From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from lists.xenproject.org (lists.xenproject.org [192.237.175.120]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id 4B051EA4E04 for ; Mon, 2 Mar 2026 14:54:38 +0000 (UTC) Received: from list by lists.xenproject.org with outflank-mailman.1244291.1543728 (Exim 4.92) (envelope-from ) id 1vx4fA-00048b-No; Mon, 02 Mar 2026 14:54:24 +0000 X-Outflank-Mailman: Message body and most headers restored to incoming version Received: by outflank-mailman (output) from mailman id 1244291.1543728; Mon, 02 Mar 2026 14:54:24 +0000 Received: from localhost ([127.0.0.1] helo=lists.xenproject.org) by lists.xenproject.org with esmtp (Exim 4.92) (envelope-from ) id 1vx4fA-00048U-Jw; Mon, 02 Mar 2026 14:54:24 +0000 Received: by outflank-mailman (input) for mailman id 1244291; 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a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1772463256; bh=xdku5nig+Gpe25ps3dCqYirfeqxPb+5bKOKhCRkUJYg=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=UPpsNxCTCH2blvH4w0w/jJ1uNxzlymqn7amt368g6EWM8QvGvID3bObS60ccOpyND HAIkiYw4t2sxYLmPVF+fHrlSfknoUysycFnsYeg2bwQNQ75N07g9A2072I8QKx1a81 gVJg1SzyUdP2vkCKMC1exl2KjMgBLttDCBDqICh0qCoFV251Ad2TpvLANGUlbvQzqY jilHRz6FHN7uCWt2iHKsuYftP8aVE9eMDcdEldNQpdBQSQ3aHnLKRckGzN+kixQ/rG mVfbKvteJbqXhTZv1DelJq2Edh3ST8tmhSv4X8cncYmcfdELLBhEAglEtkzeNNhARn ha6UbtSzTkLwA== Message-ID: <663cff60-8181-4a47-beff-204bfe01bb06@kernel.org> Date: Mon, 2 Mar 2026 15:54:12 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: Excluding init_on_free for pages for initial balloon down (Xen) To: =?UTF-8?Q?Marek_Marczykowski-G=C3=B3recki?= Cc: =?UTF-8?B?SsO8cmdlbiBHcm/Dnw==?= , xen-devel , Boris Ostrovsky , Andrew Morton , Vlastimil Babka References: <513e624f-35c1-4d43-ba3f-c96af613d400@suse.com> <224968e6-7236-4efe-bcc0-ab39ac0c6c45@kernel.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 > >> Whatever leaves the buddy shall be zeroed out. If there is a >> double-zeroing happen, the latter could get optimized out by checking >> something like user_alloc_needs_zeroing(). >> >> See mm/huge_memory.c:vma_alloc_anon_folio_pmd() as an example where we >> avoid double-zeroing. > > It isn't just reducing double-zeroing to single zeroing. It's about > avoiding zeroing such pages at all. If a domU is started with > populate-on-demand, many (sometimes most) of its pages are populated in > EPT. The idea of PoD is to start guest with high static memory size, but > low actual allocation and fake it until balloon driver kicks in and make > the domU really not use more pages than it has. When balloon driver try > to return those pages to the hypervisor, normally it would just take > unallocated page one by one and made Linux not use them. But if _any_ > zeroing is happening, each page first needs to be mapped to the guest by > the hypervisor (one trip through EPT), just to be removed from them a > moment later... The same is true for most balloon drivers, including virtio-balloon. So far nobody really cared about that, though, as init_on_free usually comes with such a high performance price tag that people in cheap VMs (where you overcommit etc) don't enable it. __GFP_BALLOON_OUT is just nasty. We could probably have a special allocation interface (not exposed to arbitrary kernel modules) and have things like mm/balloon.c consume that. IIUC, xen balloon does not use the memory balloon infrastructure, though. So we'd need some EXPORT_SYMBOL_FOR_MODULES() magic. Like an struct page *alloc_balloon_pages(gfp_t gfp, unsigned int order); Where we only support a subset of gfp flags, for example, to now having to deal with mempolicy. But it needs a bit of code to make it fly, so I am not sure if the page allocator wants to support that. -- Cheers, David