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

[chenridong]

On 2024/10/14 16:43, Muchun Song wrote:
> > On Oct 14, 2024, at 16:13, Anshuman Khandual <anshuman.khandual@xxxxxxx> wrote:
> >
> >
> >
> > On 10/14/24 08:53, 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;
> > > }
> >
> > There are two scenarios when "goto err:" gets called
> >
> > - When shrinker_info allocations fails, no kvfree() is required
> > 	- but after this change kvfree() would be called even
> > 	  when the allocation had failed originally, which does
> >   	  not sound right
>
> Yes. In this case, @info is NULL and kvfree could handle NULL.
> It seems strange but the final behaviour correct.
>
> > - shrinker_unit_alloc() fails, kvfree() is actually required
> >
> > I guess kvfree() should be called just after shrinker_unit_alloc()
> > fails but before calling into "goto err".
>
> We could do it like this, which avoids ambiguity (if someone ignores
> that kvfree could handle NULL). Something like:
>
> --- a/mm/shrinker.c
> +++ b/mm/shrinker.c
> @@ -88,13 +88,14 @@ int alloc_shrinker_info(struct mem_cgroup *memcg)
>                          goto err;
>                  info->map_nr_max = shrinker_nr_max;
>                  if (shrinker_unit_alloc(info, NULL, nid))
> -                       goto err;
> +                       goto free;
>                  rcu_assign_pointer(memcg->nodeinfo[nid]->shrinker_info, info);
>          }
>          mutex_unlock(&shrinker_mutex);
>
>          return ret;
> -
> +free:
> +       kvfree(info);
>   err:
>          mutex_unlock(&shrinker_mutex);
>          free_shrinker_info(memcg);
>
> Thanks.
>
> > But curious, should not both kvzalloc_node()/kvfree() be avoided
> > while inside mutex lock to avoid possible lockdep issues ?
How about:

diff --git a/mm/shrinker.c b/mm/shrinker.c
index dc5d2a6fcfc4..7baee7f00497 100644
--- a/mm/shrinker.c
+++ b/mm/shrinker.c
@@ -87,9 +87,9 @@ int alloc_shrinker_info(struct mem_cgroup *memcg)
                 if (!info)
                         goto err;
                 info->map_nr_max = shrinker_nr_max;
+               rcu_assign_pointer(memcg->nodeinfo[nid]->shrinker_info, 
info);
                 if (shrinker_unit_alloc(info, NULL, nid))
                         goto err;
-               rcu_assign_pointer(memcg->nodeinfo[nid]->shrinker_info, 
info);
         }
         mutex_unlock(&shrinker_mutex);

I think this is concise.

Best regards,
Ridong