linux-kernel - Re: [PATCH v2] mm/memory_hotplug: Don't allow to online/offline memory blocks with holes

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Date:   Mon, 25 Nov 2019 19:22:48 +0100
From:   David Hildenbrand <david@...hat.com>
To:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Dan Williams <dan.j.williams@...el.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Michal Hocko <mhocko@...e.com>,
        Oscar Salvador <osalvador@...e.de>,
        Pavel Tatashin <pasha.tatashin@...een.com>,
        Anshuman Khandual <anshuman.khandual@....com>
Subject: Re: [PATCH v2] mm/memory_hotplug: Don't allow to online/offline
 memory blocks with holes

On 25.11.19 18:40, Damian Tometzki wrote:
> On Tue, 19. Nov 12:52, David Hildenbrand wrote:
>> Our onlining/offlining code is unnecessarily complicated. Only memory
>> blocks added during boot can have holes (a range that is not
>> IORESOURCE_SYSTEM_RAM). Hotplugged memory never has holes (e.g., see
>> add_memory_resource()). All memory blocks that belong to boot memory are
>> already online.
>>
>> Note that boot memory can have holes and the memmap of the holes is marked
>> PG_reserved. However, also memory allocated early during boot is
>> PG_reserved - basically every page of boot memory that is not given to the
>> buddy is PG_reserved.
>>
>> Therefore, when we stop allowing to offline memory blocks with holes, we
>> implicitly no longer have to deal with onlining memory blocks with holes.
>> E.g., online_pages() will do a
>> walk_system_ram_range(..., online_pages_range), whereby
>> online_pages_range() will effectively only free the memory holes not
>> falling into a hole to the buddy. The other pages (holes) are kept
>> PG_reserved (via move_pfn_range_to_zone()->memmap_init_zone()).
>>
>> This allows to simplify the code. For example, we no longer have to
>> worry about marking pages that fall into memory holes PG_reserved when
>> onlining memory. We can stop setting pages PG_reserved completely in
>> memmap_init_zone().
>>
>> Offlining memory blocks added during boot is usually not guaranteed to work
>> either way (unmovable data might have easily ended up on that memory during
>> boot). So stopping to do that should not really hurt. Also, people are not
>> even aware of a setup where onlining/offlining of memory blocks with
>> holes used to work reliably (see [1] and [2] especially regarding the
>> hotplug path) - I doubt it worked reliably.
>>
>> For the use case of offlining memory to unplug DIMMs, we should see no
>> change. (holes on DIMMs would be weird).
>>
>> Please note that hardware errors (PG_hwpoison) are not memory holes and
>> are not affected by this change when offlining.
>>
>> [1] https://lkml.org/lkml/2019/10/22/135
>> [2] https://lkml.org/lkml/2019/8/14/1365
>>
>> Reviewed-by: Dan Williams <dan.j.williams@...el.com>
>> Cc: Andrew Morton <akpm@...ux-foundation.org>
>> Cc: Michal Hocko <mhocko@...e.com>
>> Cc: Oscar Salvador <osalvador@...e.de>
>> Cc: Pavel Tatashin <pasha.tatashin@...een.com>
>> Cc: Dan Williams <dan.j.williams@...el.com>
>> Cc: Anshuman Khandual <anshuman.khandual@....com>
>> Signed-off-by: David Hildenbrand <david@...hat.com>
>> ---
>>
>> This patch was part of:
>> 	[PATCH v1 00/10] mm: Don't mark hotplugged pages PG_reserved
>> 	(including ZONE_DEVICE)
>> 	-> https://www.spinics.net/lists/linux-driver-devel/msg130042.html
>>
>> However, before we can perform the PG_reserved changes, we have to fix
>> pfn_to_online_page() in special scenarios first (bootmem and devmem falling
>> into a single section). Dan is working on that.
>>
>> I propose to give this patch a churn in -next so we can identify if this
>> change would break any existing setup. I will then follow up with cleanups
>> and the PG_reserved changes later.
>>
>> ---
>>  mm/memory_hotplug.c | 28 ++++++++++++++++++++++++++--
>>  1 file changed, 26 insertions(+), 2 deletions(-)
>>
>> diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
>> index 46b2e056a43f..fc617ad6f035 100644
>> --- a/mm/memory_hotplug.c
>> +++ b/mm/memory_hotplug.c
>> @@ -1455,10 +1455,19 @@ static void node_states_clear_node(int node, struct memory_notify *arg)
>>  		node_clear_state(node, N_MEMORY);
>>  }
>>  
>> +static int count_system_ram_pages_cb(unsigned long start_pfn,
>> +				     unsigned long nr_pages, void *data)
>> +{
>> +	unsigned long *nr_system_ram_pages = data;
>> +
>> +	*nr_system_ram_pages += nr_pages;
>> +	return 0;
>> +}
>> +
> Hello David,
> 
> what is the meaning of the function ? The return is every time 0. 
> 
> I dont understand it ?
> 

Hi Damian,

please see how these callbacks are used within walk_system_ram_range().
A return value of 0 only means "don't stop iterating", the actual values
are returned via the "void *data" parameter in this instance.

Cheers!

-- 

Thanks,

David / dhildenb