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Wed, 24 Mar 2021 19:16:57 +0000 (UTC) Received: from [10.36.115.66] (ovpn-115-66.ams2.redhat.com [10.36.115.66]) by smtp.corp.redhat.com (Postfix) with ESMTP id DABA25D9CA; Wed, 24 Mar 2021 19:16:54 +0000 (UTC) To: Michal Hocko Cc: Oscar Salvador , Andrew Morton , Anshuman Khandual , Vlastimil Babka , Pavel Tatashin , linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <20210319092635.6214-1-osalvador@suse.de> <20210319092635.6214-2-osalvador@suse.de> <20210324101259.GB16560@linux> <3bc4168c-fd31-0c9a-44ac-88e25d524eef@redhat.com> From: David Hildenbrand Organization: Red Hat GmbH Subject: Re: [PATCH v5 1/5] mm,memory_hotplug: Allocate memmap from the added memory range Message-ID: <9591a0b8-c000-2f61-67a6-4402678fe50b@redhat.com> Date: Wed, 24 Mar 2021 20:16:53 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.8.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US X-Scanned-By: MIMEDefang 2.79 on 10.5.11.14 X-Stat-Signature: g3n9p4jpcu6gu8gy7dufbn6innqfkidi X-Rspamd-Server: rspam02 X-Rspamd-Queue-Id: 2AD4690009E9 Received-SPF: none (redhat.com>: No applicable sender policy available) receiver=imf19; identity=mailfrom; envelope-from=""; helo=us-smtp-delivery-124.mimecast.com; client-ip=170.10.133.124 X-HE-DKIM-Result: pass/pass X-HE-Tag: 1616613485-391474 Content-Transfer-Encoding: quoted-printable 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 24.03.21 17:04, Michal Hocko wrote: > On Wed 24-03-21 15:52:38, David Hildenbrand wrote: >> On 24.03.21 15:42, Michal Hocko wrote: >>> On Wed 24-03-21 13:03:29, Michal Hocko wrote: >>>> On Wed 24-03-21 11:12:59, Oscar Salvador wrote: >>> [...] >>>>> I kind of understand to be reluctant to use vmemmap_pages terminolo= gy here, but >>>>> unfortunately we need to know about it. >>>>> We could rename nr_vmemmap_pages to offset_buddy_pages or something= like that. >>>> >>>> I am not convinced. It seems you are justr trying to graft the new >>>> functionality in. But I still believe that {on,off}lining shouldn't = care >>>> about where their vmemmaps come from at all. It should be a >>>> responsibility of the code which reserves that space to compansate f= or >>>> accounting. Otherwise we will end up with a hard to maintain code >>>> because expectations would be spread at way too many places. Not to >>>> mention different pfns that the code should care about. >>> >>> The below is a quick hack on top of this patch to illustrate my >>> thinking. I have dug out all the vmemmap pieces out of the >>> {on,off}lining and hooked all the accounting when the space is reserv= ed. >>> This just compiles without any deeper look so there are likely some >>> minor problems but I haven't really encountered any major problems or >>> hacks to introduce into the code. The separation seems to be possible= . >>> The diffstat also looks promising. Am I missing something fundamental= in >>> this? >>> >> >> From a quick glimpse, this touches on two things discussed in the pas= t: >> >> 1. If the underlying memory block is offline, all sections are offline= . Zone >> shrinking code will happily skip over the vmemmap pages and you can en= d up >> with out-of-zone pages assigned to the zone. Can happen in corner case= s. >=20 > You are right. But do we really care? Those pages should be of no > interest to anybody iterating through zones/nodes anyway. Well, we were just discussing getting zone/node links + span right for=20 all pages (including for special reserved pages), because it already=20 resulted in BUGs. So I am not convinced that we *don't* have to care. However, I agree that most code that cares about node/zone spans=20 shouldn't care - e.g., never call set_pfnblock_flags_mask() on such block= s. But I guess there are corner cases where we would end up with=20 zone_is_empty() =3D=3D true, not sure what that effect would be ... at le= ast=20 the node cannot vanish as we disallow offlining it while we have a=20 memory block linked to it. Another thing that comes to my mind is that our zone shrinking code=20 currently searches in PAGES_PER_SUBSECTION (2 MiB IIRC) increments. In=20 case our vmemmap pages would be less than that, we could accidentally=20 shrink the !vmemmap part too much, as we are mis-detecting the type for=20 a PAGES_PER_SUBSECTION block. IIRC, this would apply for memory block sizes < 128 MiB. Not relevant on=20 x86 and arm64. Could be relevant for ppc64, if we'd ever want to support=20 memmap_on_memory there. Or if we'd ever reduce the section size on some=20 arch below 128 MiB. At least we would have to fence it somehow. >=20 >> There is no way to know that the memmap of these pages was initialized= and >> is of value. >> >> 2. You heavily fragment zone layout although you might end up with >> consecutive zones (e.g., online all hotplugged memory movable) >=20 > What would be consequences? IIRC, set_zone_contiguous() will leave zone->contiguous =3D false. This, in turn, will force pageblock_pfn_to_page() via the slow path,=20 turning page isolation a bit slower. Not a deal breaker, but obviously something where Oscar's original patch=20 can do better. I yet have to think again about other issues (I remember most issues we=20 discussed back then were related to having the vmemmap only within the=20 same memory block). I think 2) might be tolerable, although unfortunate.=20 Regarding 1), we'll have to dive into more details. --=20 Thanks, David / dhildenb