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 15:22:20 +0800
From:   mawupeng <mawupeng1@...wei.com>
To:     <gregkh@...uxfoundation.org>
CC:     <mawupeng1@...wei.com>, <akpm@...ux-foundation.org>,
        <david@...hat.com>, <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 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