Re: [PATCH V2] mm/page_alloc: skip debug_check_no_{obj,locks}_freed with FPI_TRYLOCK

[Vlastimil Babka <vbabka@suse.cz>]

On 2/9/26 07:26, Harry Yoo wrote:
> When CONFIG_DEBUG_OBJECTS_FREE is enabled,
> debug_check_no_{obj,locks}_freed() functions are called.
> 
> Since both of them spin on a lock, they are not safe to be called
> if the FPI_TRYLOCK flag is specified. This leads to a lockdep splat:
> 
>   ================================
>   WARNING: inconsistent lock state
>   6.19.0-rc5-slab-for-next+ #326 Tainted: G                 N
>   --------------------------------
>   inconsistent {INITIAL USE} -> {IN-NMI} usage.
>   kunit_try_catch/9046 [HC2[2]:SC0[0]:HE0:SE1] takes:
>   ffffffff84ed6bf8 (&obj_hash[i].lock){-.-.}-{2:2}, at: __debug_check_no_obj_freed+0xe0/0x300
>   {INITIAL USE} state was registered at:
>     lock_acquire+0xd9/0x2f0
>     _raw_spin_lock_irqsave+0x4c/0x80
>     __debug_object_init+0x9d/0x1f0
>     debug_object_init+0x34/0x50
>     __init_work+0x28/0x40
>     init_cgroup_housekeeping+0x151/0x210
>     init_cgroup_root+0x3d/0x140
>     cgroup_init_early+0x30/0x240
>     start_kernel+0x3e/0xcd0
>     x86_64_start_reservations+0x18/0x30
>     x86_64_start_kernel+0xf3/0x140
>     common_startup_64+0x13e/0x148
>   irq event stamp: 2998
>   hardirqs last  enabled at (2997): [<ffffffff8298b77a>] exc_nmi+0x11a/0x240
>   hardirqs last disabled at (2998): [<ffffffff8298b991>] sysvec_irq_work+0x11/0x110
>   softirqs last  enabled at (1416): [<ffffffff813c1f72>] __irq_exit_rcu+0x132/0x1c0
>   softirqs last disabled at (1303): [<ffffffff813c1f72>] __irq_exit_rcu+0x132/0x1c0
> 
>   other info that might help us debug this:
>    Possible unsafe locking scenario:
> 
>          CPU0
>          ----
>     lock(&obj_hash[i].lock);
>     <Interrupt>
>       lock(&obj_hash[i].lock);
> 
>    *** DEADLOCK ***
> 
> Rename free_pages_prepare() to __free_pages_prepare(), add an fpi_t
> parameter, and skip those checks if FPI_TRYLOCK is set. To keep the
> fpi_t definition in mm/page_alloc.c, add a wrapper function
> free_pages_prepare() that always passes FPI_NONE and use it in
> mm/compaction.c.
> 
> Fixes: 8c57b687e833 ("mm, bpf: Introduce free_pages_nolock()")
> Cc: <stable@vger.kernel.org>
> Signed-off-by: Harry Yoo <harry.yoo@oracle.com>

Reviewed-by: Vlastimil Babka <vbabka@suse.cz>

Thanks!