From: David Hildenbrand david@redhat.com Subject: drivers/base/memory.c: indicate all memory blocks as removable

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 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]

According to Nathan Fontenot, DLPAR on powerpc is nowadays no longer driven from userspace via the drmgr command (powerpc-utils). Nowadays it's managed in the kernel - including onlining/offlining of memory blocks - triggered by drmgr writing to /sys/kernel/dlpar. So the affected legacy userspace handling is only active on old kernels. Only ve= ry old versions of drmgr on a new kernel (unlikely) might execute slower - totally acceptable.

With CONFIG_MEMORY_HOTREMOVE, always indicating "removable" should not break any user space tool. We implement a very bad heuristic now. Withou= t 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

Also, this patch probably fixes a crash reported by Steve. http://lkml.kernel.org/r/CAPcyv4jpdaNvJ67SkjyUJLBnBnXXQv686BiVW042g03FUmWLXw...

Link: http://lkml.kernel.org/r/20200128093542.6908-1-david@redhat.com Signed-off-by: David Hildenbrand david@redhat.com Suggested-by: Michal Hocko mhocko@kernel.org Acked-by: Michal Hocko mhocko@suse.com Reviewed-by: Nathan Fontenot ndfont@gmail.com Reported-by: "Scargall, Steve" steve.scargall@intel.com Cc: Dan Williams dan.j.williams@intel.com Cc: Greg Kroah-Hartman gregkh@linuxfoundation.org Cc: "Rafael J. Wysocki" rafael@kernel.org Cc: Badari Pulavarty pbadari@us.ibm.com Cc: Robert Jennings rcj@linux.vnet.ibm.com Cc: Heiko Carstens heiko.carstens@de.ibm.com Cc: Karel Zak kzak@redhat.com Cc: stable@vger.kernel.org Signed-off-by: Andrew Morton akpm@linux-foundation.org ---

drivers/base/memory.c | 23 +++-------------------- 1 file changed, 3 insertions(+), 20 deletions(-)

--- a/drivers/base/memory.c~drivers-base-memoryc-indicate-all-memory-blocks-as-removable +++ a/drivers/base/memory.c @@ -97,30 +97,13 @@ static ssize_t phys_index_show(struct de }

/* - * Show whether the memory block is likely to be offlineable (or is already - * offline). Once offline, the memory block could be removed. The return - * value does, however, not indicate that there is a way to remove the - * memory block. + * Legacy interface that we cannot remove. Always indicate "removable" + * with CONFIG_MEMORY_HOTREMOVE - bad heuristic. */ static ssize_t removable_show(struct device *dev, struct device_attribute *attr, char *buf) { - struct memory_block *mem = to_memory_block(dev); - unsigned long pfn; - int ret = 1, i; - - if (mem->state != MEM_ONLINE) - goto out; - - for (i = 0; i < sections_per_block; i++) { - if (!present_section_nr(mem->start_section_nr + i)) - continue; - pfn = section_nr_to_pfn(mem->start_section_nr + i); - ret &= is_mem_section_removable(pfn, PAGES_PER_SECTION); - } - -out: - return sprintf(buf, "%d\n", ret); + return sprintf(buf, "%d\n", (int)IS_ENABLED(CONFIG_MEMORY_HOTREMOVE)); }

/* _