Re: [PATCH RFC] drivers/base/memory.c: indicate all memory blocks as removable

[David Hildenbrand <david@redhat.com>]

On 27.01.20 14:29, Michal Hocko wrote:
> On Fri 24-01-20 16:53:36, David Hildenbrand wrote:
>> We see multiple issues with the implementation/interface to compute
>> whether a memory block can be offlined (exposed via
>> /sys/devices/system/memory/memoryX/removable) and would like to simplify
>> it (remove the implementation).
>>
>> 1. It runs basically lockless. While this might be good for performance,
>>    we see possible races with memory offlining/unplug that will require
>>    at least some sort of locking to fix.
>>
>> 2. Nowadays, more false positives are possible. No arch-specific checks
>>    are performed that validate if memory offlining will not be denied
>>    right away (and such check will require locking). For example, arm64
>>    won't allow to offline any memory block that was added during boot -
>>    which will imply a very high error rate. Other archs have other
>>    constraints.
>>
>> 3. The interface is inherently racy. E.g., if a memory block is
>>    detected to be removable (and was not a false positive at that time),
>>    there is still no guarantee that offlining will actually succeed. So
>>    any caller already has to deal with false positives.
>>
>> 4. It is unclear which performance benefit this interface actually
>>    provides. The introducing commit 5c755e9fd813 ("memory-hotplug: add
>>    sysfs removable attribute for hotplug memory remove") mentioned
>> 	"A user-level agent must be able to identify which sections of
>> 	 memory are likely to be removable before attempting the
>> 	 potentially expensive operation."
>>    However, no actual performance comparison was included.
>>
>> Known users:
>> - lsmem: Will group memory blocks based on the "removable" property. [1]
>> - chmem: Indirect user. It has a RANGE mode where one can specify
>> 	 removable ranges identified via lsmem to be offlined. However, it
>> 	 also has a "SIZE" mode, which allows a sysadmin to skip the manual
>> 	 "identify removable blocks" step. [2]
>> - powerpc-utils: Uses the "removable" attribute to skip some memory
>> 		 blocks right away when trying to find some to
>> 		 offline+remove. However, with ballooning enabled, it
>> 		 already skips this information completely (because it
>> 		 once resulted in many false negatives). Therefore, the
>> 		 implementation can deal with false positives properly
>> 		 already. [3]
>>
>> With CONFIG_MEMORY_HOTREMOVE, always indicating "removable" should not
>> break any user space tool. We implement a very bad heuristic now. (in
>> contrast: always returning "not removable" would at least affect
>> powerpc-utils)
>>
>> Without CONFIG_MEMORY_HOTREMOVE we cannot offline anything, so report
>> "not removable" as before.
>>
>> Original discussion can be found in [4] ("[PATCH RFC v1] mm:
>> is_mem_section_removable() overhaul").
>>
>> Other users of is_mem_section_removable() will be removed next, so that
>> we can remove is_mem_section_removable() completely.
>>
>> [1] http://man7.org/linux/man-pages/man1/lsmem.1.html
>> [2] http://man7.org/linux/man-pages/man8/chmem.8.html
>> [3] https://github.com/ibm-power-utilities/powerpc-utils
>> [4] https://lkml.kernel.org/r/20200117105759.27905-1-david@redhat.com
>>
>> Suggested-by: Michal Hocko <mhocko@kernel.org>
>> Cc: Dan Williams <dan.j.williams@intel.com>
>> Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
>> Cc: "Rafael J. Wysocki" <rafael@kernel.org>
>> Cc: Andrew Morton <akpm@linux-foundation.org>
>> Cc: powerpc-utils-devel@googlegroups.com
>> Cc: util-linux@vger.kernel.org
>> Cc: Badari Pulavarty <pbadari@us.ibm.com>
>> Cc: Nathan Fontenot <nfont@linux.vnet.ibm.com>
>> Cc: Robert Jennings <rcj@linux.vnet.ibm.com>
>> Cc: Heiko Carstens <heiko.carstens@de.ibm.com>
>> Cc: Karel Zak <kzak@redhat.com>
>> Signed-off-by: David Hildenbrand <david@redhat.com>
> 
> Please add information provided by Nathan.
> Acked-by: Michal Hocko <mhocko@suse.com>
> 
> Minor nit below.
> 
>> +#ifdef CONFIG_MEMORY_HOTREMOVE
>> +	return sprintf(buf, "1\n");
>> +#else
>> +	return sprintf(buf, "0\n");
>> +#endif
> 	int ret = IS_ENABLED(CONFIG_MEMORY_HOTREMOVE);
> 
> 	return sprintf(buf, "%d\n", ret)
> 
> would be slightly nicer than explicit ifdefs.
> 

Indeed, thanks!

-- 
Thanks,

David / dhildenb