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 317523A35D9; Thu, 15 Jan 2026 17:41:51 +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=1768498911; cv=none; b=htVASoS0dXqmU6FIqV/A3I+uAv2UNfFSBGjyRx4Jpn2D/cx6LoH9hBuBaj5Ts679vToC4KdkFmMDjhQkTbKbMbokto/q7lE8hiU7SBQiHQcRtSAVnMsVzhaOy7yGN6GXzJgRXYyZU+vX6z3cjYJsPY0rTAl1l9GRKsmS+HfQo4A= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1768498911; c=relaxed/simple; bh=Og6i3thTN1lragtARMir4Fy1CEAvjRfPYpKglMK6jGA=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=s9olUKfHRufiZ3V0wDVeQNtbWDSOx6QGc3Q5sIzh6x5tQ/XzEsl0Qta+CtxuUjDzXeV4aAGB8PGloRrbSmfh+LpzNadeqxGVURP8UNI9BoJri/3Z01tROYrwz29Yy3j4PvHn+mqwQyhzbXye6jAIypgZoyQnfIRys4X0klAZ6+E= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=dl1EW8Bk; 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="dl1EW8Bk" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 703E1C116D0; Thu, 15 Jan 2026 17:41:46 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1768498911; bh=Og6i3thTN1lragtARMir4Fy1CEAvjRfPYpKglMK6jGA=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=dl1EW8Bk/wk7n+ked6d5dTqvGdNDbGhl8PLkyO2JwRqCbK85aViLYdw68xiFFERHu YA2RE9D2nOCr1Mp+bpDoUvE7ZdJrHdTGzhVX5fZO+KivWZQwTWaecvAtWWkglQ75W2 kZWMNaiFgu7oOn0WetGjHSnQvEaxClLl30Ku/0aUVUK0XpoBfJmO3UmbZ+c8cUkd5X eYZwNkOZrx6cP3m1cYFkY+8bNZPHXr+yQRnCYctDNvif1JySmQdEaSpxX4nDRp+yOT HOvkRaiWNPUYYljW9aSgf8nHB/6/E/dzCRGl+D4kKv3Ul673G+YRA3K8/sUN+rb77H FV/Gto8aur1OQ== Message-ID: Date: Thu, 15 Jan 2026 18:41:44 +0100 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: [PATCHv3 10/15] mm/hugetlb: Remove fake head pages To: Kiryl Shutsemau Cc: Andrew Morton , Muchun Song , Matthew Wilcox , Usama Arif , Frank van der Linden , Oscar Salvador , Mike Rapoport , Vlastimil Babka , Lorenzo Stoakes , Zi Yan , Baoquan He , Michal Hocko , Johannes Weiner , Jonathan Corbet , kernel-team@meta.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org References: <20260115144604.822702-1-kas@kernel.org> <20260115144604.822702-11-kas@kernel.org> <30ae1623-63f9-4729-9c19-9b0a9a0ae9f1@kernel.org> From: "David Hildenbrand (Red Hat)" Content-Language: en-US Autocrypt: addr=david@kernel.org; keydata= xsFNBFXLn5EBEAC+zYvAFJxCBY9Tr1xZgcESmxVNI/0ffzE/ZQOiHJl6mGkmA1R7/uUpiCjJ dBrn+lhhOYjjNefFQou6478faXE6o2AhmebqT4KiQoUQFV4R7y1KMEKoSyy8hQaK1umALTdL QZLQMzNE74ap+GDK0wnacPQFpcG1AE9RMq3aeErY5tujekBS32jfC/7AnH7I0v1v1TbbK3Gp XNeiN4QroO+5qaSr0ID2sz5jtBLRb15RMre27E1ImpaIv2Jw8NJgW0k/D1RyKCwaTsgRdwuK Kx/Y91XuSBdz0uOyU/S8kM1+ag0wvsGlpBVxRR/xw/E8M7TEwuCZQArqqTCmkG6HGcXFT0V9 PXFNNgV5jXMQRwU0O/ztJIQqsE5LsUomE//bLwzj9IVsaQpKDqW6TAPjcdBDPLHvriq7kGjt WhVhdl0qEYB8lkBEU7V2Yb+SYhmhpDrti9Fq1EsmhiHSkxJcGREoMK/63r9WLZYI3+4W2rAc UucZa4OT27U5ZISjNg3Ev0rxU5UH2/pT4wJCfxwocmqaRr6UYmrtZmND89X0KigoFD/XSeVv jwBRNjPAubK9/k5NoRrYqztM9W6sJqrH8+UWZ1Idd/DdmogJh0gNC0+N42Za9yBRURfIdKSb B3JfpUqcWwE7vUaYrHG1nw54pLUoPG6sAA7Mehl3nd4pZUALHwARAQABzSREYXZpZCBIaWxk ZW5icmFuZCA8ZGF2aWRAa2VybmVsLm9yZz7CwY0EEwEIADcWIQQb2cqtc1xMOkYN/MpN3hD3 AP+DWgUCaKYhwAIbAwUJJlgIpAILCQQVCgkIAhYCAh4FAheAAAoJEE3eEPcA/4Naa5EP/3a1 9sgS9m7oiR0uenlj+C6kkIKlpWKRfGH/WvtFaHr/y06TKnWn6cMOZzJQ+8S39GOteyCCGADh 6ceBx1KPf6/AvMktnGETDTqZ0N9roR4/aEPSMt8kHu/GKR3gtPwzfosX2NgqXNmA7ErU4puf zica1DAmTvx44LOYjvBV24JQG99bZ5Bm2gTDjGXV15/X159CpS6Tc2e3KvYfnfRvezD+alhF XIym8OvvGMeo97BCHpX88pHVIfBg2g2JogR6f0PAJtHGYz6M/9YMxyUShJfo0Df1SOMAbU1Q Op0Ij4PlFCC64rovjH38ly0xfRZH37DZs6kP0jOj4QdExdaXcTILKJFIB3wWXWsqLbtJVgjR YhOrPokd6mDA3gAque7481KkpKM4JraOEELg8pF6eRb3KcAwPRekvf/nYVIbOVyT9lXD5mJn IZUY0LwZsFN0YhGhQJ8xronZy0A59faGBMuVnVb3oy2S0fO1y/r53IeUDTF1wCYF+fM5zo14 5L8mE1GsDJ7FNLj5eSDu/qdZIKqzfY0/l0SAUAAt5yYYejKuii4kfTyLDF/j4LyYZD1QzxLC MjQl36IEcmDTMznLf0/JvCHlxTYZsF0OjWWj1ATRMk41/Q+PX07XQlRCRcE13a8neEz3F6we 08oWh2DnC4AXKbP+kuD9ZP6+5+x1H1zEzsFNBFXLn5EBEADn1959INH2cwYJv0tsxf5MUCgh Cj/CA/lc/LMthqQ773gauB9mN+F1rE9cyyXb6jyOGn+GUjMbnq1o121Vm0+neKHUCBtHyseB fDXHA6m4B3mUTWo13nid0e4AM71r0DS8+KYh6zvweLX/LL5kQS9GQeT+QNroXcC1NzWbitts 6TZ+IrPOwT1hfB4WNC+X2n4AzDqp3+ILiVST2DT4VBc11Gz6jijpC/KI5Al8ZDhRwG47LUiu Qmt3yqrmN63V9wzaPhC+xbwIsNZlLUvuRnmBPkTJwwrFRZvwu5GPHNndBjVpAfaSTOfppyKB Tccu2AXJXWAE1Xjh6GOC8mlFjZwLxWFqdPHR1n2aPVgoiTLk34LR/bXO+e0GpzFXT7enwyvF FFyAS0Nk1q/7EChPcbRbhJqEBpRNZemxmg55zC3GLvgLKd5A09MOM2BrMea+l0FUR+PuTenh 2YmnmLRTro6eZ/qYwWkCu8FFIw4pT0OUDMyLgi+GI1aMpVogTZJ70FgV0pUAlpmrzk/bLbRk F3TwgucpyPtcpmQtTkWSgDS50QG9DR/1As3LLLcNkwJBZzBG6PWbvcOyrwMQUF1nl4SSPV0L LH63+BrrHasfJzxKXzqgrW28CTAE2x8qi7e/6M/+XXhrsMYG+uaViM7n2je3qKe7ofum3s4v q7oFCPsOgwARAQABwsF8BBgBCAAmAhsMFiEEG9nKrXNcTDpGDfzKTd4Q9wD/g1oFAmic2qsF CSZYCKEACgkQTd4Q9wD/g1oq0xAAsAnw/OmsERdtdwRfAMpC74/++2wh9RvVQ0x8xXvoGJwZ rk0Jmck1ABIM//5sWDo7eDHk1uEcc95pbP9XGU6ZgeiQeh06+0vRYILwDk8Q/y06TrTb1n4n 7FRwyskKU1UWnNW86lvWUJuGPABXjrkfL41RJttSJHF3M1C0u2BnM5VnDuPFQKzhRRktBMK4 GkWBvXlsHFhn8Ev0xvPE/G99RAg9ufNAxyq2lSzbUIwrY918KHlziBKwNyLoPn9kgHD3hRBa Yakz87WKUZd17ZnPMZiXriCWZxwPx7zs6cSAqcfcVucmdPiIlyG1K/HIk2LX63T6oO2Libzz 7/0i4+oIpvpK2X6zZ2cu0k2uNcEYm2xAb+xGmqwnPnHX/ac8lJEyzH3lh+pt2slI4VcPNnz+ vzYeBAS1S+VJc1pcJr3l7PRSQ4bv5sObZvezRdqEFB4tUIfSbDdEBCCvvEMBgoisDB8ceYxO cFAM8nBWrEmNU2vvIGJzjJ/NVYYIY0TgOc5bS9wh6jKHL2+chrfDW5neLJjY2x3snF8q7U9G EIbBfNHDlOV8SyhEjtX0DyKxQKioTYPOHcW9gdV5fhSz5tEv+ipqt4kIgWqBgzK8ePtDTqRM qZq457g1/SXSoSQi4jN+gsneqvlTJdzaEu1bJP0iv6ViVf15+qHuY5iojCz8fa0= In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 1/15/26 18:23, Kiryl Shutsemau wrote: > On Thu, Jan 15, 2026 at 05:49:43PM +0100, David Hildenbrand (Red Hat) wrote: >> On 1/15/26 15:45, Kiryl Shutsemau wrote: >>> HugeTLB Vmemmap Optimization (HVO) reduces memory usage by freeing most >>> vmemmap pages for huge pages and remapping the freed range to a single >>> page containing the struct page metadata. >>> >>> With the new mask-based compound_info encoding (for power-of-2 struct >>> page sizes), all tail pages of the same order are now identical >>> regardless of which compound page they belong to. This means the tail >>> pages can be truly shared without fake heads. >>> >>> Allocate a single page of initialized tail struct pages per NUMA node >>> per order in the vmemmap_tails[] array in pglist_data. All huge pages >>> of that order on the node share this tail page, mapped read-only into >>> their vmemmap. The head page remains unique per huge page. >>> >>> This eliminates fake heads while maintaining the same memory savings, >>> and simplifies compound_head() by removing fake head detection. >>> >>> Signed-off-by: Kiryl Shutsemau >>> --- >>> include/linux/mmzone.h | 16 ++++++++++++++- >>> mm/hugetlb_vmemmap.c | 44 ++++++++++++++++++++++++++++++++++++++++-- >>> mm/sparse-vmemmap.c | 44 ++++++++++++++++++++++++++++++++++-------- >>> 3 files changed, 93 insertions(+), 11 deletions(-) >>> >>> diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h >>> index 322ed4c42cfc..2ee3eb610291 100644 >>> --- a/include/linux/mmzone.h >>> +++ b/include/linux/mmzone.h >>> @@ -82,7 +82,11 @@ >>> * currently expect (see CONFIG_HAVE_GIGANTIC_FOLIOS): with hugetlb, we expect >>> * no folios larger than 16 GiB on 64bit and 1 GiB on 32bit. >>> */ >>> -#define MAX_FOLIO_ORDER get_order(IS_ENABLED(CONFIG_64BIT) ? SZ_16G : SZ_1G) >>> +#ifdef CONFIG_64BIT >>> +#define MAX_FOLIO_ORDER (34 - PAGE_SHIFT) >>> +#else >>> +#define MAX_FOLIO_ORDER (30 - PAGE_SHIFT) >>> +#endif >> >> Where do these magic values stem from, and how do they related to the >> comment above that clearly spells out 16G vs. 1G ? > > This doesn't change the resulting value: 1UL << 34 is 16GiB, 1UL << 30 > is 1G. Subtract PAGE_SHIFT to get the order. > > The change allows the value to be used to define NR_VMEMMAP_TAILS which > is used specify size of vmemmap_tails array. get_order(IS_ENABLED(CONFIG_64BIT) ? SZ_16G : SZ_1G) should evaluate to a constant by the compiler. See __builtin_constant_p handling in get_order(). If that is not working then we have to figure out why. Was this only a specific config in where you ran into compile-time problems? -- Cheers David