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 7B4B6C46CA0 for ; Tue, 5 Dec 2023 09:34:32 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S231882AbjLEJeY convert rfc822-to-8bit (ORCPT ); Tue, 5 Dec 2023 04:34:24 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:50206 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229710AbjLEJeW (ORCPT ); Tue, 5 Dec 2023 04:34:22 -0500 Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by lindbergh.monkeyblade.net (Postfix) with ESMTP id E34679A for ; Tue, 5 Dec 2023 01:34:27 -0800 (PST) Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 381B8139F; Tue, 5 Dec 2023 01:35:14 -0800 (PST) Received: from [10.57.73.130] (unknown [10.57.73.130]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 8863E3F6C4; Tue, 5 Dec 2023 01:34:24 -0800 (PST) Message-ID: <2de0617e-d1d7-49ec-9cb8-206eaf37caed@arm.com> Date: Tue, 5 Dec 2023 09:34:23 +0000 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v8 00/10] Multi-size THP for anonymous memory Content-Language: en-GB To: Andrew Morton Cc: Matthew Wilcox , Yin Fengwei , David Hildenbrand , Yu Zhao , Catalin Marinas , Anshuman Khandual , Yang Shi , "Huang, Ying" , Zi Yan , Luis Chamberlain , Itaru Kitayama , "Kirill A. Shutemov" , John Hubbard , David Rientjes , Vlastimil Babka , Hugh Dickins , Kefeng Wang , Barry Song <21cnbao@gmail.com>, Alistair Popple , linux-mm@kvack.org, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org References: <20231204102027.57185-1-ryan.roberts@arm.com> <20231204113039.42510c23455026e40c5e2a56@linux-foundation.org> From: Ryan Roberts In-Reply-To: <20231204113039.42510c23455026e40c5e2a56@linux-foundation.org> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8BIT Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 04/12/2023 19:30, Andrew Morton wrote: > On Mon, 4 Dec 2023 10:20:17 +0000 Ryan Roberts wrote: > >> Hi All, >> >> >> Prerequisites >> ============= >> >> Some work items identified as being prerequisites are listed on page 3 at [9]. >> The summary is: >> >> | item | status | >> |:------------------------------|:------------------------| >> | mlock | In mainline (v6.7) | >> | madvise | In mainline (v6.6) | >> | compaction | v1 posted [10] | >> | numa balancing | Investigated: see below | >> | user-triggered page migration | In mainline (v6.7) | >> | khugepaged collapse | In mainline (NOP) | > > What does "prerequisites" mean here? Won't compile without? Kernel > crashes without? Nice-to-have-after? Please expand on this. Short answer: It's supposed to mean things that either need to be done to prevent the mm from regressing (both correctness and performance) when multi-size THP is present but disabled, or things that need to be done to make the mm robust (but not neccessarily optimially performant) when multi-size THP is enabled. But in reality, all of the things on the list could really be reclassified as "nice-to-have-after", IMHO; their absence will neither cause compilation nor runtime errors. Longer answer: When I first started looking at this, I was advised that there were likely a number of corners which made assumptions about large folios always being PMD-sized, and if not found and fixed, could lead to stability issues. At the time I was also pursuing a strategy of multi-size THP being a compile-time feature with no runtime control, so I decided it was important for multi-size THP to not effectively disable other features (e.g. various madvise ops used to ignore PTE-mapped large folios). This list represents all the things that I could find based on code review, as well as things suggested by others, and in the end, they all fall into that last category of "PTE-mapped large folios efectively disable existing features". But given we now have runtime controls to opt-in to multi-size THP, I'm not sure we need to classify these as prerequisites. But I didn't want to unilaterally make that decision, given this list has previously been discussed and agreed by others. It's also worth noting that in the case of compaction, that's already a problem for large folios in the page cache; large folios will be skipped. > > I looked at [9], but access is denied. Sorry about that; its owned by David Rientjes so I can't fix that for you. It's a PDF of a slide with the following table: +-------------------------------+------------------------------------------------------------------------+--------------+--------------------+ | Item | Description | Assignee | Status | +-------------------------------+------------------------------------------------------------------------+--------------+--------------------+ | mlock | Large, pte-mapped folios are ignored when mlock is requested. | Yin, Fengwei | In mainline (v6.7) | | | Code comment for mlock_vma_folio() says "...filter out pte mappings | | | | | of THPs which cannot be consistently counted: a pte mapping of the | | | | | THP head cannot be distinguished by the page alone." | | | | madvise | MADV_COLD, MADV_PAGEOUT, MADV_FREE: For large folios, code assumes | Yin, Fengwei | In mainline (v6.6) | | | exclusive only if mapcount==1, else skips remainder of operation. | | | | | For large, pte-mapped folios, exclusive folios can have mapcount | | | | | upto nr_pages and still be exclusive. Even better; don't split | | | | | the folio if it fits entirely within the range. | | | | compaction | Raised at LSFMM: Compaction skips non-order-0 pages. | Zi Yan | v1 posted | | | Already problem for page-cache pages today. | | | | numa balancing | Large, pte-mapped folios are ignored by numa-balancing code. Commit | John Hubbard | Investigated: | | | comment (e81c480): "We're going to have THP mapped with PTEs. It | | Not prerequisite | | | will confuse numabalancing. Let's skip them for now." | | | | user-triggered page migration | mm/migrate.c (migrate_pages syscall) We don't want to migrate folio | Kefeng Wang | In mainline (v6.7) | | | that is shared. | | | | khugepaged collapse | collapse small-sized THP to PMD-sized THP in khugepaged/MADV_COLLAPSE. | Ryan Roberts | In mainline (NOP) | | | Kirill thinks khugepage should already be able to collapse | | | | | small large folios to PMD-sized THP; verification required. | | | +-------------------------------+------------------------------------------------------------------------+--------------+--------------------+ Thanks, Ryan > >> [9] https://drive.google.com/file/d/1GnfYFpr7_c1kA41liRUW5YtCb8Cj18Ud/view?usp=sharing&resourcekey=0-U1Mj3-RhLD1JV6EThpyPyA > >