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Mon, 23 Nov 2020 06:16:55 -0500 Received: from mail-pg1-x542.google.com (mail-pg1-x542.google.com [IPv6:2607:f8b0:4864:20::542]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 3060CC061A4D for ; Mon, 23 Nov 2020 03:16:55 -0800 (PST) Received: by mail-pg1-x542.google.com with SMTP id t3so120969pgi.11 for ; Mon, 23 Nov 2020 03:16:55 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=bytedance-com.20150623.gappssmtp.com; s=20150623; h=mime-version:references:in-reply-to:from:date:message-id:subject:to :cc; bh=Jn8HTrSXhJ3d+fruqsHTFEas0FFS58k7ISvrDAPBJHg=; b=NNe5kOs+ZXC/nYCicSgXRLcRBZomI3DtcCTzy0ZyPPNedq8SOTU2wK9I0WtDgnqMos z4hW+w1FBImuMqZe/pxDj0g8nc7utDnVtWLObYkB9KXTPv4sWlb6G09tc/b1A04PPDXp GnHlf75BaRSkgr+gKMA5wvMAkr/Ey01OQBORVfaOVTl89HaqYZX6xiKnZeifWbSCbnye lemkXLe1DwgD7zB14qSEcRzj7WePkko9gFFeGxC/0jovAap1exa2j78sGpd4Td2kKF8U qeflptWoWISUCG6AHdMJHnFOulXgiXKsW/m7d8OqC0KNkym/MN37LeZLAEMU47Txmz/5 pyzQ== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:mime-version:references:in-reply-to:from:date :message-id:subject:to:cc; bh=Jn8HTrSXhJ3d+fruqsHTFEas0FFS58k7ISvrDAPBJHg=; b=bT4EgpTJKH1pRRX/iyITGKJNrLLF5V40Yld4S4GoilegJ8maGlRUXvALiVf4f2b6sr pYB/tRNdIeP+HJkLzog6qq74RMopcrULUDcDD2EWU8mkQbTnezfc7WoMO3OY4MRXtdWr JKDIlHGoyBALMeBG+YWVOFPti9UKkjVMALgdv3Oo/LkR9/FwgTFloXQZqFT9pFK79mZn MLpjLZaZnIPP32AJTucm9wsnxyLm4p3kLihWH2a/uIRpepslxNlnGxBQV1C4323tW9jn TkAlWBChohYe1sA7YICKkMxVPoVjUHr+19gILZ2tFZBmwFmBFZzE7enB3I6nm+7OYmnw PFuQ== X-Gm-Message-State: AOAM533PQinPxEvZ/YI2VzfRPE7lixk36pLrCIvugC2V/iojfPOgklA2 cA6+cljSTidEmSa85Nnf3MaiN4F61RQ4zR5OYV3nbg== X-Google-Smtp-Source: ABdhPJzRl4/HYHkIMffGf3YrFo/fcZ2oJNK8lGDMNHUkSLC1MnMQzjhnxEBa+jaOJ49fU+TxgH8jW0bQjamgqJEKGiw= X-Received: by 2002:a17:90b:88b:: with SMTP id bj11mr25042012pjb.229.1606130214736; Mon, 23 Nov 2020 03:16:54 -0800 (PST) MIME-Version: 1.0 References: <20201120064325.34492-1-songmuchun@bytedance.com> <20201120084202.GJ3200@dhcp22.suse.cz> <20201120131129.GO3200@dhcp22.suse.cz> <20201123074046.GB27488@dhcp22.suse.cz> <20201123094344.GG27488@dhcp22.suse.cz> <20201123104258.GJ27488@dhcp22.suse.cz> In-Reply-To: <20201123104258.GJ27488@dhcp22.suse.cz> From: Muchun Song Date: Mon, 23 Nov 2020 19:16:18 +0800 Message-ID: Subject: Re: [External] Re: [PATCH v5 00/21] Free some vmemmap pages of hugetlb page To: Michal Hocko Cc: Jonathan Corbet , Mike Kravetz , Thomas Gleixner , mingo@redhat.com, bp@alien8.de, x86@kernel.org, hpa@zytor.com, dave.hansen@linux.intel.com, luto@kernel.org, Peter Zijlstra , viro@zeniv.linux.org.uk, Andrew Morton , paulmck@kernel.org, mchehab+huawei@kernel.org, pawan.kumar.gupta@linux.intel.com, Randy Dunlap , oneukum@suse.com, anshuman.khandual@arm.com, jroedel@suse.de, Mina Almasry , David Rientjes , Matthew Wilcox , Oscar Salvador , "Song Bao Hua (Barry Song)" , Xiongchun duan , linux-doc@vger.kernel.org, LKML , Linux Memory Management List , linux-fsdevel Content-Type: text/plain; charset="UTF-8" Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Nov 23, 2020 at 6:43 PM Michal Hocko wrote: > > On Mon 23-11-20 18:36:33, Muchun Song wrote: > > On Mon, Nov 23, 2020 at 5:43 PM Michal Hocko wrote: > > > > > > On Mon 23-11-20 16:53:53, Muchun Song wrote: > > > > On Mon, Nov 23, 2020 at 3:40 PM Michal Hocko wrote: > > > > > > > > > > On Fri 20-11-20 23:44:26, Muchun Song wrote: > > > > > > On Fri, Nov 20, 2020 at 9:11 PM Michal Hocko wrote: > > > > > > > > > > > > > > On Fri 20-11-20 20:40:46, Muchun Song wrote: > > > > > > > > On Fri, Nov 20, 2020 at 4:42 PM Michal Hocko wrote: > > > > > > > > > > > > > > > > > > On Fri 20-11-20 14:43:04, Muchun Song wrote: > > > > > > > > > [...] > > > > > > > > > > > > > > > > > > Thanks for improving the cover letter and providing some numbers. I have > > > > > > > > > only glanced through the patchset because I didn't really have more time > > > > > > > > > to dive depply into them. > > > > > > > > > > > > > > > > > > Overall it looks promissing. To summarize. I would prefer to not have > > > > > > > > > the feature enablement controlled by compile time option and the kernel > > > > > > > > > command line option should be opt-in. I also do not like that freeing > > > > > > > > > the pool can trigger the oom killer or even shut the system down if no > > > > > > > > > oom victim is eligible. > > > > > > > > > > > > > > > > Hi Michal, > > > > > > > > > > > > > > > > I have replied to you about those questions on the other mail thread. > > > > > > > > > > > > > > > > Thanks. > > > > > > > > > > > > > > > > > > > > > > > > > > One thing that I didn't really get to think hard about is what is the > > > > > > > > > effect of vmemmap manipulation wrt pfn walkers. pfn_to_page can be > > > > > > > > > invalid when racing with the split. How do we enforce that this won't > > > > > > > > > blow up? > > > > > > > > > > > > > > > > This feature depends on the CONFIG_SPARSEMEM_VMEMMAP, > > > > > > > > in this case, the pfn_to_page can work. The return value of the > > > > > > > > pfn_to_page is actually the address of it's struct page struct. > > > > > > > > I can not figure out where the problem is. Can you describe the > > > > > > > > problem in detail please? Thanks. > > > > > > > > > > > > > > struct page returned by pfn_to_page might get invalid right when it is > > > > > > > returned because vmemmap could get freed up and the respective memory > > > > > > > released to the page allocator and reused for something else. See? > > > > > > > > > > > > If the HugeTLB page is already allocated from the buddy allocator, > > > > > > the struct page of the HugeTLB can be freed? Does this exist? > > > > > > > > > > Nope, struct pages only ever get deallocated when the respective memory > > > > > (they describe) is hotremoved via hotplug. > > > > > > > > > > > If yes, how to free the HugeTLB page to the buddy allocator > > > > > > (cannot access the struct page)? > > > > > > > > > > But I do not follow how that relates to my concern above. > > > > > > > > Sorry. I shouldn't understand your concerns. > > > > > > > > vmemmap pages page frame > > > > +-----------+ mapping to +-----------+ > > > > | | -------------> | 0 | > > > > +-----------+ +-----------+ > > > > | | -------------> | 1 | > > > > +-----------+ +-----------+ > > > > | | -------------> | 2 | > > > > +-----------+ +-----------+ > > > > | | -------------> | 3 | > > > > +-----------+ +-----------+ > > > > | | -------------> | 4 | > > > > +-----------+ +-----------+ > > > > | | -------------> | 5 | > > > > +-----------+ +-----------+ > > > > | | -------------> | 6 | > > > > +-----------+ +-----------+ > > > > | | -------------> | 7 | > > > > +-----------+ +-----------+ > > > > > > > > In this patch series, we will free the page frame 2-7 to the > > > > buddy allocator. You mean that pfn_to_page can return invalid > > > > value when the pfn is the page frame 2-7? Thanks. > > > > > > No I really mean that pfn_to_page will give you a struct page pointer > > > from pages which you release from the vmemmap page tables. Those pages > > > might get reused as soon sa they are freed to the page allocator. > > > > We will remap vmemmap pages 2-7 (virtual addresses) to page > > frame 1. And then we free page frame 2-7 to the buddy allocator. > > And this doesn't really happen in an atomic fashion from the pfn walker > POV, right? So it is very well possible that Yeah, you are right. But it may not be a problem for HugeTLB pages. Because in most cases, we only read the tail struct page and get the head struct page through compound_head() when the pfn is within a HugeTLB range. Right? > > struct page *page = pfn_to_page(); > // remapping happens here > // page content is no longer valid because its backing memory can be If we only read the page->compound_head. The content is also valid. Because the value of compound_head is the same for the tail page struct of HugeTLB page. > // reused for whatever purpose. > -- > Michal Hocko > SUSE Labs -- Yours, Muchun