Re: [PATCH 00/14] replace call_rcu by kfree_rcu for simple kmem_cache_free callback

Paul E. McKenney
2024-06-14T14:17:30Z

On Fri, Jun 14, 2024 at 02:35:33PM +0200, Uladzislau Rezki wrote:
> On Thu, Jun 13, 2024 at 11:13:52AM -0700, Paul E. McKenney wrote:
> > On Thu, Jun 13, 2024 at 07:58:17PM +0200, Uladzislau Rezki wrote:
> > > On Thu, Jun 13, 2024 at 10:45:59AM -0700, Paul E. McKenney wrote:
> > > > On Thu, Jun 13, 2024 at 07:38:59PM +0200, Uladzislau Rezki wrote:
> > > > > On Thu, Jun 13, 2024 at 08:06:30AM -0700, Paul E. McKenney wrote:
> > > > > > On Thu, Jun 13, 2024 at 03:06:54PM +0200, Uladzislau Rezki wrote:
> > > > > > > On Thu, Jun 13, 2024 at 05:47:08AM -0700, Paul E. McKenney wrote:
> > > > > > > > On Thu, Jun 13, 2024 at 01:58:59PM +0200, Jason A. Donenfeld wrote:
> > > > > > > > > On Wed, Jun 12, 2024 at 03:37:55PM -0700, Paul E. McKenney wrote:
> > > > > > > > > > On Wed, Jun 12, 2024 at 02:33:05PM -0700, Jakub Kicinski wrote:
> > > > > > > > > > > On Sun, 9 Jun 2024 10:27:12 +0200 Julia Lawall wrote:
> > > > > > > > > > > > Since SLOB was removed, it is not necessary to use call_rcu
> > > > > > > > > > > > when the callback only performs kmem_cache_free. Use
> > > > > > > > > > > > kfree_rcu() directly.
> > > > > > > > > > > >
> > > > > > > > > > > > The changes were done using the following Coccinelle semantic patch.
> > > > > > > > > > > > This semantic patch is designed to ignore cases where the callback
> > > > > > > > > > > > function is used in another way.
> > > > > > > > > > >
> > > > > > > > > > > How does the discussion on:
> > > > > > > > > > > [PATCH] Revert "batman-adv: prefer kfree_rcu() over call_rcu() with free-only callbacks"
> > > > > > > > > > > https://lore.kernel.org/all/20240612133357.2596-1-linus.luessing@c0d3.blue/
> > > > > > > > > > > reflect on this series? IIUC we should hold off..
> > > > > > > > > >
> > > > > > > > > > We do need to hold off for the ones in kernel modules (such as 07/14)
> > > > > > > > > > where the kmem_cache is destroyed during module unload.
> > > > > > > > > >
> > > > > > > > > > OK, I might as well go through them...
> > > > > > > > > >
> > > > > > > > > > [PATCH 01/14] wireguard: allowedips: replace call_rcu by kfree_rcu for simple kmem_cache_free callback
> > > > > > > > > > Needs to wait, see wg_allowedips_slab_uninit().
> > > > > > > > >
> > > > > > > > > Also, notably, this patch needs additionally:
> > > > > > > > >
> > > > > > > > > diff --git a/drivers/net/wireguard/allowedips.c b/drivers/net/wireguard/allowedips.c
> > > > > > > > > index e4e1638fce1b..c95f6937c3f1 100644
> > > > > > > > > --- a/drivers/net/wireguard/allowedips.c
> > > > > > > > > +++ b/drivers/net/wireguard/allowedips.c
> > > > > > > > > @@ -377,7 +377,6 @@ int __init wg_allowedips_slab_init(void)
> > > > > > > > >
> > > > > > > > > void wg_allowedips_slab_uninit(void)
> > > > > > > > > {
> > > > > > > > > - rcu_barrier();
> > > > > > > > > kmem_cache_destroy(node_cache);
> > > > > > > > > }
> > > > > > > > >
> > > > > > > > > Once kmem_cache_destroy has been fixed to be deferrable.
> > > > > > > > >
> > > > > > > > > I assume the other patches are similar -- an rcu_barrier() can be
> > > > > > > > > removed. So some manual meddling of these might be in order.
> > > > > > > >
> > > > > > > > Assuming that the deferrable kmem_cache_destroy() is the option chosen,
> > > > > > > > agreed.
> > > > > > > >
> > > > > > > <snip>
> > > > > > > void kmem_cache_destroy(struct kmem_cache *s)
> > > > > > > {
> > > > > > > int err = -EBUSY;
> > > > > > > bool rcu_set;
> > > > > > >
> > > > > > > if (unlikely(!s) || !kasan_check_byte(s))
> > > > > > > return;
> > > > > > >
> > > > > > > cpus_read_lock();
> > > > > > > mutex_lock(&slab_mutex);
> > > > > > >
> > > > > > > rcu_set = s->flags & SLAB_TYPESAFE_BY_RCU;
> > > > > > >
> > > > > > > s->refcount--;
> > > > > > > if (s->refcount)
> > > > > > > goto out_unlock;
> > > > > > >
> > > > > > > err = shutdown_cache(s);
> > > > > > > WARN(err, "%s %s: Slab cache still has objects when called from %pS",
> > > > > > > __func__, s->name, (void *)_RET_IP_);
> > > > > > > ...
> > > > > > > cpus_read_unlock();
> > > > > > > if (!err && !rcu_set)
> > > > > > > kmem_cache_release(s);
> > > > > > > }
> > > > > > > <snip>
> > > > > > >
> > > > > > > so we have SLAB_TYPESAFE_BY_RCU flag that defers freeing slab-pages
> > > > > > > and a cache by a grace period. Similar flag can be added, like
> > > > > > > SLAB_DESTROY_ONCE_FULLY_FREED, in this case a worker rearm itself
> > > > > > > if there are still objects which should be freed.
> > > > > > >
> > > > > > > Any thoughts here?
> > > > > >
> > > > > > Wouldn't we also need some additional code to later check for all objects
> > > > > > being freed to the slab, whether or not that code is initiated from
> > > > > > kmem_cache_destroy()?
> > > > > >
> > > > > Same away as SLAB_TYPESAFE_BY_RCU is handled from the kmem_cache_destroy() function.
> > > > > It checks that flag and if it is true and extra worker is scheduled to perform a
> > > > > deferred(instead of right away) destroy after rcu_barrier() finishes.
> > > >
> > > > Like this?
> > > >
> > > > SLAB_DESTROY_ONCE_FULLY_FREED
> > > >
> > > > Instead of adding a new kmem_cache_destroy_rcu()
> > > > or kmem_cache_destroy_wait() API member, instead add a
> > > > SLAB_DESTROY_ONCE_FULLY_FREED flag that can be passed to the
> > > > existing kmem_cache_destroy() function.  Use of this flag would
> > > > suppress any warnings that would otherwise be issued if there
> > > > was still slab memory yet to be freed, and it would also spawn
> > > > workqueues (or timers or whatever) to do any needed cleanup work.
> > > >
> > > >
> > > The flag is passed as all others during creating a cache:
> > >
> > > slab = kmem_cache_create(name, size, ..., SLAB_DESTROY_ONCE_FULLY_FREED | OTHER_FLAGS, NULL);
> > >
> > > the rest description is correct to me.
> >
> > Good catch, fixed, thank you!
> >
> And here we go with prototype(untested):

Thank you for putting this together! It looks way simpler than I would
have guessed, and quite a bit simpler than I would expect it would be
to extend rcu_barrier() to cover kfree_rcu().

> <snip>
> diff --git a/include/linux/slab.h b/include/linux/slab.h
> index 7247e217e21b..700b8a909f8a 100644
> --- a/include/linux/slab.h
> +++ b/include/linux/slab.h
> @@ -59,6 +59,7 @@ enum _slab_flag_bits {
> #ifdef CONFIG_SLAB_OBJ_EXT
> _SLAB_NO_OBJ_EXT,
> #endif
> + _SLAB_DEFER_DESTROY,
> _SLAB_FLAGS_LAST_BIT
> };
>
> @@ -139,6 +140,7 @@ enum _slab_flag_bits {
> */
> /* Defer freeing slabs to RCU */
> #define SLAB_TYPESAFE_BY_RCU __SLAB_FLAG_BIT(_SLAB_TYPESAFE_BY_RCU)
> +#define SLAB_DEFER_DESTROY __SLAB_FLAG_BIT(_SLAB_DEFER_DESTROY)
> /* Trace allocations and frees */
> #define SLAB_TRACE __SLAB_FLAG_BIT(_SLAB_TRACE)
>
> diff --git a/mm/slab_common.c b/mm/slab_common.c
> index 1560a1546bb1..99458a0197b5 100644
> --- a/mm/slab_common.c
> +++ b/mm/slab_common.c
> @@ -45,6 +45,11 @@ static void slab_caches_to_rcu_destroy_workfn(struct work_struct *work);
> static DECLARE_WORK(slab_caches_to_rcu_destroy_work,
> slab_caches_to_rcu_destroy_workfn);
>
> +static LIST_HEAD(slab_caches_defer_destroy);
> +static void slab_caches_defer_destroy_workfn(struct work_struct *work);
> +static DECLARE_DELAYED_WORK(slab_caches_defer_destroy_work,
> + slab_caches_defer_destroy_workfn);
> +
> /*
> * Set of flags that will prevent slab merging
> */
> @@ -448,6 +453,31 @@ static void slab_caches_to_rcu_destroy_workfn(struct work_struct *work)
> }
> }
>
> +static void
> +slab_caches_defer_destroy_workfn(struct work_struct *work)
> +{
> + struct kmem_cache *s, *s2;
> +
> + mutex_lock(&slab_mutex);
> + list_for_each_entry_safe(s, s2, &slab_caches_defer_destroy, list) {
> + if (__kmem_cache_empty(s)) {
> + /* free asan quarantined objects */
> + kasan_cache_shutdown(s);
> + (void) __kmem_cache_shutdown(s);
> +
> + list_del(&s->list);
> +
> + debugfs_slab_release(s);
> + kfence_shutdown_cache(s);
> + kmem_cache_release(s);
> + }

My guess is that there would want to be a splat if the slab stuck around
for too long, but maybe that should instead be handled elsewhere or in
some other way? I must defer to you guys on that one.

Thanx, Paul

> + }
> + mutex_unlock(&slab_mutex);
> +
> + if (!list_empty(&slab_caches_defer_destroy))
> + schedule_delayed_work(&slab_caches_defer_destroy_work, HZ);
> +}
> +
> static int shutdown_cache(struct kmem_cache *s)
> {
> /* free asan quarantined objects */
> @@ -493,6 +523,13 @@ void kmem_cache_destroy(struct kmem_cache *s)
> if (s->refcount)
> goto out_unlock;
>
> + /* Should a destroy process be deferred? */
> + if (s->flags & SLAB_DEFER_DESTROY) {
> + list_move_tail(&s->list, &slab_caches_defer_destroy);
> + schedule_delayed_work(&slab_caches_defer_destroy_work, HZ);
> + goto out_unlock;
> + }
> +
> err = shutdown_cache(s);
> WARN(err, "%s %s: Slab cache still has objects when called from %pS",
> __func__, s->name, (void *)_RET_IP_);
> <snip>