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 3ABE13CE4B6 for ; Tue, 25 Aug 2026 08:31:39 +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=1787646700; cv=none; b=NU3rdDz4dH5188+IHP0Xf/qpwTLPOSqNs6v5G0dK6O3ZqsR8MjmEg6gB4ewhKsNXbYR0sk9BRNbxOVs4Ok13ClMpCztT8Wi6kd+zlq723HhtNPFgXyywyswuCcb/o5OAtRFwqr+sBb4EtC2nviaY4N0G6n27Ni3PoQR1BnnGDus= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1787646700; c=relaxed/simple; bh=ZYhAQBcWSQN8z5GJ1aACclinzf39I8Vkm0CcDs/1js0=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=DU9wOAV3jvxfS6ID60KSxFidFiE7D6dmFMIr6L9ibsM704uZjt/As1zNQYzdy9wNlc2tf1/q5Ut/hOjgSIerA9+ADgQputvr7G9PSjDNFrzGw1qntBfx3DCOjP7fc4nmPAEblzir4K2tK/mejZERDL61I64J8DNUFMAW5qW/4VE= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=S5OCbYap; 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="S5OCbYap" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 92C731F00A3D; Tue, 25 Aug 2026 08:31:36 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1787646698; bh=6jcVuJtrzfEtM9prxYlosEdDpFr5qr6LnwE7Zah30QA=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=S5OCbYapRm0km9uPuedMjFuUoxnXo8KQTa3IlLYLUskH3GYCpqf52og9RaA8UQkPc 1YxdPmzOzNe7mvsoOMzDXvnRrkjtWMZFF+2op7RselwwXEfRU+mMrbpqfivhdCcfMK TS9anlNRbaPJAQf0+vjOKPpRhfSJjBMSC3FFd93WG81HQfuemV4ff1F0QaIRMy3EoP NKG8/JodK9AiefK/PsvbLZFPQc3qXkKdxsgqfwtku4zEBAokFTk9PKhU/Oa52Cu0Gq eFJNzvvvT9IR7FJhJwdNOxOQ5w4hdHZ/U35ix9bEH1k7s2Cf0YaVLL2YB9XcIoc9AR 6kCyHK2T6WIrg== Message-ID: Date: Tue, 25 Aug 2026 10:31:34 +0200 Precedence: bulk X-Mailing-List: linux-fsdevel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [RFC PATCH 0/4] Extend xas_split* to support splitting arbitrarily large entries To: Matthew Wilcox , Artem Lytkin Cc: Ackerley Tng , Zi Yan , Kiryl Shutsemau , Hugh Dickins , Baolin Wang , linux-mm@kvack.org, linux-fsdevel@vger.kernel.org References: <20251117224701.1279139-1-ackerleytng@google.com> <20260824194755.216000-1-iprintercanon@gmail.com> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit On 8/24/26 22:31, Matthew Wilcox wrote: > On Mon, Aug 24, 2026 at 10:47:55PM +0300, Artem Lytkin wrote: >> I'm asking because there is a second user for it that has nothing to do >> with guest_memfd. On arm64 with 64K pages HPAGE_PMD_ORDER is 13 and >> MAX_XAS_ORDER is 11, so MAX_PAGECACHE_ORDER clamps to 11 and shmem never >> offers a PMD order. MADV_COLLAPSE on tmpfs returns -EINVAL there, and >> huge=always hands out order-11 folios at best, so ShmemPmdMapped never >> leaves zero. Every other page cache file system tops out at order 11 as >> well, which is 128M and not PMD-mappable. pagemap.h admits it outright: >> "xas_split_alloc() does not support arbitrary orders. This implies no >> 512MB THP on ARM64 with 64KB base page size." Kiryl's khugepaged >> selftest series skips the shmem and file cases altogether for exactly >> this reason, and on 6 August he confirmed in that thread that uniform >> splits still go through xas_split_alloc(), so xas_try_split() on its >> own does not lift the cap. > > But why would you want it? With 64KiB pages on arm64, you need 512MiB > for a PMD-sized page, and that's just too big for current hardware. > What's your bandwidth? 40GB per DIMM? With 8 DIMMs, that's an > aggregate of 320GB bandwidth, so that restricts you to 640 page faults > per second. That's too few for almost all workloads. > > The page allocator also isn't great at doing order-13 allocations. I heard some use cases using 512M THPs on arm64 to back VMs, but it's mostly anon pages, and not really ordinary pagecache. > > Since arm64 does support 2MiB TLBs, even with a 512MiB PMD size, I'm > not sure there's a huge performance win waiting for you on the other > side of all this work. > > IOW is this driven by a real need, or just some completionist thing? > >> That case needs two new levels of nodes rather than one: 130 nodes, >> 73 KiB, a long way from the 1G to 4K split Matthew was worried about. >> Raising MAX_XAS_ORDER to 3 * XA_CHUNK_SHIFT - 1 covers it, and >> MAX_PAGECACHE_ORDER stays min()'d with HPAGE_PMD_ORDER, so no page cache >> caller can ask for the order-17 worst case that cap admits. > > 130 nodes is still a lot to ask the slab allocator for! With 64KiB > pages it's not too bad -- you get 112 per page. But still ... IMHO it would be nice if we could just support such splits. I've always been thinking of it like this: you end up consuming the exact same memory if you wouldn't have allocated THPs in the first place. But a buddy-like split is clearly preferable. (or avoiding splitting in the first place :) ) -- Cheers, David