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Subject: Re: [PATCH v2 4/8] mm/memory_hotplug: Create memory block devices
 after arch_add_memory()
To:     Wei Yang <richard.weiyang@gmail.com>
Cc:     linux-mm@kvack.org, linux-kernel@vger.kernel.org,
        linux-ia64@vger.kernel.org, linuxppc-dev@lists.ozlabs.org,
        linux-s390@vger.kernel.org, linux-sh@vger.kernel.org,
        akpm@linux-foundation.org, Dan Williams <dan.j.williams@intel.com>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        "Rafael J. Wysocki" <rafael@kernel.org>,
        "mike.travis@hpe.com" <mike.travis@hpe.com>,
        Ingo Molnar <mingo@kernel.org>,
        Andrew Banman <andrew.banman@hpe.com>,
        Oscar Salvador <osalvador@suse.de>,
        Michal Hocko <mhocko@suse.com>,
        Pavel Tatashin <pasha.tatashin@soleen.com>,
        Qian Cai <cai@lca.pw>, Arun KS <arunks@codeaurora.org>,
        Mathieu Malaterre <malat@debian.org>
References: <20190507183804.5512-1-david@redhat.com>
 <20190507183804.5512-5-david@redhat.com>
 <20190509135533.6xok3v7rxxaohc77@master>
From:   David Hildenbrand <david@redhat.com>
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Message-ID: <a8e96df6-dc6d-037f-491c-92182d4ada8d@redhat.com>
Date:   Thu, 9 May 2019 16:05:29 +0200
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On 09.05.19 15:55, Wei Yang wrote:
> On Tue, May 07, 2019 at 08:38:00PM +0200, David Hildenbrand wrote:
>> Only memory to be added to the buddy and to be onlined/offlined by
>> user space using memory block devices needs (and should have!) memory
>> block devices.
>>
>> Factor out creation of memory block devices Create all devices after
>> arch_add_memory() succeeded. We can later drop the want_memblock parameter,
>> because it is now effectively stale.
>>
>> Only after memory block devices have been added, memory can be onlined
>> by user space. This implies, that memory is not visible to user space at
>> all before arch_add_memory() succeeded.
>>
>> Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
>> Cc: "Rafael J. Wysocki" <rafael@kernel.org>
>> Cc: David Hildenbrand <david@redhat.com>
>> Cc: "mike.travis@hpe.com" <mike.travis@hpe.com>
>> Cc: Andrew Morton <akpm@linux-foundation.org>
>> Cc: Ingo Molnar <mingo@kernel.org>
>> Cc: Andrew Banman <andrew.banman@hpe.com>
>> Cc: Oscar Salvador <osalvador@suse.de>
>> Cc: Michal Hocko <mhocko@suse.com>
>> Cc: Pavel Tatashin <pasha.tatashin@soleen.com>
>> Cc: Qian Cai <cai@lca.pw>
>> Cc: Wei Yang <richard.weiyang@gmail.com>
>> Cc: Arun KS <arunks@codeaurora.org>
>> Cc: Mathieu Malaterre <malat@debian.org>
>> Signed-off-by: David Hildenbrand <david@redhat.com>
>> ---
>> drivers/base/memory.c  | 70 ++++++++++++++++++++++++++----------------
>> include/linux/memory.h |  2 +-
>> mm/memory_hotplug.c    | 15 ++++-----
>> 3 files changed, 53 insertions(+), 34 deletions(-)
>>
>> diff --git a/drivers/base/memory.c b/drivers/base/memory.c
>> index 6e0cb4fda179..862c202a18ca 100644
>> --- a/drivers/base/memory.c
>> +++ b/drivers/base/memory.c
>> @@ -701,44 +701,62 @@ static int add_memory_block(int base_section_nr)
>> 	return 0;
>> }
>>
>> +static void unregister_memory(struct memory_block *memory)
>> +{
>> +	BUG_ON(memory->dev.bus != &memory_subsys);
>> +
>> +	/* drop the ref. we got via find_memory_block() */
>> +	put_device(&memory->dev);
>> +	device_unregister(&memory->dev);
>> +}
>> +
>> /*
>> - * need an interface for the VM to add new memory regions,
>> - * but without onlining it.
>> + * Create memory block devices for the given memory area. Start and size
>> + * have to be aligned to memory block granularity. Memory block devices
>> + * will be initialized as offline.
>>  */
>> -int hotplug_memory_register(int nid, struct mem_section *section)
>> +int hotplug_memory_register(unsigned long start, unsigned long size)
>> {
>> -	int ret = 0;
>> +	unsigned long block_nr_pages = memory_block_size_bytes() >> PAGE_SHIFT;
>> +	unsigned long start_pfn = PFN_DOWN(start);
>> +	unsigned long end_pfn = start_pfn + (size >> PAGE_SHIFT);
>> +	unsigned long pfn;
>> 	struct memory_block *mem;
>> +	int ret = 0;
>>
>> -	mutex_lock(&mem_sysfs_mutex);
>> +	BUG_ON(!IS_ALIGNED(start, memory_block_size_bytes()));
>> +	BUG_ON(!IS_ALIGNED(size, memory_block_size_bytes()));
>>
>> -	mem = find_memory_block(section);
>> -	if (mem) {
>> -		mem->section_count++;
>> -		put_device(&mem->dev);
>> -	} else {
>> -		ret = init_memory_block(&mem, section, MEM_OFFLINE);
>> +	mutex_lock(&mem_sysfs_mutex);
>> +	for (pfn = start_pfn; pfn != end_pfn; pfn += block_nr_pages) {
>> +		mem = find_memory_block(__pfn_to_section(pfn));
>> +		if (mem) {
>> +			WARN_ON_ONCE(false);
> 
> One question here, the purpose of WARN_ON_ONCE(false) is? Would we trigger
> this?

Would happen if something goes terribly wrong. We might want to remove
this once we are sure this will not happen.

I replaced it in the meantime by a

if (WARN_ON_ONCE(mem)) {
	put_device(&mem->dev);
	ret = -EEXIST;
	break;
}

> 
>> +			put_device(&mem->dev);
>> +			continue;
>> +		}
>> +		ret = init_memory_block(&mem, __pfn_to_section(pfn),
>> +					MEM_OFFLINE);
>> 		if (ret)
>> -			goto out;
>> -		mem->section_count++;
>> +			break;
>> +		mem->section_count = memory_block_size_bytes() /
>> +				     MIN_MEMORY_BLOCK_SIZE;
> 
> Maybe we can leverage sections_per_block variable.

Most certainly if it does what I think it does :) thanks!


-- 

Thanks,

David / dhildenb