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Subject: Re: [PATCH v3 1/3] mm: kmemleak: Make the tool tolerant to struct
 scan_area allocation failures
To:     Catalin Marinas <catalin.marinas@arm.com>, linux-mm@kvack.org
Cc:     linux-kernel@vger.kernel.org,
        Andrew Morton <akpm@linux-foundation.org>,
        Michal Hocko <mhocko@kernel.org>,
        Matthew Wilcox <willy@infradead.org>, Qian Cai <cai@lca.pw>
References: <20190812160642.52134-1-catalin.marinas@arm.com>
 <20190812160642.52134-2-catalin.marinas@arm.com>
From:   Alexey Kardashevskiy <aik@ozlabs.ru>
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Message-ID: <2ac37341-097e-17a2-fb6b-7912da9fa38e@ozlabs.ru>
Date:   Thu, 3 Oct 2019 16:13:07 +1000
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On 13/08/2019 02:06, Catalin Marinas wrote:
> Object scan areas are an optimisation aimed to decrease the false
> positives and slightly improve the scanning time of large objects known
> to only have a few specific pointers. If a struct scan_area fails to
> allocate, kmemleak can still function normally by scanning the full
> object.
> 
> Introduce an OBJECT_FULL_SCAN flag and mark objects as such when
> scan_area allocation fails.


I came across this one while bisecting sudden drop in throughput of a 100Gbit Mellanox CX4 ethernet card in a PPC POWER9
system, the speed dropped from 100Gbit to about 40Gbit. Bisect pointed at dba82d943177, this are the relevant config
options:

[fstn1-p1 kernel]$ grep KMEMLEAK ~/pbuild/kernel-le-4g/.config
CONFIG_HAVE_DEBUG_KMEMLEAK=y
CONFIG_DEBUG_KMEMLEAK=y
CONFIG_DEBUG_KMEMLEAK_EARLY_LOG_SIZE=16000
# CONFIG_DEBUG_KMEMLEAK_TEST is not set
# CONFIG_DEBUG_KMEMLEAK_DEFAULT_OFF is not set
CONFIG_DEBUG_KMEMLEAK_AUTO_SCAN=y

Setting CONFIG_DEBUG_KMEMLEAK_MEM_POOL_SIZE=400 or even 4000 (this is what KMEMLEAK_EARLY_LOG_SIZE is now in the master)
produces soft lockups on the recent upstream (sha1 a3c0e7b1fe1f):

[c000001fde64fb60] [c000000000c24ed4] _raw_write_unlock_irqrestore+0x54/0x70
[c000001fde64fb90] [c0000000004117e4] find_and_remove_object+0xa4/0xd0
[c000001fde64fbe0] [c000000000411c74] delete_object_full+0x24/0x50
[c000001fde64fc00] [c000000000411d28] __kmemleak_do_cleanup+0x88/0xd0
[c000001fde64fc40] [c00000000012a1a4] process_one_work+0x374/0x6a0
[c000001fde64fd20] [c00000000012a548] worker_thread+0x78/0x5a0
[c000001fde64fdb0] [c000000000135508] kthread+0x198/0x1a0
[c000001fde64fe20] [c00000000000b980] ret_from_kernel_thread+0x5c/0x7c

KMEMLEAK_EARLY_LOG_SIZE=8000 works but slow.

Interestingly KMEMLEAK_EARLY_LOG_SIZE=400 on dba82d943177 still worked and I saw my 100Gbit. Disabling KMEMLEAK also
fixes the speed (apparently).

Is that something expected? Thanks,


> 
> Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
> ---
>  mm/kmemleak.c | 16 ++++++++++------
>  1 file changed, 10 insertions(+), 6 deletions(-)
> 
> diff --git a/mm/kmemleak.c b/mm/kmemleak.c
> index f6e602918dac..5ba7fad00fda 100644
> --- a/mm/kmemleak.c
> +++ b/mm/kmemleak.c
> @@ -168,6 +168,8 @@ struct kmemleak_object {
>  #define OBJECT_REPORTED		(1 << 1)
>  /* flag set to not scan the object */
>  #define OBJECT_NO_SCAN		(1 << 2)
> +/* flag set to fully scan the object when scan_area allocation failed */
> +#define OBJECT_FULL_SCAN	(1 << 3)
>  
>  #define HEX_PREFIX		"    "
>  /* number of bytes to print per line; must be 16 or 32 */
> @@ -773,12 +775,14 @@ static void add_scan_area(unsigned long ptr, size_t size, gfp_t gfp)
>  	}
>  
>  	area = kmem_cache_alloc(scan_area_cache, gfp_kmemleak_mask(gfp));
> -	if (!area) {
> -		pr_warn("Cannot allocate a scan area\n");
> -		goto out;
> -	}
>  
>  	spin_lock_irqsave(&object->lock, flags);
> +	if (!area) {
> +		pr_warn_once("Cannot allocate a scan area, scanning the full object\n");
> +		/* mark the object for full scan to avoid false positives */
> +		object->flags |= OBJECT_FULL_SCAN;
> +		goto out_unlock;
> +	}
>  	if (size == SIZE_MAX) {
>  		size = object->pointer + object->size - ptr;
>  	} else if (ptr + size > object->pointer + object->size) {
> @@ -795,7 +799,6 @@ static void add_scan_area(unsigned long ptr, size_t size, gfp_t gfp)
>  	hlist_add_head(&area->node, &object->area_list);
>  out_unlock:
>  	spin_unlock_irqrestore(&object->lock, flags);
> -out:
>  	put_object(object);
>  }
>  
> @@ -1408,7 +1411,8 @@ static void scan_object(struct kmemleak_object *object)
>  	if (!(object->flags & OBJECT_ALLOCATED))
>  		/* already freed object */
>  		goto out;
> -	if (hlist_empty(&object->area_list)) {
> +	if (hlist_empty(&object->area_list) ||
> +	    object->flags & OBJECT_FULL_SCAN) {
>  		void *start = (void *)object->pointer;
>  		void *end = (void *)(object->pointer + object->size);
>  		void *next;
> 

-- 
Alexey