4PW92zcZmiC/nn95zqyGEWZeQegjpJKyXMvl/1mTh15XMMsJjBfs 5yfcDDclRTC48qZ0kc0i+U/241pk5SeTRvFajshN4QDkC6lQ29JiROvJi5/ljgYwc0sOU +oAgFQiH9IR9hPSSGU205OZF8d+zWMwbiZk6ogT4lDshrbvkjifwWRRqFno2ZyFQL0kVb DUrYN8VwYvSnpKgyKghlRsXZis60A7ZcdycNqJiGBFWQL+GA6pB9zlW2E60cBdDqrkA== ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; dmarc=pass header.from=linux.dev; spf=pass smtp.mailfrom=linux.dev; dkim=pass header.d=linux.dev header.i=@linux.dev header.a=rsa-sha256 header.s=key1 header.b=KKaSdtvp; arc=none smtp.remote-ip=95.215.58.197 Received: from mta1.migadu.com (out-197.mta1.migadu.com [95.215.58.197]) (using TLSv1.3 with cipher TLS_AES_128_GCM_SHA256 (128/128 bits) key-exchange X25519 server-signature ECDSA (prime256v1) server-digest SHA256) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 6500B606A31D for ; Fri, 30 Oct 2026 03:03:32 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.4.2 smtp.subspace.kernel.org 6500B606A31D Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=linux.dev Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key, unprotected) header.d=linux.dev header.i=@linux.dev header.a=rsa-sha256 header.s=key1 header.b=KKaSdtvp X-Envelope-To: linux-kernel@vger.kernel.org DKIM-Signature: a=rsa-sha256; bh=A7GiL3udSSdyFKnyh+0Vow6D9krgem3lJz6Fq4u8pT4=; c=simple/simple; d=linux.dev; h=from:to:subject:date:message-id:mime-version:content-type; s=key1; t=1793329411; v=1; x=1793934211; b=KKaSdtvpu7txiAPoaxO8Yi6oAg91UieXpbfe9qIH3UNe/A6K+26PUE7c87i/lLbsBJfJcwDy HnXgdIK9fg6F+VRmcJTj7SAJUXDJfeOGWvmjFSXMhkmUpcqbaQFxIPp4jFyCXa5y/2/JK3WWnBO 6f5UQ7hbWl/4DFYCEZ9Mr+n4= X-Envelope-To: linux-kernel@vger.kernel.org Received: by mta11.migadu.com with ESMTPS id 70512d1ee8cb6ea0; Fri, 30 Oct 2026 03:03:31 +0000 X-Mizu-Trace-ID: 70512d1ee8cb6ea0 X-Migadu-Flow: FLOW_OUT Content-Type: text/plain; charset=us-ascii X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list Mime-Version: 1.0 (Mac OS X Mail 16.0 \(3901.100.1.1.12\)) Subject: Re: [PATCH] powerpc/mm: fix uninitialized ZONE_DEVICE struct pages with PMDs From: Muchun Song In-Reply-To: Date: Fri, 30 Oct 2026 11:03:08 +0800 Cc: Muchun Song , Madhavan Srinivasan , Mike Rapoport , David Hildenbrand , Andrew Morton , Michael Ellerman , Nicholas Piggin , Christophe Leroy , Venkat Rao Bagalkote , Shrikanth Hegde , Pavithra , "Aneesh Kumar K . V" , linux-mm@kvack.org, linuxppc-dev@lists.ozlabs.org, linux-kernel@vger.kernel.org, stable@vger.kernel.org Content-Transfer-Encoding: quoted-printable Message-Id: <20FE1232-A48B-4F72-8F18-89EFF70EE0FE@linux.dev> References: <20261029063601.1961415-1-songmuchun@bytedance.com> To: "Ritesh Harjani (IBM)" X-Mailer: Apple Mail (2.3901.100.1.1.12) > On Oct 29, 2026, at 23:45, Ritesh Harjani (IBM) = wrote: >=20 > Muchun Song writes: >=20 > Thanks Muchun, >=20 >> 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. >>=20 >> 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. >>=20 >> This first triggers a zone mismatch warning in __init_single_page(), = then >> faults while initializing the pageblock flags: >>=20 >> 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 >>=20 >> BUG: Kernel NULL pointer dereference on read at 0x00001400 >> __set_pfnblock_flags_mask.constprop.0+0x34/0x160 >> memmap_init_zone_device+0x358/0x430 >>=20 >> 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. >>=20 >> 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. >>=20 >=20 > Actually that would be totally new code right? It won't be a straight > forward backport, correct? ...=20 Right. Because it's impossible to backport the whole section-based = vmemmap optimization infrastructure (48 commits). >=20 >> 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. >>=20 >> Fixes: f2b79c0d7968 ("powerpc/book3s64/radix: add support for vmemmap = optimization for radix") >> Cc: stable@vger.kernel.org >=20 > ... 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. The comment following the trailer is used to guide stable maintainers to backport proper prerequisite commits. It's hard to do that. So I choose = to mention the alternative in the commit message. But with the help of LLM, I found the alternative is also not the right solution. The reason is as = below. That pmd_leaf() path only creates or reuses the vmemmap mapping. After = it returns, sparse_add_section() calls page_init_poison() on the whole = memmap, which overwrites any page state initialized there. Therefore, older kernels must pass the PMD-leaf information to memmap_init_zone_device() and let it initialize all real tail struct page entries. Initializing them immediately in the PowerPC pmd_leaf() branch would be too early. Another solution is making vmemmap_can_optimize() range-aware by passing the vmemmap start and end addresses. On PowerPC, reject the optimization when any part of the range is = already covered by a PMD leaf. This ensures that an existing PMD mapping is = reused normally and that all real tail struct page entries are initialized = later by the core code. Maybe it should be a separate patch for old kernels? Thoughts? >=20 >> Reported-by: Pavithra >> Closes: = https://lore.kernel.org/r/97f85592bb78a273be8519e6897e7f9b9bd31469.1793070= 429.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(-) >>=20 >=20 > I think, this patch should go via mm tree, due to all the dependencies > coming from there, right? Better. I hope Andrew will pick it up once I send a v2 version with more proper commit message changed. Thanks. >=20 > -ritesh