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 mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 02709C433FE for ; Thu, 11 Nov 2021 12:00:51 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id D5DF061247 for ; Thu, 11 Nov 2021 12:00:50 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S232318AbhKKMDi (ORCPT ); Thu, 11 Nov 2021 07:03:38 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:50558 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S232815AbhKKMDf (ORCPT ); Thu, 11 Nov 2021 07:03:35 -0500 Received: from mail-pg1-x52d.google.com (mail-pg1-x52d.google.com [IPv6:2607:f8b0:4864:20::52d]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id B789BC061766 for ; Thu, 11 Nov 2021 04:00:46 -0800 (PST) Received: by mail-pg1-x52d.google.com with SMTP id 200so4988381pga.1 for ; Thu, 11 Nov 2021 04:00:46 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=bytedance-com.20210112.gappssmtp.com; s=20210112; h=message-id:date:mime-version:user-agent:subject:to:cc:references :from:in-reply-to:content-transfer-encoding; bh=8mQU/a5DXQiWtemuyLrNEiXaRlt1oXHNePDS+l8sy0U=; b=JB/XcE1vimwlej+IsHSxjGqvq6J1gqTAoPf6/32k8FGdrO5dk/9yEZqxP5YzWpC0sn ndMI3VLYiweZXicPw/2yo+TbqD7LVpzDNO2IRxKr01iBpqssO1iwZ2L5lj1FbJJrRXf9 bqXNAfm1LyYUEBNwx8YdKxpCAyAHWpdUuDIIOUp38M3tMgIqjCs4OqXO6/w9gy381TSR 53y6bD4YF2PwOO+GXJFJOLjS6OlCIiDG6chzXAHJe8mxk1gQItFBBKzLu+NyJem+dorV Cuw+BeaqNHOh7qsKIsjB76NtVxhTiogoEbF2HcfAzSGpTzEBtC7sGdV1Nnn0lL4/lbVb I6HQ== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; h=x-gm-message-state:message-id:date:mime-version:user-agent:subject :to:cc:references:from:in-reply-to:content-transfer-encoding; bh=8mQU/a5DXQiWtemuyLrNEiXaRlt1oXHNePDS+l8sy0U=; b=faxMjSAO9TVMh2Wp2NiHCbsAvuYUO6k/n9wGKnsi56osWgyh+3uQRh9cFhrP+SP2Lz cGkRyTqmoO95uNNBV0wyVzAQPThKsb2HGfK/Gxrndl3KiuC9z914KQDpXyNAx5A6Q17p Y3YjR+Rt5PEN0ZzgMou107rHyC/R/mrwwKngt2YvsO5EgbBwhF/MrBM3YUKIJEPpCHHZ xJmNJOTDSaVPc/jZfMV9huWqjYNmtxopNFcHiAVKC6nG6MOzUgkg+tjhfkXT9AhqiL9u Jd5w2aNkpdXi3mbGzjao4wOPQpwDelDHyYC8TmuZO1468WlYTp/z1WO9qPU2fZYumhT1 S3fA== X-Gm-Message-State: AOAM533RSIh24e5bSiRvwiD2GVO5ubSZZ4IwiHJnivN4ZPeiOJunJZ0z HAItEsDrsT0fn1eXIUC20Qrldw== X-Google-Smtp-Source: ABdhPJwvSL9McBDK/ZE696u4dfpcg5dfnlyb0sSdeeEP2MYOstUkgZb0CfeENfOU5AzL+hcjd2fT2Q== X-Received: by 2002:a05:6a00:238d:b0:47c:2232:80d8 with SMTP id f13-20020a056a00238d00b0047c223280d8mr6117575pfc.12.1636632014970; Thu, 11 Nov 2021 04:00:14 -0800 (PST) Received: from [10.254.173.217] ([139.177.225.248]) by smtp.gmail.com with ESMTPSA id l11sm8064855pjg.22.2021.11.11.04.00.09 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 11 Nov 2021 04:00:14 -0800 (PST) Message-ID: <2e19ad1b-15f3-7508-c5d5-6c31765f26d3@bytedance.com> Date: Thu, 11 Nov 2021 20:00:06 +0800 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:91.0) Gecko/20100101 Thunderbird/91.3.0 Subject: Re: [PATCH v3 00/15] Free user PTE page table pages To: David Hildenbrand , Jason Gunthorpe Cc: akpm@linux-foundation.org, tglx@linutronix.de, kirill.shutemov@linux.intel.com, mika.penttila@nextfour.com, linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, songmuchun@bytedance.com, zhouchengming@bytedance.com References: <20211110105428.32458-1-zhengqi.arch@bytedance.com> <20211110125601.GQ1740502@nvidia.com> <8d0bc258-58ba-52c5-2e0d-a588489f2572@redhat.com> <20211110143859.GS1740502@nvidia.com> <6ac9cc0d-7dea-0e19-51b3-625ec6561ac7@redhat.com> <20211110163925.GX1740502@nvidia.com> <7c97d86f-57f4-f764-3e92-1660690a0f24@redhat.com> <60515562-5f93-11cd-6c6a-c7cc92ff3bf8@bytedance.com> <9ee06b52-4844-7996-fa34-34fc7d4fdc10@bytedance.com> <27d73395-70b4-fe4a-4c8d-415b43ff9c1f@redhat.com> From: Qi Zheng In-Reply-To: <27d73395-70b4-fe4a-4c8d-415b43ff9c1f@redhat.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 11/11/21 7:19 PM, David Hildenbrand wrote: > On 11.11.21 12:08, Qi Zheng wrote: >> >> >> On 11/11/21 5:22 PM, David Hildenbrand wrote: >>> On 11.11.21 04:58, Qi Zheng wrote: >>>> >>>> >>>> On 11/11/21 1:37 AM, David Hildenbrand wrote: >>>>>>> It would still be a fairly coarse-grained locking, I am not sure if that >>>>>>> is a step into the right direction. If you want to modify *some* page >>>>>>> table in your process you have exclude each and every page table walker. >>>>>>> Or did I mis-interpret what you were saying? >>>>>> >>>>>> That is one possible design, it favours fast walking and penalizes >>>>>> mutation. We could also stick a lock in the PMD (instead of a >>>>>> refcount) and still logically be using a lock instead of a refcount >>>>>> scheme. Remember modify here is "want to change a table pointer into a >>>>>> leaf pointer" so it isn't an every day activity.. >>>>> >>>>> It will be if we somewhat frequent when reclaim an empty PTE page table >>>>> as soon as it turns empty. This not only happens when zapping, but also >>>>> during writeback/swapping. So while writing back / swapping you might be >>>>> left with empty page tables to reclaim. >>>>> >>>>> Of course, this is the current approach. Another approach that doesn't >>>>> require additional refcounts is scanning page tables for empty ones and >>>>> reclaiming them. This scanning can either be triggered manually from >>>>> user space or automatically from the kernel. >>>> >>>> Whether it is introducing a special rwsem or scanning an empty page >>>> table, there are two problems as follows: >>>> >>>> #1. When to trigger the scanning or releasing? >>> >>> For example when reclaiming memory, when scanning page tables in >>> khugepaged, or triggered by user space (note that this is the approach I >>> originally looked into). But it certainly requires more locking thought >>> to avoid stopping essentially any page table walker. >>> >>>> #2. Every time to release a 4K page table page, 512 page table >>>> entries need to be scanned. >>> >>> It would happen only when actually trigger reclaim of page tables >>> (again, someone has to trigger it), so it's barely an issue. >>> >>> For example, khugepaged already scans the page tables either way. >>> >>>> >>>> For #1, if the scanning is triggered manually from user space, the >>>> kernel is relatively passive, and the user does not fully know the best >>>> timing to scan. If the scanning is triggered automatically from the >>>> kernel, that is great. But the timing is not easy to confirm, is it >>>> scanned and reclaimed every time zap or try_to_unmap? >>>> >>>> For #2, refcount has advantages. >>>> >>>>> >>>>>> >>>>>> There is some advantage with this thinking because it harmonizes well >>>>>> with the other stuff that wants to convert tables into leafs, but has >>>>>> to deal with complicated locking. >>>>>> >>>>>> On the other hand, refcounts are a degenerate kind of rwsem and only >>>>>> help with freeing pages. It also puts more atomics in normal fast >>>>>> paths since we are refcounting each PTE, not read locking the PMD. >>>>>> >>>>>> Perhaps the ideal thing would be to stick a rwsem in the PMD. read >>>>>> means a table cannot be come a leaf. I don't know if there is space >>>>>> for another atomic in the PMD level, and we'd have to use a hitching >>>>>> post/hashed waitq scheme too since there surely isn't room for a waitq >>>>>> too.. >>>>>> >>>>>> I wouldn't be so quick to say one is better than the other, but at >>>>>> least let's have thought about a locking solution before merging >>>>>> refcounts :) >>>>> >>>>> Yes, absolutely. I can see the beauty in the current approach, because >>>>> it just reclaims "automatically" once possible -- page table empty and >>>>> nobody is walking it. The downside is that it doesn't always make sense >>>>> to reclaim an empty page table immediately once it turns empty. >>>>> >>>>> Also, it adds complexity for something that is only a problem in some >>>>> corner cases -- sparse memory mappings, especially relevant for some >>>>> memory allocators after freeing a lot of memory or running VMs with >>>>> memory ballooning after inflating the balloon. Some of these use cases >>>>> might be good with just triggering page table reclaim manually from user >>>>> space. >>>>> >>>> >>>> Yes, this is indeed a problem. Perhaps some flags can be introduced so >>>> that the release of page table pages can be delayed in some cases. >>>> Similar to the lazyfree mechanism in MADV_FREE? >>> >>> The issue AFAIU is that once your refcount hits 0 (no more references, >>> no more entries), the longer you wait with reclaim, the longer others >>> have to wait for populating a fresh page table because the "page table >>> to be reclaimed" is still stuck around. You'd have to keep the refcount >>> increased for a while, and only drop it after a while. But when? And >>> how? IMHO it's not trivial, but maybe there is an easy way to achieve it. >>> >> >> For running VMs with memory ballooning after inflating the balloon, is >> this a hot behavior? Even if it is, it is already facing the release and >> reallocation of physical pages. The overhead after introducing >> pte_refcount is that we need to release and re-allocate page table page. >> But 2MB physical pages only corresponds to 4KiB of PTE page table page. >> So maybe the overhead is not big. > > The cases that come to my mind are > > a) Swapping on shared memory with concurrent access > b) Reclaim on file-backed memory with concurrent access > c) Free page reporting as implemented by virtio-balloon > > In all of these cases, you can have someone immediately re-access the > page table and re-populate it. In the performance test shown on the cover, we repeatedly performed touch and madvise(MADV_DONTNEED) actions, which simulated the case you said above. We did find a small amount of performance regression, but I think it is acceptable, and no new perf hotspots have been added. > > For something mostly static (balloon inflation, memory allocator), it's > not that big of a deal I guess. >