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

[Michal Hocko]

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@xxxxxxxxxx
> 
> Suggested-by: Michal Hocko <mhocko@xxxxxxxxxx>
> Cc: Dan Williams <dan.j.williams@xxxxxxxxx>
> Cc: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
> Cc: "Rafael J. Wysocki" <rafael@xxxxxxxxxx>
> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Cc: powerpc-utils-devel@xxxxxxxxxxxxxxxx
> Cc: util-linux@xxxxxxxxxxxxxxx
> Cc: Badari Pulavarty <pbadari@xxxxxxxxxx>
> Cc: Nathan Fontenot <nfont@xxxxxxxxxxxxxxxxxx>
> Cc: Robert Jennings <rcj@xxxxxxxxxxxxxxxxxx>
> Cc: Heiko Carstens <heiko.carstens@xxxxxxxxxx>
> Cc: Karel Zak <kzak@xxxxxxxxxx>
> Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>

Please add information provided by Nathan.
Acked-by: Michal Hocko <mhocko@xxxxxxxx>

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.
-- 
Michal Hocko
SUSE Labs