Re: [PATCH -next] mm/slub: fix a slab missed to be freed problem

[Vlastimil Babka <vbabka@suse.cz>]

On 9/30/22 13:51, Feng Tang wrote:
> On Fri, Sep 30, 2022 at 07:25:54PM +0800, Vlastimil Babka wrote:
>> 
>> On 9/30/22 12:07, Feng Tang wrote:
>> > When enable kasan and kfence's in-kernel kunit test with slub_debug on,
>> > it caught a problem (in linux-next tree):
>> > 
>> >  ------------[ cut here ]------------
>> >  kmem_cache_destroy test: Slab cache still has objects when called from test_exit+0x1a/0x30
>> >  WARNING: CPU: 3 PID: 240 at mm/slab_common.c:492 kmem_cache_destroy+0x16c/0x170
>> 
>> Assuming the warning was preceded by some kunit test failures?
>> I don't see how leaving more empty slabs on free list than needed would
>> cause this warning, the shutdown should just drop the empty slab.
> 
> The previous code only call remove_partial() to dequeue the slab from
> partial list, and miss to call discard_slab() for it.
> 
> From the debug dump, the n->nr_partils stays at 5, while n->nr_slabs
> keeps increasing. And during shutdown, the free_partial() only free
> the 5 slabs on partial list, and n->nr_slabs still has a big numbers
> of orphan slabs

Thanks, finally I get the exact cause now. I've added the more detailed
explanation to commit log and the result is here:
https://git.kernel.org/pub/scm/linux/kernel/git/vbabka/slab.git/commit/?h=for-6.1/slub_validation_locking&id=b731e3575f7a45a46512708f9fdf953b40c46a53

>> >  Modules linked in:
>> >  CPU: 3 PID: 240 Comm: kunit_try_catch Tainted: G    B            N 6.0.0-rc7-next-20220929 #52
>> >  Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.15.0-1 04/01/2014
>> >  RIP: 0010:kmem_cache_destroy+0x16c/0x170
>> >  Code: 41 5c 41 5d e9 a5 04 0b 00 c3 cc cc cc cc 48 8b 55 60 48 8b 4c 24 20 48 c7 c6 40 37 d2 82 48 c7 c7 e8 a0 33 83 e8 4e d7 14 01 <0f> 0b eb a7 41 56 41 89 d6 41 55 49 89 f5 41 54 49 89 fc 55 48 89
>> >  RSP: 0000:ffff88800775fea0 EFLAGS: 00010282
>> >  RAX: 0000000000000000 RBX: ffffffff83bdec48 RCX: 0000000000000000
>> >  RDX: 0000000000000001 RSI: 1ffff11000eebf9e RDI: ffffed1000eebfc6
>> >  RBP: ffff88804362fa00 R08: ffffffff81182e58 R09: ffff88800775fbdf
>> >  R10: ffffed1000eebf7b R11: 0000000000000001 R12: 000000008c800d00
>> >  R13: ffff888005e78040 R14: 0000000000000000 R15: ffff888005cdfad0
>> >  FS:  0000000000000000(0000) GS:ffff88807ed00000(0000) knlGS:0000000000000000
>> >  CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
>> >  CR2: 0000000000000000 CR3: 000000000360e001 CR4: 0000000000370ee0
>> >  DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
>> >  DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
>> >  Call Trace:
>> >   <TASK>
>> >   test_exit+0x1a/0x30
>> >   kunit_try_run_case+0xad/0xc0
>> >   kunit_generic_run_threadfn_adapter+0x26/0x50
>> >   kthread+0x17b/0x1b0
>> > 
>> > It was biscted to commit c7323a5ad078 ("mm/slub: restrict sysfs
>> > validation to debug caches and make it safe")
>> > 
>> > The problem is inside free_debug_processing(), in one path, the slab
>> > on partial list is missed to be freed when partial list is full.
>> > 
>> > Signed-off-by: Feng Tang <feng.tang@intel.com>
>> > ---
>> > 
>> > Hi reviewers,
>> > 
>> > Sorry for the late reporting, but it's curious that this problem didn't
>> > show up in my earlier test (which caught some other problems).
>> 
>> I think we can reuse the slab_free and don't need a new bool?
> 
> Yes, much simpler!
> 
> Thanks,
> Feng
> 
>> diff --git a/mm/slub.c b/mm/slub.c
>> index 5c3c31a154ba..a63953f649ed 100644
>> --- a/mm/slub.c
>> +++ b/mm/slub.c
>> @@ -2886,22 +2886,25 @@ static noinline void free_debug_processing(
>>                 set_freepointer(s, tail, prior);
>>                 slab->freelist = head;
>>  
>> -               /* Do we need to remove the slab from full or partial list? */
>> +               /*
>> +                * If the slab is empty, and node's partial list is full,
>> +                * it should be discarded anyway no matter it's on full or
>> +                * partial list.
>> +                */
>> +               if (slab->inuse == 0 && n->nr_partial >= s->min_partial)
>> +                       slab_free = slab;
>> +
>>                 if (!prior) {
>> +                       /* was on full list */
>>                         remove_full(s, n, slab);
>> -               } else if (slab->inuse == 0 &&
>> -                          n->nr_partial >= s->min_partial) {
>> +                       if (!slab_free) {
>> +                               add_partial(n, slab, DEACTIVATE_TO_TAIL);
>> +                               stat(s, FREE_ADD_PARTIAL);
>> +                       }
>> +               } else if (slab_free) {
>>                         remove_partial(n, slab);
>>                         stat(s, FREE_REMOVE_PARTIAL);
>>                 }
>> -
>> -               /* Do we need to discard the slab or add to partial list? */
>> -               if (slab->inuse == 0 && n->nr_partial >= s->min_partial) {
>> -                       slab_free = slab;
>> -               } else if (!prior) {
>> -                       add_partial(n, slab, DEACTIVATE_TO_TAIL);
>> -                       stat(s, FREE_ADD_PARTIAL);
>> -               }
>>         }
>>  
>>         if (slab_free) {