linux-kernel - Re: [PATCH v2] kmemleak: survive in a low-memory situation

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Message-ID: <9197d86b-a684-c7f4-245b-63c890f1104f@lca.pw>
Date:   Thu, 3 Jan 2019 11:51:57 -0500
From:   Qian Cai <cai@....pw>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     catalin.marinas@....com, akpm@...ux-foundation.org, cl@...ux.com,
        penberg@...nel.org, rientjes@...gle.com, iamjoonsoo.kim@....com,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] kmemleak: survive in a low-memory situation

On 1/3/19 4:32 AM, Michal Hocko wrote:
> On Wed 02-01-19 13:06:19, Qian Cai wrote:
> [...]
>> diff --git a/mm/kmemleak.c b/mm/kmemleak.c
>> index f9d9dc250428..9e1aa3b7df75 100644
>> --- a/mm/kmemleak.c
>> +++ b/mm/kmemleak.c
>> @@ -576,6 +576,16 @@ static struct kmemleak_object *create_object(unsigned long ptr, size_t size,
>>  	struct rb_node **link, *rb_parent;
>>  
>>  	object = kmem_cache_alloc(object_cache, gfp_kmemleak_mask(gfp));
>> +#ifdef CONFIG_PREEMPT_COUNT
>> +	if (!object) {
>> +		/* last-ditch effort in a low-memory situation */
>> +		if (irqs_disabled() || is_idle_task(current) || in_atomic())
>> +			gfp = GFP_ATOMIC;
>> +		else
>> +			gfp = gfp_kmemleak_mask(gfp) | __GFP_DIRECT_RECLAIM;
>> +		object = kmem_cache_alloc(object_cache, gfp);
>> +	}
>> +#endif
> 
> I do not get it. How can this possibly help when gfp_kmemleak_mask()
> adds __GFP_NOFAIL modifier to the given gfp mask? Or is this not the
> case anymore in some tree?

Well, __GFP_NOFAIL can still fail easily without __GFP_DIRECT_RECLAIM in a
low-memory situation.

__alloc_pages_slowpath():

/* Caller is not willing to reclaim, we can't balance anything */
if (!can_direct_reclaim)
	goto nopage;

nopage:

/*
 * All existing users of the __GFP_NOFAIL are blockable, so
 * warn of any new users that actually require GFP_NOWAIT
 */
if (WARN_ON_ONCE(!can_direct_reclaim))
	goto fail;