From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (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 6FBDB86353; Thu, 5 Feb 2026 13:10:48 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1770297048; cv=none; b=X3WSCoj8RZ5fKpuK19Bz0vvgZyDORpr57w3aOcP5XOzS2TP4FvXQ1pb/qvjcK0JwAV4wd3+7vt9axewwfuFB8+ZdcJERL25uqhTkrgDuaoZReBGdCLvRWOaMvyFyxXZKyj/rkZDroFDzBA00ISjmL9NtyPdDzkoPEC9XEbnWCLk= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1770297048; c=relaxed/simple; bh=YNe9Nz9GlYTd64O1KTiTpwt9/xT1kD1kmstkqnvmOSI=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=JauozM0mUpMCVotAiIWWsqQgC+/OQSoZAr2vUyOBLniA/HqeJp28iMFRRC3Br9S8mlSzZNuKEChRQdMdpSQos9ISLH3I2QK4WzE8F+XhrhPNhAjDNpe5xqJqaCU/ZjToS1Dc7/+DSqxemry0rWDg+LbeGFb6IwDHqetpx8/a6f4= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=pNYfAeQa; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="pNYfAeQa" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 1EFC0C4CEF7; Thu, 5 Feb 2026 13:10:41 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1770297048; bh=YNe9Nz9GlYTd64O1KTiTpwt9/xT1kD1kmstkqnvmOSI=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=pNYfAeQaHzQS4JAIIf5RMJS4M1jrbWEl3KdV5wQ1CVfmAFYOtRX+59SIn8fmPO8Ex tkfXAh3SXYC3mzPHaYTYbNsTm8rtvbpWyuIcuhhDdcNdq377k6vzTZdarqvtilL3D4 Xf7+yiULCpU1+aEzuPb0FLB9CQSTB8MvWMkcFMDB5mJWXPUZ0ommmMGYPHmT6kL3rY qyMG/T2JtOKIm/hWkO7u/asLSIi4rUjArZSx8eVYVe3SeX/U9MUJb01oH/jG6auYNX sdh+Fq9oU+SuPthWrlrJpd2rDQNhsEp6q9NPXTzVBkBmuBLwpHUR29VvLiXSZaTSkg 51OS4qEEhW6Pg== Message-ID: Date: Thu, 5 Feb 2026 14:10:40 +0100 Precedence: bulk X-Mailing-List: linux-doc@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCHv6 08/17] mm: Make page_zonenum() use head page To: Kiryl Shutsemau , Andrew Morton , Muchun Song , Matthew Wilcox , Usama Arif , Frank van der Linden Cc: Oscar Salvador , Mike Rapoport , Vlastimil Babka , Lorenzo Stoakes , Zi Yan , Baoquan He , Michal Hocko , Johannes Weiner , Jonathan Corbet , Huacai Chen , WANG Xuerui , Palmer Dabbelt , Paul Walmsley , Albert Ou , Alexandre Ghiti , kernel-team@meta.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org, loongarch@lists.linux.dev, linux-riscv@lists.infradead.org References: <20260202155634.650837-1-kas@kernel.org> <20260202155634.650837-9-kas@kernel.org> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 2/2/26 16:56, Kiryl Shutsemau wrote: > With the upcoming changes to HVO, a single page of tail struct pages > will be shared across all huge pages of the same order on a node. Since > huge pages on the same node may belong to different zones, the zone > information stored in shared tail page flags would be incorrect. > > Always fetch zone information from the head page, which has unique and > correct zone flags for each compound page. > > Signed-off-by: Kiryl Shutsemau > Acked-by: Zi Yan > --- > include/linux/mmzone.h | 1 + > 1 file changed, 1 insertion(+) > > diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h > index be8ce40b5638..192143b5cdc0 100644 > --- a/include/linux/mmzone.h > +++ b/include/linux/mmzone.h > @@ -1219,6 +1219,7 @@ static inline enum zone_type memdesc_zonenum(memdesc_flags_t flags) > > static inline enum zone_type page_zonenum(const struct page *page) > { > + page = compound_head(page); > return memdesc_zonenum(page->flags); We end up calling page_zonenum() without holding a reference. Given that _compound_head() does a READ_ONCE(), this should work even if we see concurrent page freeing etc. However, this change implies that we now perform a compound page lookup for every PageHighMem() [meh], page_zone() [quite some users in the buddy, including for pageblock access and page freeing]. That's a nasty compromise for making HVO better? :) We should likely limit that special casing to kernels that really rquire it (HVO). -- Cheers, David