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[209.132.183.28]) by mx.google.com with ESMTPS id i53si11199451qta.136.2019.06.26.01.11.24 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Wed, 26 Jun 2019 01:11:24 -0700 (PDT) Received-SPF: pass (google.com: domain of david@redhat.com designates 209.132.183.28 as permitted sender) client-ip=209.132.183.28; Authentication-Results: mx.google.com; spf=pass (google.com: domain of david@redhat.com designates 209.132.183.28 as permitted sender) smtp.mailfrom=david@redhat.com; dmarc=pass (p=NONE sp=NONE dis=NONE) header.from=redhat.com Received: from smtp.corp.redhat.com (int-mx04.intmail.prod.int.phx2.redhat.com [10.5.11.14]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 39AD4811D8; Wed, 26 Jun 2019 08:11:10 +0000 (UTC) Received: from [10.36.116.174] (ovpn-116-174.ams2.redhat.com [10.36.116.174]) by smtp.corp.redhat.com (Postfix) with ESMTP id 14D545D9C6; Wed, 26 Jun 2019 08:11:06 +0000 (UTC) Subject: Re: [PATCH v2 0/5] Allocate memmap from hotadded memory To: Oscar Salvador Cc: akpm@linux-foundation.org, mhocko@suse.com, dan.j.williams@intel.com, pasha.tatashin@soleen.com, Jonathan.Cameron@huawei.com, anshuman.khandual@arm.com, vbabka@suse.cz, linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <20190625075227.15193-1-osalvador@suse.de> <2ebfbd36-11bd-9576-e373-2964c458185b@redhat.com> <20190626080249.GA30863@linux> From: David Hildenbrand Openpgp: preference=signencrypt Autocrypt: addr=david@redhat.com; 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Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.7.0 MIME-Version: 1.0 In-Reply-To: <20190626080249.GA30863@linux> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 8bit X-Scanned-By: MIMEDefang 2.79 on 10.5.11.14 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.27]); Wed, 26 Jun 2019 08:11:23 +0000 (UTC) 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 26.06.19 10:03, Oscar Salvador wrote: > On Tue, Jun 25, 2019 at 10:25:48AM +0200, David Hildenbrand wrote: >>> [Coverletter] >>> >>> This is another step to make memory hotplug more usable. The primary >>> goal of this patchset is to reduce memory overhead of the hot-added >>> memory (at least for SPARSEMEM_VMEMMAP memory model). The current way we use >>> to populate memmap (struct page array) has two main drawbacks: > > First off, thanks for looking into this :-) Thanks for working on this ;) > >> >> Mental note: How will it be handled if a caller specifies "Allocate >> memmap from hotadded memory", but we are running under SPARSEMEM where >> we can't do this. > > In add_memory_resource(), we have a call to mhp_check_correct_flags(), which is > in charge of checking if the flags passed are compliant with our configuration > among other things. > It also checks if both flags were passed (_MEMBLOCK|_DEVICE). > > If a) any of the flags were specified and we are not on CONFIG_SPARSEMEM_VMEMMAP, > b) the flags are colliding with each other or c) the flags just do not make sense, > we print out a warning and drop the flags to 0, so we just ignore them. > > I just realized that I can adjust the check even more (something for the next > version). > > But to answer your question, flags are ignored under !CONFIG_SPARSEMEM_VMEMMAP. So it is indeed a hint only. > >> >>> >>> a) it consumes an additional memory until the hotadded memory itself is >>> onlined and >>> b) memmap might end up on a different numa node which is especially true >>> for movable_node configuration. >>> >>> a) it is a problem especially for memory hotplug based memory "ballooning" >>> solutions when the delay between physical memory hotplug and the >>> onlining can lead to OOM and that led to introduction of hacks like auto >>> onlining (see 31bc3858ea3e ("memory-hotplug: add automatic onlining >>> policy for the newly added memory")). >>> >>> b) can have performance drawbacks. >>> >>> Another minor case is that I have seen hot-add operations failing on archs >>> because they were running out of order-x pages. >>> E.g On powerpc, in certain configurations, we use order-8 pages, >>> and given 64KB base pagesize, that is 16MB. >>> If we run out of those, we just fail the operation and we cannot add >>> more memory. >> >> At least for SPARSEMEM, we fallback to vmalloc() to work around this >> issue. I haven't looked into the populate_section_memmap() internals >> yet. Can you point me at the code that performs this allocation? > > Yes, on SPARSEMEM we first try to allocate the pages physical configuous, and > then fallback to vmalloc. > This is because on CONFIG_SPARSEMEM memory model, the translations pfn_to_page/ > page_to_pfn do not expect the memory to be contiguous. > > But that is not the case on CONFIG_SPARSEMEM_VMEMMAP. > We do expect the memory to be physical contigous there, that is why a simply > pfn_to_page/page_to_pfn is a matter of adding/substracting vmemmap/pfn. Yeas, I explored that last week but didn't figure out where the actual vmmap population code resided - thanks :) > > Powerpc code is at: > > https://elixir.bootlin.com/linux/v5.2-rc6/source/arch/powerpc/mm/init_64.c#L175 > > > >> So, assuming we add_memory(1GB, MHP_MEMMAP_DEVICE) and then >> remove_memory(128MB) of the added memory, this will work? > > No, MHP_MEMMAP_DEVICE is meant to be used when hot-adding and hot-removing work > in the same granularity. > This is because all memmap pages will be stored at the beginning of the memory > range. > Allowing hot-removing in a different granularity on MHP_MEMMAP_DEVICE would imply > a lot of extra work. > For example, we would have to parse the vmemmap-head of the hot-removed range, > and punch a hole in there to clear the vmemmap pages, and then be very carefull > when deleting those pagetables. > > So I followed Michal's advice, and I decided to let the caller specify if he > either wants to allocate per memory block or per hot-added range(device). > Where per memory block, allows us to do: > > add_memory(1GB, MHP_MEMMAP_MEMBLOCK) > remove_memory(128MB) Back then, I already mentioned that we might have some users that remove_memory() they never added in a granularity it wasn't added. My concerns back then were never fully sorted out. arch/powerpc/platforms/powernv/memtrace.c - Will remove memory in memory block size chunks it never added - What if that memory resides on a DIMM added via MHP_MEMMAP_DEVICE? Will it at least bail out? Or simply break? IOW: I am not yet 100% convinced that MHP_MEMMAP_DEVICE is save to be introduced. -- Thanks, David / dhildenb