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From: Roman Gushchin To: Vlastimil Babka Cc: Muchun Song , chenridong , Anshuman Khandual , "Kirill A. Shutemov" , akpm@linux-foundation.org, david@fromorbit.com, zhengqi.arch@bytedance.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org, wangweiyang2@huawei.com Subject: Re: [PATCH v2] mm: shrinker: avoid memleak in alloc_shrinker_info Message-ID: References: <1BD74B20-879A-4159-B957-1223553217C1@linux.dev> <0b883f9e-451f-41c2-805f-7f5bc7eebee2@suse.cz> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <0b883f9e-451f-41c2-805f-7f5bc7eebee2@suse.cz> X-Migadu-Flow: FLOW_OUT On Wed, Oct 16, 2024 at 07:02:23PM +0200, Vlastimil Babka wrote: > On 10/16/24 16:08, Muchun Song wrote: > > > > > >> On Oct 16, 2024, at 19:43, Vlastimil Babka wrote: > >> On 10/16/24 04:21, Muchun Song wrote: > >>> > >>> > >>>> On Oct 16, 2024, at 09:25, chenridong wrote: > >>>> 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 > >>>>>>> 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): > >>>>>>> [] __kmalloc_node_noprof+0x394/0x470 > >>>>>>> [] alloc_shrinker_info+0x7b/0x1a0 > >>>>>>> [] mem_cgroup_css_online+0x11a/0x3b0 > >>>>>>> [] online_css+0x29/0xa0 > >>>>>>> [] cgroup_apply_control_enable+0x20d/0x360 > >>>>>>> [] cgroup_mkdir+0x168/0x5f0 > >>>>>>> [] kernfs_iop_mkdir+0x5e/0x90 > >>>>>>> [] vfs_mkdir+0x144/0x220 > >>>>>>> [] do_mkdirat+0x87/0x130 > >>>>>>> [] __x64_sys_mkdir+0x49/0x70 > >>>>>>> [] do_syscall_64+0x68/0x140 > >>>>>>> [] 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 > >>>>>>> --- > >>>>>>> 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? > >>> > >>> I insist on my opinion, not mixing two different approaches > >>> to do release resources. > >> > >> So instead we mix the cleanup of the whole function with the cleanup of what > >> is effectively a per-iteration temporary variable? > >> > >> The fact there was already a confusion in this thread about whether it's > >> safe and relies on kvfree(NULL) to be a no-op, should be a hint. > > > > Yes. I think someone is confused about my opinion. > > I don’t care about whether we should apply this hit. > > If we think the hint is tricky, we could add another > > label to fix it like I suggested previously. Because > > we already use goto-based approaches to > > cleanup the resources, why not keeping > > consistent? > > I think we're rather pragmatic than striving to be consistent for the sake > of consistency. goto is not the nicest thing in the world, but we (unlike > other projects) use it where it makes sense to avoid if/else nesting > explosion. Here for the info it's not the most pragmatic option. > > > It will be easier for us to add a new > > "if" statement and handle the failure case in the future. > > Let's not overengineer things for hypothetical future. > > > For example, if we use his v1 proposal, we should do > > the cleanups again for info. But for goto-based > > version, we just add another label to do the > > cleanups and go to the new label for failure case. goto-based fix is what I insisted on. I copied my previous suggested fix here to clarify my opinion. > > Again, info is a loop-iteration-local variable, v1 fix making it truly local > is the way to go. If there are further cleanups added in the loop itself in > the future, they could hopefully keep being local to the loop as well. > Cleanup of info outside the loop iteration is breaking its real scope. +1 to that. I don't think it's such a big deal and both versions are ok, but I strongly prefer the original version (without introduction of a new label). Please, feel free to use Acked-by: Roman Gushchin with the original version. Thanks!