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b=qHbu6oASLVcSZ08Zpc4nEsoVAmHkOx6JfyjrPg4mtkqF1lLtz1PDsn/tKY0d2CM30 kgm7w96xhaFBSneyNYbU/dEPE0cYQI/OxIpZAi4XBDyQvJwUIDhKT3Fsl2WmNnUO64 8aYT6byMe3dMiAB0jL/9YfxpzgzlI/XBAXczASO/UCSI0j9fxzUN9h3vMVrGdKlwD4 uYPtJ77sKC7Lo8LhDqsuacaHxcVdoOmYWw6rLHkwpiBVH0o7Le7x3BqYMEGfZrEKY/ qIm5aPg9j0h4uuCHaK1rWPEim3c35dnkRrnGkBWVTU60ptfWEYaMVOKlxUYUXYjZ9c 2V7ah/aO9veJg== Message-ID: <1253ca14-69de-418f-8f94-b08e8105e924@kernel.org> Date: Fri, 24 Apr 2026 10:52:06 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v3] mm/page_alloc: replace kernel_init_pages() with batch page clearing To: "Salunke, Hrushikesh" , Andrew Morton Cc: ljs@kernel.org, Liam.Howlett@oracle.com, vbabka@kernel.org, rppt@kernel.org, surenb@google.com, mhocko@suse.com, jackmanb@google.com, hannes@cmpxchg.org, ziy@nvidia.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org, rkodsara@amd.com, bharata@amd.com, ankur.a.arora@oracle.com, shivankg@amd.com References: <20260422102729.166599-1-hsalunke@amd.com> <20260423041249.156eb95889696ccfaf23dca1@linux-foundation.org> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit X-Stat-Signature: otco65acmjbdnca1k73gyk75nubdc9fa X-Rspam-User: X-Rspamd-Queue-Id: 41FCD80008 X-Rspamd-Server: rspam05 X-HE-Tag: 1777020734-537748 X-HE-Meta: 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 Du1YvhNZ jP7swsx/GvrX+vKy+j2WuDBmoJrje4RvT433EH8qAzIALObJY/z7anr1iU95/tj4vaIoUFGP9Bs9hrO4SzDJth9qL1bDpf4q7Yv3CZlhQn1lJSbw2yDVx4Mp94nMULn4xbuhnnziM0BfHaNBcgrsPOeaHoSeRLOOd9p+VoYmmEYZ40Cb7Tzl7pqTR6QwT82YCQZvuwdoREp6N67kBbILmX4Sju910PrXZv7AoG66MwSRHlY8AvBvG6SOSaK+hyY3cZOR1h+1vBxhvDmCF/Zw37bYc0S93VT80eTHIjCjNKphzgpcxnuEZiX0a8sEosLIeq0g/PrH5ktFO31k= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 4/24/26 10:42, Salunke, Hrushikesh wrote: > > On 23-04-2026 16:42, Andrew Morton wrote: >> Caution: This message originated from an External Source. Use proper caution when opening attachments, clicking links, or responding. >> >> >> On Wed, 22 Apr 2026 10:26:58 +0000 Hrushikesh Salunke wrote: >> >>> When init_on_alloc is enabled, kernel_init_pages() clears every page >>> one at a time via clear_highpage_kasan_tagged(), which incurs per-page >>> kmap_local_page()/kunmap_local() overhead and prevents the architecture >>> clearing primitive from operating on contiguous ranges. >>> >>> Introduce clear_highpages_kasan_tagged() in highmem.h, a batch >>> clearing helper that calls clear_pages() for the full contiguous range >>> on !HIGHMEM systems, bypassing the per-page kmap overhead and allowing >>> a single invocation of the arch clearing primitive across the entire >>> allocation. The HIGHMEM path falls back to per-page clearing since >>> those pages require kmap. >>> >>> Replace kernel_init_pages() with direct calls to the new helper, as it >>> becomes a trivial wrapper. >>> >>> Allocating 8192 x 2MB HugeTLB pages (16GB) with init_on_alloc=1: >>> >>> Before: 0.445s >>> After: 0.166s (-62.7%, 2.68x faster) >> Nice. >> >>> Kernel time (sys) reduction per workload with init_on_alloc=1: >>> >>> Workload Before After Change >>> Graph500 64C128T 30m 41.8s 15m 14.8s -50.3% >>> Graph500 16C32T 15m 56.7s 9m 43.7s -39.0% >>> Pagerank 32T 1m 58.5s 1m 12.8s -38.5% >>> Pagerank 128T 2m 36.3s 1m 40.4s -35.7% >>> >>> ... >>> >>> --- a/include/linux/highmem.h >>> +++ b/include/linux/highmem.h >>> @@ -345,6 +345,21 @@ static inline void clear_highpage_kasan_tagged(struct page *page) >>> kunmap_local(kaddr); >>> } >>> >>> +static inline void clear_highpages_kasan_tagged(struct page *page, int numpages) >>> +{ >>> + /* s390's use of memset() could override KASAN redzones. */ >>> + kasan_disable_current(); >>> + if (!IS_ENABLED(CONFIG_HIGHMEM)) { >>> + clear_pages(kasan_reset_tag(page_address(page)), numpages); >>> + } else { >>> + int i; >>> + >>> + for (i = 0; i < numpages; i++) >>> + clear_highpage_kasan_tagged(page + i); >>> + } >>> + kasan_enable_current(); >>> +} >> Why was it globally published and inlined? Is there any expectation >> that this will be used outside of page_alloc.c? >> >> Both of the callsites are themselves inlined. The patch adds 330 bytes >> to my arm allmodcnfig page_alloc.o - did we gain anything from that? >> > Hi Andrew, > > The idea was to keep it alongside clear_highpage_kasan_tagged() as its > batch counterpart, but currently it is only used by page_alloc.c. Right. Looking at init_vmalloc_pages(), I wonder if it could also benefit from batching if we find that pages are actually contiguous. That would require looking up multiple pages at once. vmalloc_to_pages() or sth like that. Surely, doing such an optimized page table walk could be beneficial by itself. > > Your concern about the code size increase is valid. Would you prefer if > I move it to page_alloc.c as a static function and drop the inline > in v4? If an external user comes along later it can always be moved > back to the header. What is exactly is responsible for the code increase? Two calls in clear_highpages_kasan_tagged()? Surely the compiler would just inline kernel_init_pages() already? So my best guess that the 330 bytes are just clear_pages() overhead or some code layout changes? -- Cheers, David