Re: [PATCH v2] lib/stackdepot: Double DEPOT_POOLS_CAP if KASAN is enabled

[Lorenzo Stoakes]

On Wed, Aug 07, 2024 at 10:32:15PM GMT, Waiman Long 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.
>
> 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.
>
> 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.
>
> Also use the MIN() macro for defining DEPOT_MAX_POOLS to clarify the
> intention.
>
> Fixes: fc60e0caa94d ("lib/stackdepot: use fixed-sized slots for stack records")
> Signed-off-by: Waiman Long <longman@xxxxxxxxxx>
> ---
>  lib/stackdepot.c | 7 ++++---
>  1 file changed, 4 insertions(+), 3 deletions(-)
>
> diff --git a/lib/stackdepot.c b/lib/stackdepot.c
> index 5ed34cc963fc..beeb70b57710 100644
> --- a/lib/stackdepot.c
> +++ b/lib/stackdepot.c
> @@ -36,11 +36,12 @@
>  #include <linux/memblock.h>
>  #include <linux/kasan-enabled.h>
>
> -#define DEPOT_POOLS_CAP 8192
> +/* KASAN is a big user of stackdepot, double the cap if KASAN is enabled */
> +#define DEPOT_POOLS_CAP (8192 * (IS_ENABLED(CONFIG_KASAN) ? 2 : 1))
> +
>  /* The pool_index is offset by 1 so the first record does not have a 0 handle. */
>  #define DEPOT_MAX_POOLS \
> -	(((1LL << (DEPOT_POOL_INDEX_BITS)) - 1 < DEPOT_POOLS_CAP) ? \
> -	 (1LL << (DEPOT_POOL_INDEX_BITS)) - 1 : DEPOT_POOLS_CAP)
> +	MIN((1LL << (DEPOT_POOL_INDEX_BITS)) - 1, DEPOT_POOLS_CAP)

This breaks the build for the mm-unstable branch in Andrew's mm tree which this
patch has been taken to.

This is because we don't appear to have commit 1a251f52cfdc ("minmax: make
generic MIN() and MAX() macros available everywhere").

This commit is in Linus's tree so perhaps the easiest solution is to pull this
into the mm-unstable branch?

Thanks!

>
>  static bool stack_depot_disabled;
>  static bool __stack_depot_early_init_requested __initdata = IS_ENABLED(CONFIG_STACKDEPOT_ALWAYS_INIT);
> --
> 2.43.5
>
>