Re: [PATCH v4] kmemleak: survive in a low-memory situation

[Qian Cai <cai@lca.pw>]

On 3/27/19 4:44 AM, Michal Hocko wrote:
>> diff --git a/mm/kmemleak.c b/mm/kmemleak.c
>> index a2d894d3de07..7f4545ab1f84 100644
>> --- a/mm/kmemleak.c
>> +++ b/mm/kmemleak.c
>> @@ -580,7 +580,16 @@ static struct kmemleak_object *create_object(unsigned long ptr, size_t size,
>>  	struct rb_node **link, *rb_parent;
>>  	unsigned long untagged_ptr;
>>  
>> -	object = kmem_cache_alloc(object_cache, gfp_kmemleak_mask(gfp));
>> +	/*
>> +	 * The tracked memory was allocated successful, if the kmemleak object
>> +	 * failed to allocate for some reasons, it ends up with the whole
>> +	 * kmemleak disabled, so try it harder.
>> +	 */
>> +	gfp = (in_atomic() || irqs_disabled()) ?
>> +	       gfp_kmemleak_mask(gfp) | GFP_ATOMIC :
>> +	       gfp_kmemleak_mask(gfp) | __GFP_DIRECT_RECLAIM;
> 
> 
> The comment for in_atomic says:
>  * Are we running in atomic context?  WARNING: this macro cannot
>  * always detect atomic context; in particular, it cannot know about
>  * held spinlocks in non-preemptible kernels.  Thus it should not be
>  * used in the general case to determine whether sleeping is possible.
>  * Do not use in_atomic() in driver code.

That is why it needs both in_atomic() and irqs_disabled(), so irqs_disabled()
can detect kernel functions held spinlocks even in non-preemptible kernels.

According to [1],

"This [2] is useful if you know that the data in question is only ever
manipulated from a "process context", ie no interrupts involved."

Since kmemleak only deal with kernel context, if a spinlock was held, it always
has local interrupt disabled.

ftrace is in the same boat where this commit was merged a while back that has
the same check.

ef99b88b16be
tracing: Handle ftrace_dump() atomic context in graph_trace_open()

[1] https://www.kernel.org/doc/Documentation/locking/spinlocks.txt
[2]
	spin_lock(&lock);
	...
	spin_unlock(&lock);