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    SubjectRe: [PATCH V5 1/3] mm/hotplug: Reorder arch_remove_memory() call in __remove_memory()
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    On 05/30/2019 04:07 PM, Mark Rutland wrote:
    > On Wed, May 29, 2019 at 02:46:25PM +0530, 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 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.
    >>
    >> Acked-by: Michal Hocko <mhocko@suse.com>
    >> Reviewed-by: David Hildenbrand <david@redhat.com>
    >> Reviewed-by: Oscar Salvador <osalvador@suse.de>
    >> Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.com>
    >
    > Acked-by: Mark Rutland <mark.rutland@arm.com>

    Hello Andrew,

    Will it be possible for this particular patch of the series to be merged alone.
    I am still reworking arm64 hot-remove parts as per the suggestions from Mark.
    Just wondering if this patch which has been reviewed and acked for a while now
    can be out of our way.

    Also because this has some conflict with David's series which can be sorted out
    earlier before arm64 hot-remove V6 series comes in.

    From my previous response on this series last week, the following can resolve
    the conflict with David's [v3, 09/11] patch.

    C) Rebase (https://patchwork.kernel.org/patch/10962589/) [v3, 09/11]

    - hot-remove series moves arch_remove_memory() before memblock_[free|remove]()
    - So remove_memory_block_devices() should be moved before arch_remove_memory()
    in it's new position

    It will be great if this patch can be merged alone.

    - Anshuman

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