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[2003:cb:c705:f700:352b:d857:b95d:9072]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-4316f5c311bsm81820335e9.29.2024.10.22.02.33.07 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 22 Oct 2024 02:33:09 -0700 (PDT) Message-ID: <1238f2a3-88a2-4996-92f2-05735801002b@redhat.com> Date: Tue, 22 Oct 2024 11:33:07 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [RFC] Virtualizing tagged disaggregated memory capacity (app specific, multi host shared) To: Gregory Price Cc: Jonathan Cameron , linux-mm@kvack.org, linux-cxl@vger.kernel.org, Davidlohr Bueso , Ira Weiny , John Groves , virtualization@lists.linux.dev, Oscar Salvador , qemu-devel@nongnu.org, Dave Jiang , Dan Williams , linuxarm@huawei.com, wangkefeng.wang@huawei.com, John Groves , Fan Ni , Navneet Singh , =?UTF-8?B?4oCcTWljaGFlbCBTLiBUc2lya2lu4oCd?= , Igor Mammedov , =?UTF-8?Q?Philippe_Mathieu-Daud=C3=A9?= References: <20240815172223.00001ca7@Huawei.com> From: David Hildenbrand Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Rspamd-Server: rspam06 X-Rspamd-Queue-Id: 0F465A000A X-Stat-Signature: xiqpmxb6t4wx7uahrq5pejqsbc9kkiay X-Rspam-User: X-HE-Tag: 1729589577-471988 X-HE-Meta: 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 7XOpWMwv 4QyvztsACcuLtfreaCh4prC0WZmVwaBQ7hbq2YsiF7/+atFfznbUMMSqXplMD6S2NSMrfrP0dRdwuOUB0JZwD/dR5gMfV3W6fEVckauFrNqy+Yb5WFlA8m4rZ/wsHyWwBMy4O9fXa7SyORtkVREuOCaAOPsxOgH+SkwWW6BUJXJIHsAyQ1zss8I+ZzIf6tfiD+Jdofers4cXiIGNMa8yJk+772H711wS/gucAh6Uhy1AC0Y+QPrQab9kmw0P05SYIlAjSvSwR8LbXyLG4FiIIiQHMetHc5G7eAR9nkLodpy0/16iZNWi9Sd0CIUhAguBPRKtBsYKuFFb6wEUN51lZCkgdxencrW2RwnOjxBYPaRMwF8m+5i9pLTgAPfV50eG0r5pjCK96vVPY5Gy/dNqQ1WECAQ== 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: List-Subscribe: List-Unsubscribe: On 20.09.24 11:06, Gregory Price wrote: >>> 2. Coarse grained memory increases for 'normal' memory. >>> Can use memory hot-plug. Recovery of capacity likely to only be possible on >>> VM shutdown. >> >> Is there are reason "movable" (ZONE_MOVABLE) is not an option, at least in >> some setups? If not, why? >> > > > This seems like a bit of a muddied conversation. Cleaning up my inbox ... well at least trying :) > > "'normal' memory" has no defined meaning - so lets clear this up a bit > > There is: > * System-RAM (memory managed by kernel allocators) > * Special Purpose Memory (generally presented as DAX) > > System-RAM is managed as zones - the relevant ones are > * ZONE_NORMAL allows both movable and non-movable allocations .. except in corner cases like MIGRATE_CMA :) > * ZONE_MOVABLE only allows non-movable allocations > (Caveat: this generally only applies to allocation, you can > violate this with stuff like pinning) Note that long-term pinning is forbidden on MOVABLE, just like it is on MIGRATE_CMA. So we try that common use cases cannot violate this. > > Hotplug can be thought of as two discrete mechanisms > * Exposing capacity to the kernel (CXL DCD Transactions) > * Exposing capacity to allocators (mm/memory-hotplug.c) > > 1) if the intent is to primarily utilize dynamic capacity for VMs, then > the host does not need (read: should not need) to map the memory as > System-RAM in the host. The VMM should be made to consume it directly > via DAX or otherwise. > > That capacity is almost by definition "Capital G Guaranteed" to be > reclaimable regardless of what the guest does. A VMM can force a guest > to let go of resources - that's its job. > > 2) if the intent is to provide dynamic capacity to a host as System-RAM, then > recoverability is dictated by system usage of that capacity. If onlined > into ZONE_MOVABLE, then if the system has avoided doing things like pinning > those pages it should *generally* be recoverable (but not guaranteed). There is, of course, the use case of memory overcommit -- in which case you would want 2). But likely that's out of the picture for this tagged memory. > > > For the virtualization discussion: > > Hotplug and recoverability is a non-issue. The capacity should never be > exposed to system allocators and the VMM should be made to consume special > purpose memory directly. That's on the VMM/orchestration software to get right. > > > For the host System-RAM discussion: > > Auto-onlined hotplug capacity presently defaults to ZONE_NORMAL, but we > discussed (yesterday, at Plumbers) changing this default to ZONE_MOVABLE. > > The only concern is when insufficient ZONE_NORMAL exists to support > ZONE_MOVABLE capacity - but this is unlikely to be the general scenario AND > can be mitigated w/ existing mechanisms. It might be worthwhile looking at Documentation/admin-guide/mm/memory-hotplug.rst "auto-movable" memory onlining polciy. It might not fit all sue cases, though (just like ZONE_MOVABLE doesn't) > > Manually onlined capacity defaults to ZONE_MOVABLE. > > It would be nice to make this behavior consistent, since the general opinion > appears to be that this capacity should default to ZONE_MOVABLE. It's much easier to shoot yourself into the foot with ZONE_MOVABLE, that's why the default can be adjusted manually using "online_movable" with e.g., memhp_default_state. It's all a bit complicated, because there are various use cases and mechanisms for memory hotplug ... IIRC RHEL defaults with its udev rules to "ZONE_MOVABLE" on bare metal and "ZONE_NORMAL" in VMs. Except on s390, where we default to "offline" (standby memory ....). I once worked on a systemd unit to make this configuration easier (and avoid udev rules), and possibly more "automatic" depending on the detected environment. -- Cheers, David / dhildenb