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 6E87E429CE4 for ; Tue, 11 Aug 2026 12:23:23 +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=1786451004; cv=none; b=UYdJ5+OFoxxjgQmE/Y29TJMxffVjJ4APjp9u8S63vhnpXzpsyPKuLZ/bTz7Y+5liWuYmFDhzLNO/vfhW++ebxnDHRPvvxowJSkRmfRvN1uem/zggeFt7RkkGpelo/G4JtANulBixMuG5pj/djbzwGFMyQV6BzRT4wUU82Dl7ndM= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1786451004; c=relaxed/simple; bh=ac9vA00UBKJGYC8TkqLqFStOyhbLZyu0X2xbBWppp9o=; h=Message-ID:Date:MIME-Version:Subject:From:To:Cc:References: In-Reply-To:Content-Type; b=lTrTbT/v0kCasoAZcCZsKkFMJqfWIq6WEmZV4hnxdpylXnpzKnQxNvnsXLiMGxry78IAVZqlIuL+r0VQVY41LCl8g0iwLo2z3fnjimBgioNfdIePsbyAGGNfLizyCcw03n9RgPcTm5A6sgkRVVvhxJcv/DRGZ/lR7I2U0XOMuOM= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=HISixaP+; 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="HISixaP+" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 4BFCC1F000E9; Tue, 11 Aug 2026 12:23:20 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1786451003; bh=io/KtamoJ2U8IcpfdYH6Lz63cdY0XnNwSLtsC8XuWNc=; h=Date:Subject:From:To:Cc:References:In-Reply-To; b=HISixaP+3dJjYZpRGTLwWs5c4DeZQ5UuzQMDDSF4z8+Wh5yUWazwJSmKxDZvD7b5+ IJe0VuDrlYeWL3SsNaKKHjC7ZRg4VhAEXREF1v9UUq7pJ/zAxiQ/TOS3nN7p7W2Btb 2afTp2Ydfl63D2uH66cwors28CTK44qcJKpY2iQI5cvv1L/u0mjWcSSk8E1NtqakEr T08GCvpIk/KutIWSj9h3xzWDo7b5WPjL5ypwMk+3NLjF97DVLRhhMVWl4jeNzckEPB MbeL9lVSClWrLiPrKx1ED/+z12AhvA6nuGxhkS6bjrRLmxiJvBugLpTEd3vdySIT/i HLnjhZD6v5kNg== Message-ID: <01795519-54e8-4ed8-a94e-2b77780abc3a@kernel.org> Date: Tue, 11 Aug 2026 14:23:18 +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 v6 1/2] mm/memory_hotplug: optimize zone contiguous check when changing pfn range From: "David Hildenbrand (Arm)" To: "Liu, Yuan1" , Oscar Salvador , Mike Rapoport , Wei Yang Cc: "linux-mm@kvack.org" , "Zou, Nanhai" , "Deng, Pan" , "Li, Tianyou" , Chen Zhang , "Zeng, Jason" , "linux-kernel@vger.kernel.org" References: <20260723084946.189392-1-yuan1.liu@intel.com> <20260723084946.189392-2-yuan1.liu@intel.com> <2cbd42e1-b5bb-474c-b0e7-ce3f46541891@kernel.org> <0b02ef3e-363f-4b49-a208-54cdfea56c75@kernel.org> Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit On 8/10/26 16:04, David Hildenbrand (Arm) wrote: >> >> Hi David >> >> My understanding is that init_unavailable_range() initializes all >> PFNs that satisfy pfn_valid(), but not all of them satisfy >> pfn_to_online_page(), since some PFNs belong to subsections that are >> not online. > > Thanks for reminding me. I think the right direction is to finally clean up the > pfn_valid() handling. > >> >> You previously mentioned: >> >> pfn_valid() says early sections always have a full memmap, so even invalid >> subsections have a memmap. pfn_to_online_page() says an invalid subsection >> cannot be online and its content must be stale. for_each_valid_pfn() follows >> pfn_valid() semantics, and we use it to initialize memmap that is not going >> to be online and account it as pages_with_online_memmap, which is wrong. >> >> The cleanest approach is to avoid allocating memmap for subsections, which >> also removes the special early-section handling from pfn_valid() and >> for_each_valid_pfn(). >> >> I also share the concerns raised by Sashiko in the analysis below [1]: >> >> Scanners like isolate_migratepages_block() will then blindly iterate through >> the pageblock and access the completely uninitialized struct pages of the hole, >> leading to functional errors or kernel panics when reading these zero-filled >> structures via macros like PageHuge() or page_zone(). >> >> That's why we went with the current approach in v6 instead of your >> earlier suggestion. I'd really appreciate your guidance on which >> direction you think would be more appropriate. > Let me take a stab at just having pfn_valid() / for_each_valid_pfn() respecting > the subsection map. ... and that turns complicated very quickly. The problem is that we have some users, in particular the buddy, that just assumes that MAX_PAGE_ORDER regions are fully accessible. The fun begins once we have MAX_PAGE_ORDER span multiple subsections. So we'd actually want to initialize the memmap. The pfn_valid() vs. pfn_to_online_page() inconsistency is really nasty :( I mean, in init_unavailable_range() we could actually figure out fairly easily whether we are dealing with holes where pfn_to_online_page() would succeed. diff --git a/mm/mm_init.c b/mm/mm_init.c index e9c4204b73adb..54e71e17f2c3c 100644 --- a/mm/mm_init.c +++ b/mm/mm_init.c @@ -843,11 +843,13 @@ static void __init init_unavailable_range(unsigned long spfn, int zone, int node) { unsigned long pfn; - u64 pgcnt = 0; + u64 pgcnt = 0, online_pgcnt = 0; for_each_valid_pfn(pfn, spfn, epfn) { __init_single_page(pfn_to_page(pfn), pfn, zone, node); __SetPageReserved(pfn_to_page(pfn)); + if (pfn_to_online_page(pfn)) + online_pgcnt++; pgcnt++; } If it's a problem performance-wise, we can always try optimizing by skipping checks within the same (sub)section. -- Cheers, David