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 vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id E7B70C6FD18 for ; Wed, 29 Mar 2023 08:13:31 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S230037AbjC2INa (ORCPT ); Wed, 29 Mar 2023 04:13:30 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:42170 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229970AbjC2IN0 (ORCPT ); Wed, 29 Mar 2023 04:13:26 -0400 Received: from smtp-out2.suse.de (smtp-out2.suse.de [IPv6:2001:67c:2178:6::1d]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 02A4626A5 for ; Wed, 29 Mar 2023 01:13:25 -0700 (PDT) Received: from imap2.suse-dmz.suse.de (imap2.suse-dmz.suse.de [192.168.254.74]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (P-521) server-digest SHA512) (No client certificate requested) by smtp-out2.suse.de (Postfix) with ESMTPS id B00141FDF3; Wed, 29 Mar 2023 08:13:23 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=suse.com; s=susede1; t=1680077603; h=from:from:reply-to:date:date:message-id:message-id:to:to:cc:cc: mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=4uKJsI6kcmXX5PcI4vzLQ7gfadOE59ADRMPHNu9uM4g=; b=KgmXk3WDiPzBXzdj45vKo4Vxz9vi4sI6hNTwjCUsKoQDpB57b7/5//GKc082Or6lCMx/b9 ZKDzzzwTxi4ao+87xcxzxrVyODS5gjn/Lku2in5ezddE1za04vzIij6QMQpnfKlTJeGdIU mG87qg70katMuzooi3uJR+33T3GLTfE= Received: from imap2.suse-dmz.suse.de (imap2.suse-dmz.suse.de [192.168.254.74]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (P-521) server-digest SHA512) (No client certificate requested) by imap2.suse-dmz.suse.de (Postfix) with ESMTPS id A26B0139D3; Wed, 29 Mar 2023 08:13:23 +0000 (UTC) Received: from dovecot-director2.suse.de ([192.168.254.65]) by imap2.suse-dmz.suse.de with ESMTPSA id CwdQJyPzI2TmWQAAMHmgww (envelope-from ); Wed, 29 Mar 2023 08:13:23 +0000 Date: Wed, 29 Mar 2023 10:13:23 +0200 From: Michal Hocko To: Mike Rapoport Cc: linux-mm@kvack.org, Andrew Morton , Dave Hansen , Peter Zijlstra , Rick Edgecombe , Song Liu , Thomas Gleixner , Vlastimil Babka , linux-kernel@vger.kernel.org, x86@kernel.org Subject: Re: [RFC PATCH 1/5] mm: intorduce __GFP_UNMAPPED and unmapped_alloc() Message-ID: References: <20230308094106.227365-1-rppt@kernel.org> <20230308094106.227365-2-rppt@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed 29-03-23 10:28:02, Mike Rapoport wrote: > On Tue, Mar 28, 2023 at 05:24:49PM +0200, Michal Hocko wrote: > > On Tue 28-03-23 18:11:20, Mike Rapoport wrote: > > > On Tue, Mar 28, 2023 at 09:39:37AM +0200, Michal Hocko wrote: > > [...] > > > > OK, so you want to reduce that direct map fragmentation? > > > > > > Yes. > > > > > > > Is that a real problem? > > > > > > A while ago Intel folks published report [1] that showed better performance > > > with large pages in the direct map for majority of benchmarks. > > > > > > > My impression is that modules are mostly static thing. BPF > > > > might be a different thing though. I have a recollection that BPF guys > > > > were dealing with direct map fragmention as well. > > > > > > Modules are indeed static, but module_alloc() used by anything that > > > allocates code pages, e.g. kprobes, ftrace and BPF. Besides, Thomas > > > mentioned that having code in 2M pages reduces iTLB pressure [2], but > > > that's not only about avoiding the splits in the direct map but also about > > > using large mappings in the modules address space. > > > > > > BPF guys suggested an allocator for executable memory [3] mainly because > > > they've seen performance improvement of 0.6% - 0.9% in their setups [4]. > > > > These are fair arguments and it would have been better to have them in > > the RFC. Also it is not really clear to me what is the actual benefit of > > the unmapping for those usecases. I do get they want to benefit from > > caching on the same permission setup but do they need unmapping as well? > > The pages allocated with module_alloc() get different permissions depending > on whether they belong to text, rodata, data etc. The permissions are > updated in both vmalloc address space and in the direct map. The updates to > the direct map cause splits of the large pages. That much is clear (wouldn't hurt to mention that in the changelog though). > If we cache large pages as > unmapped we take out the entire 2M page from the direct map and then > if/when it becomes free it can be returned to the direct map as a 2M page. > > Generally, the unmapped allocations are intended for use-cases that anyway > map the memory elsewhere than direct map and need to modify direct mappings > of the memory, be it modules_alloc(), secretmem, PKS page tables or maybe > even some of the encrypted VM memory. I believe we are still not on the same page here. I do understand that you want to re-use the caching capability of the unmapped_pages_alloc for modules allocations as well. My question is whether module_alloc really benefits from the fact that the memory is unmapped? I guess you want to say that it does because it wouldn't have to change the permission for the direct map but I do not see that anywhere in the patch... Also consinder the slow path where module_alloc needs to allocate a fresh (huge)page and unmap it. Does the overhead of the unmapping suits needs of all module_alloc users? Module loader is likely not interesting as it tends to be rather static but what about BPF programs? [...] > > > I also think vmalloc with unmmapped pages can provide backing pages for > > > execmem_alloc() Song proposed. > > > > Why would you need to have execmem_alloc have its memory virtually > > mapped into vmalloc space? > > Currently all the memory allocated for code is managed in a subset of > vmalloc() space. The intention of execmem_alloc() was to replace > module_alloc() for the code pages, so it's natural that it will use the > same virtual ranges. Ohh, sorry my bad. I have only now realized I have misread and thought about secretmem_alloc... -- Michal Hocko SUSE Labs