Re: [PATCH v2] mm: shrinker: avoid memleak in alloc_shrinker_info

[chenridong]

On 2024/10/15 14:55, Anshuman Khandual wrote:
> On 10/14/24 16:59, Kirill A. Shutemov wrote:
> > On Mon, Oct 14, 2024 at 03:23:36AM +0000, Chen Ridong wrote:
> > > From: Chen Ridong <chenridong@xxxxxxxxxx>
> > >
> > > A memleak was found as bellow:
> > >
> > > unreferenced object 0xffff8881010d2a80 (size 32):
> > >    comm "mkdir", pid 1559, jiffies 4294932666
> > >    hex dump (first 32 bytes):
> > >      00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
> > >      40 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  @...............
> > >    backtrace (crc 2e7ef6fa):
> > >      [<ffffffff81372754>] __kmalloc_node_noprof+0x394/0x470
> > >      [<ffffffff813024ab>] alloc_shrinker_info+0x7b/0x1a0
> > >      [<ffffffff813b526a>] mem_cgroup_css_online+0x11a/0x3b0
> > >      [<ffffffff81198dd9>] online_css+0x29/0xa0
> > >      [<ffffffff811a243d>] cgroup_apply_control_enable+0x20d/0x360
> > >      [<ffffffff811a5728>] cgroup_mkdir+0x168/0x5f0
> > >      [<ffffffff8148543e>] kernfs_iop_mkdir+0x5e/0x90
> > >      [<ffffffff813dbb24>] vfs_mkdir+0x144/0x220
> > >      [<ffffffff813e1c97>] do_mkdirat+0x87/0x130
> > >      [<ffffffff813e1de9>] __x64_sys_mkdir+0x49/0x70
> > >      [<ffffffff81f8c928>] do_syscall_64+0x68/0x140
> > >      [<ffffffff8200012f>] entry_SYSCALL_64_after_hwframe+0x76/0x7e
> > >
> > > In the alloc_shrinker_info function, when shrinker_unit_alloc return
> > > err, the info won't be freed. Just fix it.
> > >
> > > Fixes: 307bececcd12 ("mm: shrinker: add a secondary array for shrinker_info::{map, nr_deferred}")
> > > Signed-off-by: Chen Ridong <chenridong@xxxxxxxxxx>
> > > ---
> > >   mm/shrinker.c | 1 +
> > >   1 file changed, 1 insertion(+)
> > >
> > > diff --git a/mm/shrinker.c b/mm/shrinker.c
> > > index dc5d2a6fcfc4..92270413190d 100644
> > > --- a/mm/shrinker.c
> > > +++ b/mm/shrinker.c
> > > @@ -97,6 +97,7 @@ int alloc_shrinker_info(struct mem_cgroup *memcg)
> > >   
> > >   err:
> > >   	mutex_unlock(&shrinker_mutex);
> > > +	kvfree(info);
> > >   	free_shrinker_info(memcg);
> > >   	return -ENOMEM;
> > >   }
> >
> > NAK. If in the future there going to one more error case after
> > rcu_assign_pointer() we will end up with double free.
> >
> > This should be safer:
> >
> > diff --git a/mm/shrinker.c b/mm/shrinker.c
> > index dc5d2a6fcfc4..763fd556bc7d 100644
> > --- a/mm/shrinker.c
> > +++ b/mm/shrinker.c
> > @@ -87,8 +87,10 @@ int alloc_shrinker_info(struct mem_cgroup *memcg)
> >   		if (!info)
> >   			goto err;
> >   		info->map_nr_max = shrinker_nr_max;
> > -		if (shrinker_unit_alloc(info, NULL, nid))
> > +		if (shrinker_unit_alloc(info, NULL, nid)) {
> > +			kvfree(info);
> >   			goto err;
> > +		}
> >   		rcu_assign_pointer(memcg->nodeinfo[nid]->shrinker_info, info);
> >   	}
> >   	mutex_unlock(&shrinker_mutex);
>
> Agreed, this is what I mentioned earlier as well.
>
> ------------------------------------------------------------------
> I guess kvfree() should be called just after shrinker_unit_alloc()
> fails but before calling into "goto err"
> ------------------------------------------------------------------

After discussion, it seems that v1 is acceptable.
Hi, Muchun, do you have any other opinions?

Best regards,
Ridong