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

[chenridong]

On 2024/10/14 17:20, Muchun Song wrote:
> > On Oct 14, 2024, at 17:04, chenridong <chenridong@xxxxxxxxxx> wrote:
> >
> >
> >
> > 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);
>
> No. We should make sure the @info is fully initialized before others
> could see it. That's why rcu_assign_pointer is used here.

Thank you, it seems that 'goto free' is a better choice.
Will update.

Thanks,
Ridong
> > I think this is concise.
> >
> > Best regards,
> > Ridong