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

[Greg KH <gregkh@linuxfoundation.org>]

On Mon, Jun 19, 2023 at 03:53:40PM +0800, mawupeng wrote:
> 
> 
> On 2023/6/19 15:41, David Hildenbrand wrote:
> > 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@redhat.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@linux.alibaba.com>
> >>>> Cc: "Michael S. Tsirkin" <mst@redhat.com>
> >>>> Cc: Jason Wang <jasowang@redhat.com>
> >>>> Cc: Pankaj Gupta <pankaj.gupta.linux@gmail.com>
> >>>> Cc: Michal Hocko <mhocko@kernel.org>
> >>>> Cc: Oscar Salvador <osalvador@suse.de>
> >>>> Cc: Wei Yang <richard.weiyang@linux.alibaba.com>
> >>>> Cc: Andrew Morton <akpm@linux-foundation.org>
> >>>> Signed-off-by: David Hildenbrand <david@redhat.com>
> >>>> Link: https://lore.kernel.org/r/20201112133815.13332-28-david@redhat.com
> >>>> Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
> >>>> Acked-by: Andrew Morton <akpm@linux-foundation.org>
> >>>> Signed-off-by: Ma Wupeng <mawupeng1@huawei.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.
> 
> Since the new patch does not introduce any kabi change, maybe we can merge this one？

stable kernels never care about "kabi", that is a made up thing that
some distros work to enforce only.  It has nothing to do with the
community.

And I will always prefer to take the real commit that is in Linus's tree
over any "custom" patch, as 90%+ of the time, custom changes are almost
always wrong.

thanks,

greg k-h