Subject: Re: [PATCH V7 1/3] mm/hotplug: Reorder memblock_[free|remove]() calls in try_remove_memory()

From: David Hildenbrand <[email-masked]>
Date: Wed, 4 Sep 2019 10:16:28 +0200

On 03.09.19 11:45, 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.

Since

commit 60bb462fc7adb06ebee3beb5a4af6c7e6182e248
Author: David Hildenbrand <[email-masked]>
Date:   Wed Aug 28 13:57:15 2019 +1000

    drivers/base/node.c: simplify unregister_memory_block_under_nodes()

that problem should be gone. There is no get_nid_for_pfn() call anymore.

So this patch should no longer be necessary - but as I said during
earlier versions of this patch, the re-ordering might still make sense
for consistency (removing stuff in the reverse order they were added).
You'll have to rephrase the description then.

> 
> [   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 memblock_[free|remove]() with arch_remove_memory() 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 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.
> 
> Reviewed-by: David Hildenbrand <[email-masked]>
> Reviewed-by: Oscar Salvador <[email-masked]>
> Acked-by: Mark Rutland <[email-masked]>
> Acked-by: Michal Hocko <[email-masked]>
> Signed-off-by: Anshuman Khandual <[email-masked]>
> ---
>  mm/memory_hotplug.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
> index c73f09913165..355c466e0621 100644
> --- a/mm/memory_hotplug.c
> +++ b/mm/memory_hotplug.c
> @@ -1770,13 +1770,13 @@ static int __ref try_remove_memory(int nid, u64 start, u64 size)
>  
>  	/* remove memmap entry */
>  	firmware_map_remove(start, start + size, "System RAM");
> -	memblock_free(start, size);
> -	memblock_remove(start, size);
>  
>  	/* remove memory block devices before removing memory */
>  	remove_memory_block_devices(start, size);
>  
>  	arch_remove_memory(nid, start, size, NULL);
> +	memblock_free(start, size);
> +	memblock_remove(start, size);
>  	__release_memory_resource(start, size);
>  
>  	try_offline_node(nid);
> 

-- 

Thanks,

David / dhildenb

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