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 kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by smtp.lore.kernel.org (Postfix) with ESMTP id EB9FAC433EF for ; Tue, 3 May 2022 04:44:59 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id DB0766B0071; Tue, 3 May 2022 00:44:58 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id D5F256B0073; Tue, 3 May 2022 00:44:58 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id C27736B0074; Tue, 3 May 2022 00:44:58 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from relay.hostedemail.com (relay.hostedemail.com [64.99.140.27]) by kanga.kvack.org (Postfix) with ESMTP id ACE4E6B0071 for ; Tue, 3 May 2022 00:44:58 -0400 (EDT) Received: from smtpin10.hostedemail.com (a10.router.float.18 [10.200.18.1]) by unirelay10.hostedemail.com (Postfix) with ESMTP id 8429EFF0 for ; Tue, 3 May 2022 04:44:58 +0000 (UTC) X-FDA: 79423191876.10.38651F5 Received: from dfw.source.kernel.org (dfw.source.kernel.org [139.178.84.217]) by imf16.hostedemail.com (Postfix) with ESMTP id 42035180027 for ; Tue, 3 May 2022 04:44:52 +0000 (UTC) Received: from smtp.kernel.org (relay.kernel.org [52.25.139.140]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by dfw.source.kernel.org (Postfix) with ESMTPS id 0F8DE6152B; Tue, 3 May 2022 04:44:57 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 779C6C385A4; Tue, 3 May 2022 04:44:55 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1651553096; bh=eIdjs4XODuZ7/ywGfqv6zqyfmOwof/f8BJ/NNK9JEy0=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=CL7JZyokbdHXQ2/8cwW60cI6U3wDZB6DI8RpKc/QwVC84fl9U/0ObwYuJdKMKrA0B iWe2ZiqQgOy9i1CHcSoZZn6ESi9hZUD3gE1fTi3QAGxprYRBCMEhjc2KMP05eAOrpq 8dwYTA/6lJxDG0NAghPF1z6qEoT5BTcJEOnklfEPgUMJgUgDcq5D8HMgRz5X7UMixk eoQEKm364GbzEy3dUrvpGFDSySRNuFFbdC5atPfO2FluBFvnLcUjtMmFD/V2Hh6NC2 ShJjiwTaYQSWw/E7hA3yX+wNDNE9XfxZoG5+wX95spUZszJxhpqH4MNu/yrnT+Isrx 3TRaX4L7RNdvg== Date: Mon, 2 May 2022 21:44:48 -0700 From: Mike Rapoport To: Hyeonggon Yoo <42.hyeyoo@gmail.com> Cc: linux-mm@kvack.org, Andrew Morton , Andy Lutomirski , Dave Hansen , Ira Weiny , Kees Cook , Mike Rapoport , Peter Zijlstra , Rick Edgecombe , Vlastimil Babka , linux-kernel@vger.kernel.org, x86@kernel.org Subject: Re: [RFC PATCH 0/3] Prototype for direct map awareness in page allocator Message-ID: References: <20220127085608.306306-1-rppt@kernel.org> <20220430134415.GA25819@ip-172-31-27-201.ap-northeast-1.compute.internal> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20220430134415.GA25819@ip-172-31-27-201.ap-northeast-1.compute.internal> Authentication-Results: imf16.hostedemail.com; dkim=pass header.d=kernel.org header.s=k20201202 header.b=CL7JZyok; spf=pass (imf16.hostedemail.com: domain of rppt@kernel.org designates 139.178.84.217 as permitted sender) smtp.mailfrom=rppt@kernel.org; dmarc=pass (policy=none) header.from=kernel.org X-Rspam-User: X-Rspamd-Server: rspam05 X-Rspamd-Queue-Id: 42035180027 X-Stat-Signature: jyzp9qzouqiu63hpsouffa7tnm3y3c7r X-HE-Tag: 1651553092-473052 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Sat, Apr 30, 2022 at 01:44:16PM +0000, Hyeonggon Yoo wrote: > On Tue, Apr 26, 2022 at 06:21:57PM +0300, Mike Rapoport wrote: > > Hello Hyeonggon, > > > > On Tue, Apr 26, 2022 at 05:54:49PM +0900, Hyeonggon Yoo wrote: > > > On Thu, Jan 27, 2022 at 10:56:05AM +0200, Mike Rapoport wrote: > > > > From: Mike Rapoport > > > > > > > > Hi, > > > > > > > > This is a second attempt to make page allocator aware of the direct map > > > > layout and allow grouping of the pages that must be mapped at PTE level in > > > > the direct map. > > > > > > > > > > Hello mike, It may be a silly question... > > > > > > Looking at implementation of set_memory*(), they only split > > > PMD/PUD-sized entries. But why not _merge_ them when all entries > > > have same permissions after changing permission of an entry? > > > > > > I think grouping __GFP_UNMAPPED allocations would help reducing > > > direct map fragmentation, but IMHO merging split entries seems better > > > to be done in those helpers than in page allocator. > > > > Maybe, I didn't got as far as to try merging split entries in the direct > > map. IIRC, Kirill sent a patch for collapsing huge pages in the direct map > > some time ago, but there still was something that had to initiate the > > collapse. > > But in this case buddy allocator's view of direct map is quite limited. > It cannot merge 2M entries to 1G entry as it does not support > big allocations. Also it cannot merge entries of pages freed in boot process > as they weren't allocated from page allocator. > > And it will become harder when pages in MIGRATE_UNMAPPED is borrowed > from another migrate type.... > > So it would be nice if we can efficiently merge mappings in > change_page_attr_set(). this approach can handle cases above. > > I think in this case grouping allocations and merging mappings > should be done separately. I've added the provision to merge the mappings in __free_one_page() because at that spot we know for sure we can replace multiple PTEs with a single PMD. I'm not saying there should be no additional mechanism for collapsing direct map pages, but I don't know when and how it should be invoked. > > > For example: > > > 1) set_memory_ro() splits 1 RW PMD entry into 511 RW PTE > > > entries and 1 RO PTE entry. > > > > > > 2) before freeing the pages, we call set_memory_rw() and we have > > > 512 RW PTE entries. Then we can merge it to 1 RW PMD entry. > > > > For this we need to check permissions of all 512 pages to make sure we can > > use a PMD entry to map them. > > Of course that may be slow. Maybe one way to optimize this is using some bits > in struct page, something like: each bit of page->direct_map_split (unsigned long) > is set when at least one entry in (PTRS_PER_PTE = 512)/(BITS_PER_LONG = 64) = 8 entries > has special permissions. > > Then we just need to set the corresponding bit when splitting mappings and > iterate 8 entries when changing permission back again. (and then unset the bit when 8 entries has > usual permissions). we can decide to merge by checking if page->direct_map_split is zero. > > When scanning, 8 entries would fit into one cacheline. > > Any other ideas? > > > Not sure that doing the scan in each set_memory call won't cause an overall > > slowdown. > > I think we can evaluate it by measuring boot time and bpf/module > load/unload time. > > Is there any other workload that is directly affected > by performance of set_memory*()? > > > > 3) after 2) we can do same thing about PMD-sized entries > > > and merge them into 1 PUD entry if 512 PMD entries have > > > same permissions. > > > [...] > > > > Mike Rapoport (3): > > > > mm/page_alloc: introduce __GFP_UNMAPPED and MIGRATE_UNMAPPED > > > > mm/secretmem: use __GFP_UNMAPPED to allocate pages > > > > EXPERIMENTAL: x86/module: use __GFP_UNMAPPED in module_alloc > > > -- > > > Thanks, > > > Hyeonggon > > > > -- > > Sincerely yours, > > Mike. -- Sincerely yours, Mike.