Re: [PATCH v3] lib/stackdepot: Double DEPOT_POOLS_CAP if KASAN is enabled

[Waiman Long]

On 8/8/24 12:12, Andrey Konovalov wrote:
> On Thu, Aug 8, 2024 at 2:54 PM Waiman Long <longman@xxxxxxxxxx> wrote:
> > As said in commit 02754e0a484a ("lib/stackdepot.c: bump stackdepot
> > capacity from 16MB to 128MB"), KASAN uses stackdepot to memorize stacks
> > for all kmalloc/kfree calls. So stackdepot capacity was increased 8
> > times to accommodate KASAN usage even thought it was claimed 4X should
> > be enough at that time.
> >
> > With commit fc60e0caa94d ("lib/stackdepot: use fixed-sized slots
> > for stack records"), all stackdepot records uses a fixed size with
> > CONFIG_STACKDEPOT_MAX_FRAMES (default=64) entries. This is merged to
> > support evictable KASAN stack records. Commit 31639fd6cebd ("stackdepot:
> > use variable size records for non-evictable entries") re-enabled
> > the use of variable size records for non-KASAN use cases, but KASAN
> > (generic mode) still uses the large fixed size stack records.
> No, since commit 711d349174fd ("kasan: revert eviction of stack traces
> in generic mode") Generic KASAN doesn't use fixed-sized slots.
>
> > With the default CONFIG_STACKDEPOT_MAX_FRAMES of 64, KASAN use of
> > stackdepot space had been more than double than before. Assuming an
> > average stack frame size of 16, a KASAN stack record is almost 4X the
> > size of a non-KASAN one.
> So this is not correct.
Yes, you are right. I missed the KASAN commit 711d349174fd in my 
analysis. I will update the commit log in the next version.
> > When a wide variety of workloads are run on a debug kernel with KASAN
> > enabled, the following warning may sometimes be printed.
> >
> >   [ 6818.650674] Stack depot reached limit capacity
> >   [ 6818.650730] WARNING: CPU: 1 PID: 272741 at lib/stackdepot.c:252 depot_alloc_stack+0x39e/0x3d0
> >     :
> >   [ 6818.650907] Call Trace:
> >   [ 6818.650909]  [<00047dd453d84b92>] depot_alloc_stack+0x3a2/0x3d0
> >   [ 6818.650916]  [<00047dd453d85254>] stack_depot_save_flags+0x4f4/0x5c0
> >   [ 6818.650920]  [<00047dd4535872c6>] kasan_save_stack+0x56/0x70
> >   [ 6818.650924]  [<00047dd453587328>] kasan_save_track+0x28/0x40
> >   [ 6818.650927]  [<00047dd45358a27a>] kasan_save_free_info+0x4a/0x70
> >   [ 6818.650930]  [<00047dd45358766a>] __kasan_slab_free+0x12a/0x1d0
> >   [ 6818.650933]  [<00047dd45350deb4>] kmem_cache_free+0x1b4/0x580
> >   [ 6818.650938]  [<00047dd452c520da>] __put_task_struct+0x24a/0x320
> >   [ 6818.650945]  [<00047dd452c6aee4>] delayed_put_task_struct+0x294/0x350
> >   [ 6818.650949]  [<00047dd452e9066a>] rcu_do_batch+0x6ea/0x2090
> >   [ 6818.650953]  [<00047dd452ea60f4>] rcu_core+0x474/0xa90
> >   [ 6818.650956]  [<00047dd452c780c0>] handle_softirqs+0x3c0/0xf90
> >   [ 6818.650960]  [<00047dd452c76fbe>] __irq_exit_rcu+0x35e/0x460
> >   [ 6818.650963]  [<00047dd452c79992>] irq_exit_rcu+0x22/0xb0
> >   [ 6818.650966]  [<00047dd454bd8128>] do_ext_irq+0xd8/0x120
> >   [ 6818.650972]  [<00047dd454c0ddd0>] ext_int_handler+0xb8/0xe8
> >   [ 6818.650979]  [<00047dd453589cf6>] kasan_check_range+0x236/0x2f0
> >   [ 6818.650982]  [<00047dd453378cf0>] filemap_get_pages+0x190/0xaa0
> >   [ 6818.650986]  [<00047dd453379940>] filemap_read+0x340/0xa70
> >   [ 6818.650989]  [<00047dd3d325d226>] xfs_file_buffered_read+0x2c6/0x400 [xfs]
> >   [ 6818.651431]  [<00047dd3d325dfe2>] xfs_file_read_iter+0x2c2/0x550 [xfs]
> >   [ 6818.651663]  [<00047dd45364710c>] vfs_read+0x64c/0x8c0
> >   [ 6818.651669]  [<00047dd453648ed8>] ksys_read+0x118/0x200
> >   [ 6818.651672]  [<00047dd452b6cf5a>] do_syscall+0x27a/0x380
> >   [ 6818.651676]  [<00047dd454bd7e74>] __do_syscall+0xf4/0x1a0
> >   [ 6818.651680]  [<00047dd454c0db58>] system_call+0x70/0x98
> >
> > With all the recent changes in stackdepot to support new KASAN features,
> > it is obvious that the current DEPOT_POOLS_CAP of 8192 may not be
> > enough when KASAN is enabled. Fix this stackdepot capability issue
> > by doubling DEPOT_POOLS_CAP if KASAN is enabled. With 4k pages, the
> > maximum stackdepot capacity is doubled to 256 MB with KASAN enabled.
> It is possible that the stack depot runs out of space due to a truly
> large number of unique stack traces, but I would first make sure that
> is indeed the case. The one thing to check would be to dump all the
> stack traces from the stack depot when it overflows, and check whether
> they make sense. There have been cases in the past, when e.g. the task
> context part of a stack trace from an interrupt didn't get stripped
> properly, and thus almost each stack trace from an interrupt was
> considered unique by the stack depot. Perhaps, something similar
> started happening again.

It could be.

Anyway, I got a bug report from our QE team and they have seen it once 
so far. So it is not an easily reproducible problem. I need to do 
further investigation first. Thanks for your quick response.

Cheers,
Longman