Re: [PATCH 1/1] sched/numa: Fix memory leak due to the overwritten vma->numab_state

[Raghavendra K T <raghavendra.kt@amd.com>]

On 11/8/2024 7:01 PM, Adrian Huang wrote:
> From: Adrian Huang <ahuang12@lenovo.com>
> 
> [Problem Description]
> When running the hackbench program of LTP, the following memory leak is
> reported by kmemleak.
> 
>    # /opt/ltp/testcases/bin/hackbench 20 thread 1000
>    Running with 20*40 (== 800) tasks.
> 
>    # dmesg | grep kmemleak
>    ...
>    kmemleak: 480 new suspected memory leaks (see /sys/kernel/debug/kmemleak)
>    kmemleak: 665 new suspected memory leaks (see /sys/kernel/debug/kmemleak)
> 
>    # cat /sys/kernel/debug/kmemleak
>    unreferenced object 0xffff888cd8ca2c40 (size 64):
>      comm "hackbench", pid 17142, jiffies 4299780315
>      hex dump (first 32 bytes):
>        ac 74 49 00 01 00 00 00 4c 84 49 00 01 00 00 00  .tI.....L.I.....
>        00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
>      backtrace (crc bff18fd4):
>        [<ffffffff81419a89>] __kmalloc_cache_noprof+0x2f9/0x3f0
>        [<ffffffff8113f715>] task_numa_work+0x725/0xa00
>        [<ffffffff8110f878>] task_work_run+0x58/0x90
>        [<ffffffff81ddd9f8>] syscall_exit_to_user_mode+0x1c8/0x1e0
>        [<ffffffff81dd78d5>] do_syscall_64+0x85/0x150
>        [<ffffffff81e0012b>] entry_SYSCALL_64_after_hwframe+0x76/0x7e
>    ...
> 
>    This issue can be consistently reproduced on three different servers:
>      * a 448-core server
>      * a 256-core server
>      * a 192-core server
> 
> [Root Cause]
> Since multiple threads are created by the hackbench program (along with
> the command argument 'thread'), a shared vma might be accessed by two or
> more cores simultaneously. When two or more cores observe that
> vma->numab_state is NULL at the same time, vma->numab_state will be
> overwritten.
> 

Thanks for reporting.

IIRC, This is not the entire scenario. Chunk above the vma->numab code
ideally ensures, only one thread descend down to scan the VMA's in a
single 'numa_scan_period'

     migrate = mm->numa_next_scan;
         if (time_before(now, migrate))
                 return;
         next_scan = now + msecs_to_jiffies(p->numa_scan_period);
         if (!try_cmpxchg(&mm->numa_next_scan, &migrate, next_scan))
                 return;

However since there are 800 threads, I see there might be an opportunity
for another thread to enter in the next 'numa_scan_period' while
we have not gotten till numab_state allocation.

There should be simpler ways to overcome like Vlastimil already pointed
in the other thread, and having lock is an overkill.

for e.g.,
numab_state = kzalloc(..)

if we see that some other thread able to successfully assign
vma->numab_state with their allocation (with cmpxchg), simply
free your allocation.

Can you please check if my understanding is correct?

Thanks
- Raghu

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