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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 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 X-Rspam-User: X-Rspamd-Server: rspam11 X-Rspamd-Queue-Id: 379321A000F X-Stat-Signature: 5w3ci1xmwb7761dsoyftc9qes536n7gb X-HE-Tag: 1786451004-108695 X-HE-Meta: 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 OsTwr5du XLUZpFlIt2dduZPDHnQtnjuo6FJ7ExY49eknku8nBpKT12gFAd0cK+xmorY0ToW6v3K41XEUGNFyixPPQLRJ1DV0LzmMCu+v1HC4LiqSN1GdY6euNYpiAw9Dmhuchm4xhnX5b96bBD2EvXb8zmuR4QUUqt6PJPNAOYdYHnyV00a/rprz7IxB2GODR/t58yGpv+nM5PiiaN8haoQtDnTp4qDgW7bv3XJOUmmM2dr2k8MOU7Qzl/8CftoGiBJLbK5WHhSgc/lHLChuAZraWOgA1lU43teQgiogL2w05CVavgu6svsfZhtCNHat9W+E4OLZ+xye4ar7ysuFyiyqjp3z954BrVB2fOBvnOF5ZBebR3PsLC9jXwiSY6UIxKZWt3VGAb2iZmxnFZHeop6GSlIhO5SSTAJSs4dz6aGXTQFodNVEZC1A= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: 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