Re: [PATCH v1] mm/memory_hotplug: Don't take the cpu_hotplug_lock

[Michal Hocko]

On Wed 25-09-19 12:01:02, Qian Cai wrote:
> On Wed, 2019-09-25 at 09:02 +0200, David Hildenbrand wrote:
> > On 24.09.19 20:54, Qian Cai wrote:
> > > On Tue, 2019-09-24 at 17:11 +0200, Michal Hocko wrote:
> > > > On Tue 24-09-19 11:03:21, Qian Cai wrote:
> > > > [...]
> > > > > While at it, it might be a good time to rethink the whole locking over there, as
> > > > > it right now read files under /sys/kernel/slab/ could trigger a possible
> > > > > deadlock anyway.
> > > > > 
> > > > 
> > > > [...]
> > > > > [  442.452090][ T5224] -> #0 (mem_hotplug_lock.rw_sem){++++}:
> > > > > [  442.459748][ T5224]        validate_chain+0xd10/0x2bcc
> > > > > [  442.464883][ T5224]        __lock_acquire+0x7f4/0xb8c
> > > > > [  442.469930][ T5224]        lock_acquire+0x31c/0x360
> > > > > [  442.474803][ T5224]        get_online_mems+0x54/0x150
> > > > > [  442.479850][ T5224]        show_slab_objects+0x94/0x3a8
> > > > > [  442.485072][ T5224]        total_objects_show+0x28/0x34
> > > > > [  442.490292][ T5224]        slab_attr_show+0x38/0x54
> > > > > [  442.495166][ T5224]        sysfs_kf_seq_show+0x198/0x2d4
> > > > > [  442.500473][ T5224]        kernfs_seq_show+0xa4/0xcc
> > > > > [  442.505433][ T5224]        seq_read+0x30c/0x8a8
> > > > > [  442.509958][ T5224]        kernfs_fop_read+0xa8/0x314
> > > > > [  442.515007][ T5224]        __vfs_read+0x88/0x20c
> > > > > [  442.519620][ T5224]        vfs_read+0xd8/0x10c
> > > > > [  442.524060][ T5224]        ksys_read+0xb0/0x120
> > > > > [  442.528586][ T5224]        __arm64_sys_read+0x54/0x88
> > > > > [  442.533634][ T5224]        el0_svc_handler+0x170/0x240
> > > > > [  442.538768][ T5224]        el0_svc+0x8/0xc
> > > > 
> > > > I believe the lock is not really needed here. We do not deallocated
> > > > pgdat of a hotremoved node nor destroy the slab state because an
> > > > existing slabs would prevent hotremove to continue in the first place.
> > > > 
> > > > There are likely details to be checked of course but the lock just seems
> > > > bogus.
> > > 
> > > Check 03afc0e25f7f ("slab: get_online_mems for
> > > kmem_cache_{create,destroy,shrink}"). It actually talk about the races during
> > > memory as well cpu hotplug, so it might even that cpu_hotplug_lock removal is
> > > problematic?
> > > 
> > 
> > Which removal are you referring to? get_online_mems() does not mess with
> > the cpu hotplug lock (and therefore this patch).
> 
> The one in your patch. I suspect there might be races among the whole NUMA node
> hotplug, kmem_cache_create, and show_slab_objects(). See bfc8c90139eb ("mem-
> hotplug: implement get/put_online_mems")
> 
> "kmem_cache_{create,destroy,shrink} need to get a stable value of cpu/node
> online mask, because they init/destroy/access per-cpu/node kmem_cache parts,
> which can be allocated or destroyed on cpu/mem hotplug."

I still have to grasp that code but if the slub allocator really needs
a stable cpu mask then it should be using the explicit cpu hotplug
locking rather than rely on side effect of memory hotplug locking.

> Both online_pages() and show_slab_objects() need to get a stable value of
> cpu/node online mask.

Could tou be more specific why online_pages need a stable cpu online
mask? I do not think that show_slab_objects is a real problem because a
potential race shouldn't be critical.

-- 
Michal Hocko
SUSE Labs