Hello, Joel.
>
> On Thu, Oct 29, 2020 at 05:50:18PM +0100, Uladzislau Rezki (Sony) wrote:
> > Given that CONFIG_PREEMPT_COUNT is unconditionally enabled by the
> > earlier commits in this series, the preemptible() macro now properly
> > detects preempt-disable code regions even in kernels built with
> > CONFIG_PREEMPT_NONE.
> >
> > This commit therefore uses preemptible() to determine whether allocation
> > is possible at all for double-argument kvfree_rcu(). If !preemptible(),
> > then allocation is not possible, and kvfree_rcu() falls back to using
> > the less cache-friendly rcu_head approach. Even when preemptible(),
> > the caller might be involved in reclaim, so the GFP_ flags used by
> > double-argument kvfree_rcu() must avoid invoking reclaim processing.
> >
> > Note that single-argument kvfree_rcu() must be invoked in sleepable
> > contexts, and that its fallback is the relatively high latency
> > synchronize_rcu(). Single-argument kvfree_rcu() therefore uses
> > GFP_KERNEL|__GFP_RETRY_MAYFAIL to allow limited sleeping within the
> > memory allocator.
> >
> > [ paulmck: Add add_ptr_to_bulk_krc_lock header comment per Michal Hocko. ]
> > Signed-off-by: Uladzislau Rezki (Sony) <urezki@xxxxxxxxx>
> > Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
> > ---
> > kernel/rcu/tree.c | 48 ++++++++++++++++++++++++++++++-----------------
> > 1 file changed, 31 insertions(+), 17 deletions(-)
> >
> > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> > index f2da2a1cc716..3f9b016a44dc 100644
> > --- a/kernel/rcu/tree.c
> > +++ b/kernel/rcu/tree.c
> > @@ -3406,37 +3406,55 @@ run_page_cache_worker(struct kfree_rcu_cpu *krcp)
> > }
> > }
> >
> > +// Record ptr in a page managed by krcp, with the pre-krc_this_cpu_lock()
> > +// state specified by flags. If can_sleep is true, the caller must
> > +// be schedulable and not be holding any locks or mutexes that might be
> > +// acquired by the memory allocator or anything that it might invoke.
> > +// If !can_sleep, then if !preemptible() no allocation will be undertaken,
> > +// otherwise the allocation will use GFP_ATOMIC to avoid the remainder of
> > +// the aforementioned deadlock possibilities. Returns true if ptr was
> > +// successfully recorded, else the caller must use a fallback.
> > static inline bool
> > -kvfree_call_rcu_add_ptr_to_bulk(struct kfree_rcu_cpu *krcp, void *ptr)
> > +add_ptr_to_bulk_krc_lock(struct kfree_rcu_cpu **krcp,
> > + unsigned long *flags, void *ptr, bool can_sleep)
> > {
> > struct kvfree_rcu_bulk_data *bnode;
> > + bool can_alloc_page = preemptible();
> > + gfp_t gfp = (can_sleep ? GFP_KERNEL | __GFP_RETRY_MAYFAIL :
> > + GFP_ATOMIC) | __GFP_NOWARN;
> > int idx;
> >
> > - if (unlikely(!krcp->initialized))
> > + *krcp = krc_this_cpu_lock(flags);
> > + if (unlikely(!(*krcp)->initialized))
> > return false;
> >
> > - lockdep_assert_held(&krcp->lock);
> > idx = !!is_vmalloc_addr(ptr);
> >
> > /* Check if a new block is required. */
>
> Maybe convert this comment also to //... like the new ones you added (and the
> ones below).
>
No, problem. I can convert it.
> > - if (!krcp->bkvhead[idx] ||
> > - krcp->bkvhead[idx]->nr_records == KVFREE_BULK_MAX_ENTR) {
> > - bnode = get_cached_bnode(krcp);
> > - /* Switch to emergency path. */
> > + if (!(*krcp)->bkvhead[idx] ||
> > + (*krcp)->bkvhead[idx]->nr_records == KVFREE_BULK_MAX_ENTR) {
> > + bnode = get_cached_bnode(*krcp);
> > + if (!bnode && can_alloc_page) {
>
> I think you can directly put preemptible() here with a comment saying
> allocate only if preemptible and get rid of can_alloc_page.
>
Not really. We check preemtable() before acquiring the internal lock,
otherwise it will always return "false". Thus, it is checked on the
entry in the beginning.
> > + krc_this_cpu_unlock(*krcp, *flags);
> > + bnode = (struct kvfree_rcu_bulk_data *)
> > + __get_free_page(gfp);
> > + *krcp = krc_this_cpu_lock(flags);
> > + }
> > +
>
> I think the "Switch to emergency path" comment should stay here before the
> if().
>
Right. We need to keep it. Will take it back.
--
Vlad Rezki
