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

[Greg KH]

On Mon, Jun 19, 2023 at 03:22:20PM +0800, 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@xxxxxxxxxx>
> >>
> >> 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@xxxxxxxxxxxxxxxxx>
> >> Cc: "Michael S. Tsirkin" <mst@xxxxxxxxxx>
> >> Cc: Jason Wang <jasowang@xxxxxxxxxx>
> >> Cc: Pankaj Gupta <pankaj.gupta.linux@xxxxxxxxx>
> >> Cc: Michal Hocko <mhocko@xxxxxxxxxx>
> >> Cc: Oscar Salvador <osalvador@xxxxxxx>
> >> Cc: Wei Yang <richard.weiyang@xxxxxxxxxxxxxxxxx>
> >> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> >> Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>
> >> Link: https://lore.kernel.org/r/20201112133815.13332-28-david@xxxxxxxxxx
> >> Signed-off-by: Michael S. Tsirkin <mst@xxxxxxxxxx>
> >> Acked-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> >> Signed-off-by: Ma Wupeng <mawupeng1@xxxxxxxxxx>
> >> ---
> >>  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

Ok, thanks for the information, now queued up.

greg k-h