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SubjectRe: [PATCH v3 1/3] mm: kmemleak: Make the tool tolerant to struct scan_area allocation failures
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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

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