From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 1FFA23603EE for ; Thu, 18 Jun 2026 09:04:50 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1781773492; cv=none; b=sLc0KNSwMjvX1WRhZWhJQmsQ1FLJjuiCYaGmFWDnyPZPh6wXqaKTLOg3/t6BBpy0huwsniFTQRTKcZUPhfjSjEw+NF+SFMOpXJL4Q5WmmM+6jis9xigjdzMr2jmGWc2OTwvyUzsDhr2cEFYoiUrSSm6CeHDW7achgqN+Yk4xzAs= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1781773492; c=relaxed/simple; bh=z0G24aTXbwrzjfDqxMchDFnfKs519byu9+GtdTJ7r50=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=ZPW24lCV8wlaaXnKZtUafcm/FLtTGZP1yPm+oPyQrJCMyg7EgmMpdv9lUGpuyIJjIBsnVi7F1c2YzPm7I6E3YGqqINYxCJ/qSWLp1oJttD7O6G5UqOTIddPWkEh9IYVN5/DiLGQUE/aJ2D+eyMSH4hHFsMlc0wS7pysd9/BmjoM= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=haKYBsbc; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="haKYBsbc" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 26A2E1F000E9; Thu, 18 Jun 2026 09:04:47 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1781773490; bh=brT7Ln5skWkl+WmDDmlxUMKf7E5NAPsYv0yiDxFLo0I=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=haKYBsbc9gCVaQo39LR+3EyQo8ZJfVFQDN0oWO8bdm33HUoFFfgmQ0Q/bku4jiMwd NPZqNARZ1j/ki3MoKAq3l7Ta8UPdWIbOrvrtlpQUrUXW/HRNsyIYAuIhm9UWTaN57d ftSVq/75FRg3nLmmtiEgTDavTJVDvAHlNgRxKsNWO7Tf+FUE4uT6Jp1TZpzi+RSmkx dD9tGP+GaZe9SANOX54wGUJm9W8fw1FDm+nQK7zRhJbeC5Kyc/McYKoDijYZUSknSH oRyjelG+ljKff+kiKs9jgVXutRj6H94fkp8/Ud5LQ1jf63rcUzWEF/XDbw0QX/XySh ChdSu6axxE/Vg== Message-ID: <1dbebf73-6c46-44e1-ac6a-f5ffa9849b37@kernel.org> Date: Thu, 18 Jun 2026 11:04:47 +0200 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] mm: page_ext: validate section in for_each_page_ext iterator To: Ketan , Andrew Morton , Vlastimil Babka , Suren Baghdasaryan , Michal Hocko , Brendan Jackman , Johannes Weiner , Zi Yan , Luiz Capitulino Cc: kernel@oss.qualcomm.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <20260617-page_ext-v1-1-37ad802b1a38@oss.qualcomm.com> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit On 6/17/26 17:09, Ketan wrote: > The page_ext iteration API do not validate if the PFN still > belongs to a valid section while advancing the iterator. > > When dynamically adding memory in hotplug path, it can lead > to a NULL pointer dereference during page_ext_lookup at the > boundary of the last valid section. > > [ 14.555124][ T846] Call trace: > [ 14.555125][ T846] lookup_page_ext+0x6c/0x108 (P) > [ 14.555127][ T846] page_ext_lookup+0x30/0x3c > [ 14.555129][ T846] +0x11c/0x260 > [ 14.571201][ T846] __free_pages_ok+0x5e8/0x8e0 > [ 14.571204][ T846] __free_pages_core+0x78/0xf0 > [ 14.571206][ T846] generic_online_page+0x14/0x24 > [ 14.597782][ T846] online_pages+0x178/0x30c > [ 14.597784][ T846] memory_block_change_state+0x284/0x32c > [ 14.597787][ T846] memory_subsys_online+0x4c/0x64 > [ 14.597789][ T846] device_online+0x88/0xb0 > [ 14.597791][ T846] online_memory_block+0x30/0x40 > [ 14.597793][ T846] walk_memory_blocks+0xac/0xe8 > [ 14.597794][ T846] add_memory_resource+0x280/0x298 > [ 14.656161][ T846] add_memory+0x60/0x98 So, we do allocate the page_ext in memory_notify(MEM_GOING_ONLINE) -> ... page_ext_callback() -> online_page_ext(). Which makes sure that all page_ext is actually allocated (for the full section). So when we later end up in generic_online_page(), the page_ext should be there and the memory section should be valid. How can online_pages(), which onlines memory part of present memory sections, online (free) memory that suddenly does not belong to a present memory section? Something doesn't make sense here. > > Add a valid-section check before looking up the next page_ext > so the iterator stops cleanly at section boundaries. > > Fixes: 9039b9096ea2 ("mm: page_owner: use new iteration API") If the problem is real, we'd want to CC stable. > Signed-off-by: Ketan Kishore > --- > mm/page_ext.c | 12 ++++++++++++ > 1 file changed, 12 insertions(+) > > diff --git a/mm/page_ext.c b/mm/page_ext.c > index e2e92bd27ebd..74067ea740fe 100644 > --- a/mm/page_ext.c > +++ b/mm/page_ext.c > @@ -253,6 +253,18 @@ static struct page_ext *lookup_page_ext(const struct page *page) > { > unsigned long pfn = page_to_pfn(page); > struct mem_section *section = __pfn_to_section(pfn); > + > + /* > + * section can be NULL when the page_ext iterator's for-loop increment > + * computes a PFN one step beyond the last registered section. This > + * occurs because pfn_to_page() uses __nr_to_section() which succeeds > + * for unregistered sections that share a root array with registered > + * sections,while __pfn_to_section() returns NULL for them. > + * > + */ > + if (!section) > + return NULL; > + > struct page_ext *page_ext = READ_ONCE(section->page_ext); In addition to what Zi says, which will end up as: if (!section) return NULL; page_ext = READ_ONCE(section->page_ext); drop that comment, not required. And likely best to just add a unlikely(), because this doesn't usually happen. -- Cheers, David