Re: [PATCH V5 - Rebased] mm/hotplug: Reorder memblock_[free|remove]() calls in try_remove_memory()

[Andrew Morton]

On Tue, 11 Jun 2019 16:56:13 +0530 Anshuman Khandual <anshuman.khandual@xxxxxxx> 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

This seems like a serious problem.  Once which should be fixed in 5.2
and perhaps the various -stable kernels as well.

> 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.
> 
> ...
>
> 
> - Rebased on linux-next (next-20190611)

Yet the patch you've prepared is designed for 5.3.  Was that
deliberate, or should we be targeting earlier kernels?