From: Michal Hocko <mhocko@suse.com>
To: Oscar Salvador <osalvador@suse.de>
Cc: Andrew Morton <akpm@linux-foundation.org>,
David Hildenbrand <david@redhat.com>,
Anshuman Khandual <anshuman.khandual@arm.com>,
Vlastimil Babka <vbabka@suse.cz>,
Pavel Tatashin <pasha.tatashin@soleen.com>,
linux-mm@kvack.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v5 3/5] mm,memory_hotplug: Add kernel boot option to enable memmap_on_memory
Date: Wed, 24 Mar 2021 10:02:59 +0100 [thread overview]
Message-ID: <YFsAQxZFCE0L3Sor@dhcp22.suse.cz> (raw)
In-Reply-To: <20210324084457.GA16560@linux>
On Wed 24-03-21 09:45:01, Oscar Salvador wrote:
> On Tue, Mar 23, 2021 at 11:47:53AM +0100, Michal Hocko wrote:
> > On Fri 19-03-21 10:26:33, Oscar Salvador wrote:
> > > Self stored memmap leads to a sparse memory situation which is unsuitable
> > > for workloads that requires large contiguous memory chunks, so make this
> > > an opt-in which needs to be explicitly enabled.
> > >
> > > To control this, let memory_hotplug have its own memory space, as suggested
> > > by David, so we can add memory_hotplug.memmap_on_memory parameter.
> > >
> > > Signed-off-by: Oscar Salvador <osalvador@suse.de>
> > > Reviewed-by: David Hildenbrand <david@redhat.com>
> >
> > Acked-by: Michal Hocko <mhocko@suse.com>
> >
> > I would just rephrased the help text to be less low level
> ...
> > When enabled, runtime hotplugged memory will
> > allocate its internal metadata (struct pages)
> > from the hotadded memory which will allow to
> > hotadd a lot of memory without requiring
> > additional memory to do so.
> > This feature is disabled by default because it
> > has some implication on large (e.g. GB)
> > allocations in some configurations (e.g. small
> > memory blocks).
>
> Ok, this sounds good as well, and I guess it might suit best for what admin-guide
> is about.
>
> > The memmap_on_memory can be dropped from the 1st patch IIUC and only
> > introduce it now.
>
> It could be done, and I __think__ in some previous persion it was that way, but
> I am leaning to not do it.
> In the 1st patch, memmap_on_memory is false by default, so I see it as a preparatory
> step for later (this patchset) till it might be enabled.
>
> Moreover, the big comment from mhp_support_memmap_on_memory() should change to not
> mention it, and change here again to reflect it.
>
> All in all, I think it can stay, but maybe place a comment in the 1st patch above
> the variable saying something like "This is a noop now, it will be enabled later on"
I will leave that up to you. This is likely not worth a larger
discussion but it seems quite pointless to add a variable which never
changes. The resulting code might look different than you expect because
compiler is allowed to simply drop the whole condition.
> > > +
> > > +/*
> > > + * memory_hotplug.memmap_on_memory parameter
> > > + */
> > > +static bool memmap_on_memory __ro_after_init;
> > > +#ifdef CONFIG_MHP_MEMMAP_ON_MEMORY
> > > +module_param(memmap_on_memory, bool, 0444);
> > > +MODULE_PARM_DESC(memmap_on_memory, "Enable memmap on memory for memory hotplug");
> > > +#endif
> >
> > I am not very much familiar with the machinery. Does this expose the
> > state to the userspace?
>
> Kind of:
>
> # ls /sys/module/memory_hotplug/parameters
> memmap_on_memory
> # cat /sys/module/memory_hotplug/parameters/memmap_on_memory
> Y
>
> But that is not really the state, but rather it shows whether the user
> opted-in the feature by passing "memory_hotplug.memmap_on_memory=yes".
> It might be that the user opted-in the feature, but it cannot be used at
> at runtime (e.g: mhp_support_memmap_on_memory() return false due to size !=
> memory_block_size())
Thanks for the clarification.
--
Michal Hocko
SUSE Labs
next prev parent reply other threads:[~2021-03-24 9:03 UTC|newest]
Thread overview: 63+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-03-19 9:26 [PATCH v5 0/5] Allocate memmap from hotadded memory (per device) Oscar Salvador
2021-03-19 9:26 ` [PATCH v5 1/5] mm,memory_hotplug: Allocate memmap from the added memory range Oscar Salvador
2021-03-19 10:20 ` David Hildenbrand
2021-03-19 10:31 ` Oscar Salvador
2021-03-19 12:04 ` David Hildenbrand
2021-03-23 10:11 ` Michal Hocko
2021-03-24 10:12 ` Oscar Salvador
2021-03-24 12:03 ` Michal Hocko
2021-03-24 12:10 ` Michal Hocko
2021-03-24 12:23 ` David Hildenbrand
2021-03-24 12:37 ` Michal Hocko
2021-03-24 13:13 ` David Hildenbrand
2021-03-24 13:40 ` Michal Hocko
2021-03-24 14:05 ` David Hildenbrand
2021-03-24 13:27 ` Oscar Salvador
2021-03-24 14:42 ` Michal Hocko
2021-03-24 14:52 ` David Hildenbrand
2021-03-24 16:04 ` Michal Hocko
2021-03-24 19:16 ` David Hildenbrand
2021-03-25 8:07 ` Oscar Salvador
2021-03-25 9:17 ` Michal Hocko
2021-03-25 10:55 ` Oscar Salvador
2021-03-25 11:08 ` David Hildenbrand
2021-03-25 11:23 ` Oscar Salvador
2021-03-25 12:35 ` Michal Hocko
2021-03-25 12:40 ` David Hildenbrand
2021-03-25 14:08 ` Michal Hocko
2021-03-25 14:09 ` David Hildenbrand
2021-03-25 14:34 ` Michal Hocko
2021-03-25 14:46 ` David Hildenbrand
2021-03-25 15:12 ` Michal Hocko
2021-03-25 15:19 ` David Hildenbrand
2021-03-25 15:35 ` Michal Hocko
2021-03-25 15:40 ` David Hildenbrand
2021-03-25 16:07 ` Michal Hocko
2021-03-25 16:20 ` David Hildenbrand
2021-03-25 16:36 ` Michal Hocko
2021-03-25 16:47 ` Michal Hocko
2021-03-25 16:55 ` David Hildenbrand
2021-03-25 22:06 ` Oscar Salvador
2021-03-26 8:35 ` Michal Hocko
2021-03-26 8:52 ` David Hildenbrand
2021-03-26 8:57 ` Oscar Salvador
2021-03-26 12:15 ` Oscar Salvador
2021-03-26 13:36 ` David Hildenbrand
2021-03-26 14:38 ` Michal Hocko
2021-03-26 14:53 ` David Hildenbrand
2021-03-26 15:31 ` Michal Hocko
2021-03-26 16:03 ` David Hildenbrand
2021-03-26 8:55 ` Oscar Salvador
2021-03-26 9:11 ` Michal Hocko
2021-03-25 18:08 ` David Hildenbrand
2021-03-25 12:26 ` Michal Hocko
2021-03-25 14:02 ` Oscar Salvador
2021-03-25 14:40 ` Michal Hocko
2021-03-19 9:26 ` [PATCH v5 2/5] acpi,memhotplug: Enable MHP_MEMMAP_ON_MEMORY when supported Oscar Salvador
2021-03-23 10:40 ` Michal Hocko
2021-03-19 9:26 ` [PATCH v5 3/5] mm,memory_hotplug: Add kernel boot option to enable memmap_on_memory Oscar Salvador
2021-03-23 10:47 ` Michal Hocko
2021-03-24 8:45 ` Oscar Salvador
2021-03-24 9:02 ` Michal Hocko [this message]
2021-03-19 9:26 ` [PATCH v5 4/5] x86/Kconfig: Introduce ARCH_MHP_MEMMAP_ON_MEMORY_ENABLE Oscar Salvador
2021-03-19 9:26 ` [PATCH v5 5/5] arm64/Kconfig: " Oscar Salvador
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