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SubjectRe: [PATCH v4] kmemleak: survive in a low-memory situation
On Wed 27-03-19 17:29:57, Catalin Marinas wrote:
[...]
> Quick attempt below and it needs some more testing (pretty random pick
> of the EMERGENCY_POOL_SIZE value). Also, with __GFP_NOFAIL removed, are
> the other flags safe or we should trim them further?

I would be still careful about __GFP_NORETRY. I pressume that the
primary purpose is to prevent from the OOM killer but this makes the
allocation failure much more likely. So if anything __GFP_RETRY_MAYFAIL
would suite better for that purpose. But I am not really sure that this
is worth bothering.

> ---------------8<-------------------------------
> >From dc4194539f8191bb754901cea74c86e7960886f8 Mon Sep 17 00:00:00 2001
> From: Catalin Marinas <catalin.marinas@arm.com>
> Date: Wed, 27 Mar 2019 17:20:57 +0000
> Subject: [PATCH] mm: kmemleak: Add an emergency allocation pool for kmemleak
> objects
>
> This patch adds an emergency pool for struct kmemleak_object in case the
> normal kmem_cache_alloc() fails under the gfp constraints passed by the
> slab allocation caller. The patch also removes __GFP_NOFAIL which does
> not play well with other gfp flags (introduced by commit d9570ee3bd1d,
> "kmemleak: allow to coexist with fault injection").

Thank you! This is definitely a step into the right direction. Maybe the
pool allocation logic will need some tuning - e.g. does it make sense to
allocate into the pool from sleepable allocations - or is it sufficient
to refill on free. Something for the real workloads to tell, I guess.

> Suggested-by: Michal Hocko <mhocko@kernel.org>
> Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
> ---
> mm/kmemleak.c | 59 +++++++++++++++++++++++++++++++++++++++++++++++++--
> 1 file changed, 57 insertions(+), 2 deletions(-)
>
> diff --git a/mm/kmemleak.c b/mm/kmemleak.c
> index 6c318f5ac234..366a680cff7c 100644
> --- a/mm/kmemleak.c
> +++ b/mm/kmemleak.c
> @@ -127,7 +127,7 @@
> /* GFP bitmask for kmemleak internal allocations */
> #define gfp_kmemleak_mask(gfp) (((gfp) & (GFP_KERNEL | GFP_ATOMIC)) | \
> __GFP_NORETRY | __GFP_NOMEMALLOC | \
> - __GFP_NOWARN | __GFP_NOFAIL)
> + __GFP_NOWARN)
>
> /* scanning area inside a memory block */
> struct kmemleak_scan_area {
> @@ -191,11 +191,16 @@ struct kmemleak_object {
> #define HEX_ASCII 1
> /* max number of lines to be printed */
> #define HEX_MAX_LINES 2
> +/* minimum emergency pool size */
> +#define EMERGENCY_POOL_SIZE (NR_CPUS * 4)
>
> /* the list of all allocated objects */
> static LIST_HEAD(object_list);
> /* the list of gray-colored objects (see color_gray comment below) */
> static LIST_HEAD(gray_list);
> +/* emergency pool allocation */
> +static LIST_HEAD(emergency_list);
> +static int emergency_pool_size;
> /* search tree for object boundaries */
> static struct rb_root object_tree_root = RB_ROOT;
> /* rw_lock protecting the access to object_list and object_tree_root */
> @@ -467,6 +472,43 @@ static int get_object(struct kmemleak_object *object)
> return atomic_inc_not_zero(&object->use_count);
> }
>
> +/*
> + * Emergency pool allocation and freeing. kmemleak_lock must not be held.
> + */
> +static struct kmemleak_object *emergency_alloc(void)
> +{
> + unsigned long flags;
> + struct kmemleak_object *object;
> +
> + write_lock_irqsave(&kmemleak_lock, flags);
> + object = list_first_entry_or_null(&emergency_list, typeof(*object), object_list);
> + if (object) {
> + list_del(&object->object_list);
> + emergency_pool_size--;
> + }
> + write_unlock_irqrestore(&kmemleak_lock, flags);
> +
> + return object;
> +}
> +
> +/*
> + * Return true if object added to the emergency pool, false otherwise.
> + */
> +static bool emergency_free(struct kmemleak_object *object)
> +{
> + unsigned long flags;
> +
> + if (emergency_pool_size >= EMERGENCY_POOL_SIZE)
> + return false;
> +
> + write_lock_irqsave(&kmemleak_lock, flags);
> + list_add(&object->object_list, &emergency_list);
> + emergency_pool_size++;
> + write_unlock_irqrestore(&kmemleak_lock, flags);
> +
> + return true;
> +}
> +
> /*
> * RCU callback to free a kmemleak_object.
> */
> @@ -485,7 +527,8 @@ static void free_object_rcu(struct rcu_head *rcu)
> hlist_del(&area->node);
> kmem_cache_free(scan_area_cache, area);
> }
> - kmem_cache_free(object_cache, object);
> + if (!emergency_free(object))
> + kmem_cache_free(object_cache, object);
> }
>
> /*
> @@ -577,6 +620,8 @@ static struct kmemleak_object *create_object(unsigned long ptr, size_t size,
> unsigned long untagged_ptr;
>
> object = kmem_cache_alloc(object_cache, gfp_kmemleak_mask(gfp));
> + if (!object)
> + object = emergency_alloc();
> if (!object) {
> pr_warn("Cannot allocate a kmemleak_object structure\n");
> kmemleak_disable();
> @@ -2127,6 +2172,16 @@ void __init kmemleak_init(void)
> kmemleak_warning = 0;
> }
> }
> +
> + /* populate the emergency allocation pool */
> + while (emergency_pool_size < EMERGENCY_POOL_SIZE) {
> + struct kmemleak_object *object;
> +
> + object = kmem_cache_alloc(object_cache, GFP_KERNEL);
> + if (!object)
> + break;
> + emergency_free(object);
> + }
> }
>
> /*

--
Michal Hocko
SUSE Labs

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