linux-kernel - Re: [PATCH stable 5.10] mm/memory_hotplug: extend offline_and_remove_memory() to handle more than one memory block

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Date:   Mon, 19 Jun 2023 09:41:16 +0200
From:   David Hildenbrand <david@...hat.com>
To:     mawupeng <mawupeng1@...wei.com>, gregkh@...uxfoundation.org
Cc:     akpm@...ux-foundation.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, stable@...r.kernel.org,
        richard.weiyang@...ux.alibaba.com, mst@...hat.com,
        jasowang@...hat.com, pankaj.gupta.linux@...il.com,
        mhocko@...nel.org, osalvador@...e.de
Subject: Re: [PATCH stable 5.10] mm/memory_hotplug: extend
 offline_and_remove_memory() to handle more than one memory block

On 19.06.23 09:22, mawupeng wrote:
> 
> 
> On 2023/6/19 15:16, Greg KH wrote:
>> On Mon, Jun 19, 2023 at 02:51:21PM +0800, Wupeng Ma wrote:
>>> From: David Hildenbrand <david@...hat.com>
>>>
>>> commit 8dc4bb58a146655eb057247d7c9d19e73928715b upstream.
>>>
>>> virtio-mem soon wants to use offline_and_remove_memory() memory that
>>> exceeds a single Linux memory block (memory_block_size_bytes()). Let's
>>> remove that restriction.
>>>
>>> Let's remember the old state and try to restore that if anything goes
>>> wrong. While re-onlining can, in general, fail, it's highly unlikely to
>>> happen (usually only when a notifier fails to allocate memory, and these
>>> are rather rare).
>>>
>>> This will be used by virtio-mem to offline+remove memory ranges that are
>>> bigger than a single memory block - for example, with a device block
>>> size of 1 GiB (e.g., gigantic pages in the hypervisor) and a Linux memory
>>> block size of 128MB.
>>>
>>> While we could compress the state into 2 bit, using 8 bit is much
>>> easier.
>>>
>>> This handling is similar, but different to acpi_scan_try_to_offline():
>>>
>>> a) We don't try to offline twice. I am not sure if this CONFIG_MEMCG
>>> optimization is still relevant - it should only apply to ZONE_NORMAL
>>> (where we have no guarantees). If relevant, we can always add it.
>>>
>>> b) acpi_scan_try_to_offline() simply onlines all memory in case
>>> something goes wrong. It doesn't restore previous online type. Let's do
>>> that, so we won't overwrite what e.g., user space configured.
>>>
>>> Reviewed-by: Wei Yang <richard.weiyang@...ux.alibaba.com>
>>> Cc: "Michael S. Tsirkin" <mst@...hat.com>
>>> Cc: Jason Wang <jasowang@...hat.com>
>>> Cc: Pankaj Gupta <pankaj.gupta.linux@...il.com>
>>> Cc: Michal Hocko <mhocko@...nel.org>
>>> Cc: Oscar Salvador <osalvador@...e.de>
>>> Cc: Wei Yang <richard.weiyang@...ux.alibaba.com>
>>> Cc: Andrew Morton <akpm@...ux-foundation.org>
>>> Signed-off-by: David Hildenbrand <david@...hat.com>
>>> Link: https://lore.kernel.org/r/20201112133815.13332-28-david@redhat.com
>>> Signed-off-by: Michael S. Tsirkin <mst@...hat.com>
>>> Acked-by: Andrew Morton <akpm@...ux-foundation.org>
>>> Signed-off-by: Ma Wupeng <mawupeng1@...wei.com>
>>> ---
>>>   mm/memory_hotplug.c | 105 +++++++++++++++++++++++++++++++++++++-------
>>>   1 file changed, 89 insertions(+), 16 deletions(-)
>>>
>>
>> Why is this needed in 5.10.y?  Looks like a new feature to me, what
>> problem does it solve there?
>>
>> thanks,
>>
>> greg k-h
> 
> It do introduce a new feature. But at the same time, it fix a memleak introduced
> in Commit 08b3acd7a68f ("mm/memory_hotplug: Introduce offline_and_remove_memory()"
> 
> Our test find a memleak in init_memory_block, it is clear that mem is never
> been released due to wrong refcount. Commit 08b3acd7a68f ("mm/memory_hotplug:
> Introduce offline_and_remove_memory()") failed to dec refcount after
> find_memory_block which fail to dec refcount to zero in remove memory
> causing the leak.
> 
> Commit 8dc4bb58a146 ("mm/memory_hotplug: extend offline_and_remove_memory()
> to handle more than one memory block") introduce walk_memory_blocks to
> replace find_memory_block which dec refcount by calling put_device after
> find_memory_block_by_id. In the way, the memleak is fixed.
> 
> Here is the simplified calltrace:
> 
>    kmem_cache_alloc_trace+0x664/0xed0
>    init_memory_block+0x8c/0x170
>    create_memory_block_devices+0xa4/0x150
>    add_memory_resource+0x188/0x530
>    __add_memory+0x78/0x104
>    add_memory+0x6c/0xb0
> 

Makes sense to me. Of course, we could think about a simplified stable 
fix that only drops the ref.

-- 
Cheers,

David / dhildenb