Re: [PATCH 4/6] mm/hotplug: Reorder arch_remove_memory() call in __remove_memory()

[Michal Hocko <mhocko@kernel.org>]

On Wed 03-04-19 10:00:04, Anshuman Khandual wrote:
> Memory hot remove uses get_nid_for_pfn() while tearing down linked sysfs
> entries between memory block and node. It first checks pfn validity with
> pfn_valid_within() before fetching nid. With CONFIG_HOLES_IN_ZONE config
> (arm64 has this enabled) pfn_valid_within() calls pfn_valid().
> 
> pfn_valid() is an arch implementation on arm64 (CONFIG_HAVE_ARCH_PFN_VALID)
> which scans all mapped memblock regions with memblock_is_map_memory(). This
> creates a problem in memory hot remove path which has already removed given
> memory range from memory block with memblock_[remove|free] before arriving
> at unregister_mem_sect_under_nodes(). Hence get_nid_for_pfn() returns -1
> skipping subsequent sysfs_remove_link() calls leaving node <-> memory block
> sysfs entries as is. Subsequent memory add operation hits BUG_ON() because
> of existing sysfs entries.
> 
> [   62.007176] NUMA: Unknown node for memory at 0x680000000, assuming node 0
> [   62.052517] ------------[ cut here ]------------
> [   62.053211] kernel BUG at mm/memory_hotplug.c:1143!
> [   62.053868] Internal error: Oops - BUG: 0 [#1] PREEMPT SMP
> [   62.054589] Modules linked in:
> [   62.054999] CPU: 19 PID: 3275 Comm: bash Not tainted 5.1.0-rc2-00004-g28cea40b2683 #41
> [   62.056274] Hardware name: linux,dummy-virt (DT)
> [   62.057166] pstate: 40400005 (nZcv daif +PAN -UAO)
> [   62.058083] pc : add_memory_resource+0x1cc/0x1d8
> [   62.058961] lr : add_memory_resource+0x10c/0x1d8
> [   62.059842] sp : ffff0000168b3ce0
> [   62.060477] x29: ffff0000168b3ce0 x28: ffff8005db546c00
> [   62.061501] x27: 0000000000000000 x26: 0000000000000000
> [   62.062509] x25: ffff0000111ef000 x24: ffff0000111ef5d0
> [   62.063520] x23: 0000000000000000 x22: 00000006bfffffff
> [   62.064540] x21: 00000000ffffffef x20: 00000000006c0000
> [   62.065558] x19: 0000000000680000 x18: 0000000000000024
> [   62.066566] x17: 0000000000000000 x16: 0000000000000000
> [   62.067579] x15: ffffffffffffffff x14: ffff8005e412e890
> [   62.068588] x13: ffff8005d6b105d8 x12: 0000000000000000
> [   62.069610] x11: ffff8005d6b10490 x10: 0000000000000040
> [   62.070615] x9 : ffff8005e412e898 x8 : ffff8005e412e890
> [   62.071631] x7 : ffff8005d6b105d8 x6 : ffff8005db546c00
> [   62.072640] x5 : 0000000000000001 x4 : 0000000000000002
> [   62.073654] x3 : ffff8005d7049480 x2 : 0000000000000002
> [   62.074666] x1 : 0000000000000003 x0 : 00000000ffffffef
> [   62.075685] Process bash (pid: 3275, stack limit = 0x00000000d754280f)
> [   62.076930] Call trace:
> [   62.077411]  add_memory_resource+0x1cc/0x1d8
> [   62.078227]  __add_memory+0x70/0xa8
> [   62.078901]  probe_store+0xa4/0xc8
> [   62.079561]  dev_attr_store+0x18/0x28
> [   62.080270]  sysfs_kf_write+0x40/0x58
> [   62.080992]  kernfs_fop_write+0xcc/0x1d8
> [   62.081744]  __vfs_write+0x18/0x40
> [   62.082400]  vfs_write+0xa4/0x1b0
> [   62.083037]  ksys_write+0x5c/0xc0
> [   62.083681]  __arm64_sys_write+0x18/0x20
> [   62.084432]  el0_svc_handler+0x88/0x100
> [   62.085177]  el0_svc+0x8/0xc
> 
> Re-ordering arch_remove_memory() with memblock_[free|remove] solves the
> problem on arm64 as pfn_valid() behaves correctly and returns positive
> as memblock for the address range still exists. arch_remove_memory()
> removes applicable memory sections from zone with __remove_pages() and
> tears down kernel linear mapping. Removing memblock regions afterwards
> is consistent.

consistent with what? Anyway, I believe you wanted to mention that this
is safe because there is no other memblock (bootmem) allocator user that
late. So nobody is going to allocate from the removed range just to blow
up later. Also nobody should be using the bootmem allocated range else
we wouldn't allow to remove it. So reordering is indeed safe.
 
> Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.com>

With a changelog updated to explain why this is safe
Acked-by: Michal Hocko <mhocko@suse.com>

> ---
>  mm/memory_hotplug.c | 3 +--
>  1 file changed, 1 insertion(+), 2 deletions(-)
> 
> diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
> index 0082d69..71d0d79 100644
> --- a/mm/memory_hotplug.c
> +++ b/mm/memory_hotplug.c
> @@ -1872,11 +1872,10 @@ void __ref __remove_memory(int nid, u64 start, u64 size)
>  
>  	/* remove memmap entry */
>  	firmware_map_remove(start, start + size, "System RAM");
> +	arch_remove_memory(nid, start, size, NULL);
>  	memblock_free(start, size);
>  	memblock_remove(start, size);
>  
> -	arch_remove_memory(nid, start, size, NULL);
> -
>  	try_offline_node(nid);
>  
>  	mem_hotplug_done();
> -- 
> 2.7.4
> 

-- 
Michal Hocko
SUSE Labs