linux-kernel - Re: [PATCH v3 1/7] mm/hotplug: fix hot remove failure in SPARSEMEM|!VMEMMAP case

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Message-ID: <6446ffe5-0eb6-f4d7-40ab-c164d8661e0c@redhat.com>
Date:   Mon, 9 Mar 2020 14:22:02 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Baoquan He <bhe@...hat.com>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        akpm@...ux-foundation.org, mhocko@...e.com,
        richardw.yang@...ux.intel.com, dan.j.williams@...el.com,
        osalvador@...e.de, rppt@...ux.ibm.com
Subject: Re: [PATCH v3 1/7] mm/hotplug: fix hot remove failure in
 SPARSEMEM|!VMEMMAP case

On 09.03.20 14:18, Baoquan He wrote:
> On 03/09/20 at 09:58am, David Hildenbrand wrote:
>> On 07.03.20 09:42, Baoquan He wrote:
>>> In section_deactivate(), pfn_to_page() doesn't work any more after
>>> ms->section_mem_map is resetting to NULL in SPARSEMEM|!VMEMMAP case.
>>> It caused hot remove failure:
>>>
>>> kernel BUG at mm/page_alloc.c:4806!
>>> invalid opcode: 0000 [#1] SMP PTI
>>> CPU: 3 PID: 8 Comm: kworker/u16:0 Tainted: G        W         5.5.0-next-20200205+ #340
>>> Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 0.0.0 02/06/2015
>>> Workqueue: kacpi_hotplug acpi_hotplug_work_fn
>>> RIP: 0010:free_pages+0x85/0xa0
>>> Call Trace:
>>>  __remove_pages+0x99/0xc0
>>>  arch_remove_memory+0x23/0x4d
>>>  try_remove_memory+0xc8/0x130
>>>  ? walk_memory_blocks+0x72/0xa0
>>>  __remove_memory+0xa/0x11
>>>  acpi_memory_device_remove+0x72/0x100
>>>  acpi_bus_trim+0x55/0x90
>>>  acpi_device_hotplug+0x2eb/0x3d0
>>>  acpi_hotplug_work_fn+0x1a/0x30
>>>  process_one_work+0x1a7/0x370
>>>  worker_thread+0x30/0x380
>>>  ? flush_rcu_work+0x30/0x30
>>>  kthread+0x112/0x130
>>>  ? kthread_create_on_node+0x60/0x60
>>>  ret_from_fork+0x35/0x40
>>>
>>> Let's move the ->section_mem_map resetting after depopulate_section_memmap()
>>> to fix it.
>>>
>>> Fixes: ba72b4c8cf60 ("mm/sparsemem: support sub-section hotplug")
>>> Signed-off-by: Baoquan He <bhe@...hat.com>
>>> Cc: stable@...r.kernel.org
>>> ---
>>>  mm/sparse.c | 8 ++++++--
>>>  1 file changed, 6 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/mm/sparse.c b/mm/sparse.c
>>> index 42c18a38ffaa..1b50c15677d7 100644
>>> --- a/mm/sparse.c
>>> +++ b/mm/sparse.c
>>> @@ -734,6 +734,7 @@ static void section_deactivate(unsigned long pfn, unsigned long nr_pages,
>>>  	struct mem_section *ms = __pfn_to_section(pfn);
>>>  	bool section_is_early = early_section(ms);
>>>  	struct page *memmap = NULL;
>>> +	bool empty = false;
>>
>> Oh, one NIT: no need to initialize empty to false.
> 
> Thanks for careful reviewing, David.
> 
> Not very sure about this, do you have a doc or discussion thread about
> not initializing local variable? Maybe Andrew can help update it if this
> is not suggested. 

The general rule is to no initialize what will always be initialized
later. Compare with most other code in-tree - e.g., sparse_init_nid.

Makes the code usually easier to follow.

-- 
Thanks,

David / dhildenb