On Tue, Dec 5, 2023 at 2:55 AM Vlastimil Babka <vbabka@xxxxxxx> wrote:
>
> On 12/3/23 10:23, Hyeonggon Yoo wrote:
> > On Thu, Nov 2, 2023 at 12:25 PM <chengming.zhou@xxxxxxxxx> wrote:
> >>
> >> From: Chengming Zhou <zhouchengming@xxxxxxxxxxxxx>
> >>
> >> Since the introduce of unfrozen slabs on cpu partial list, we don't
> >> need to synchronize the slab frozen state under the node list_lock.
> >>
> >> The caller of deactivate_slab() and the caller of __slab_free() won't
> >> manipulate the slab list concurrently.
> >>
> >> So we can get node list_lock in the last stage if we really need to
> >> manipulate the slab list in this path.
> >>
> >> Signed-off-by: Chengming Zhou <zhouchengming@xxxxxxxxxxxxx>
> >> Reviewed-by: Vlastimil Babka <vbabka@xxxxxxx>
> >> Tested-by: Hyeonggon Yoo <42.hyeyoo@xxxxxxxxx>
> >> ---
> >> mm/slub.c | 79 ++++++++++++++++++-------------------------------------
> >> 1 file changed, 26 insertions(+), 53 deletions(-)
> >>
> >> diff --git a/mm/slub.c b/mm/slub.c
> >> index bcb5b2c4e213..d137468fe4b9 100644
> >> --- a/mm/slub.c
> >> +++ b/mm/slub.c
> >> @@ -2468,10 +2468,8 @@ static void init_kmem_cache_cpus(struct kmem_cache *s)
> >> static void deactivate_slab(struct kmem_cache *s, struct slab *slab,
> >> void *freelist)
> >> {
> >> - enum slab_modes { M_NONE, M_PARTIAL, M_FREE, M_FULL_NOLIST };
> >> struct kmem_cache_node *n = get_node(s, slab_nid(slab));
> >> int free_delta = 0;
> >> - enum slab_modes mode = M_NONE;
> >> void *nextfree, *freelist_iter, *freelist_tail;
> >> int tail = DEACTIVATE_TO_HEAD;
> >> unsigned long flags = 0;
> >> @@ -2509,65 +2507,40 @@ static void deactivate_slab(struct kmem_cache *s, struct slab *slab,
> >> /*
> >> * Stage two: Unfreeze the slab while splicing the per-cpu
> >> * freelist to the head of slab's freelist.
> >> - *
> >> - * Ensure that the slab is unfrozen while the list presence
> >> - * reflects the actual number of objects during unfreeze.
> >> - *
> >> - * We first perform cmpxchg holding lock and insert to list
> >> - * when it succeed. If there is mismatch then the slab is not
> >> - * unfrozen and number of objects in the slab may have changed.
> >> - * Then release lock and retry cmpxchg again.
> >> */
> >> -redo:
> >> -
> >> - old.freelist = READ_ONCE(slab->freelist);
> >> - old.counters = READ_ONCE(slab->counters);
> >> - VM_BUG_ON(!old.frozen);
> >> -
> >> - /* Determine target state of the slab */
> >> - new.counters = old.counters;
> >> - if (freelist_tail) {
> >> - new.inuse -= free_delta;
> >> - set_freepointer(s, freelist_tail, old.freelist);
> >> - new.freelist = freelist;
> >> - } else
> >> - new.freelist = old.freelist;
> >> -
> >> - new.frozen = 0;
> >> + do {
> >> + old.freelist = READ_ONCE(slab->freelist);
> >> + old.counters = READ_ONCE(slab->counters);
> >> + VM_BUG_ON(!old.frozen);
> >> +
> >> + /* Determine target state of the slab */
> >> + new.counters = old.counters;
> >> + new.frozen = 0;
> >> + if (freelist_tail) {
> >> + new.inuse -= free_delta;
> >> + set_freepointer(s, freelist_tail, old.freelist);
> >> + new.freelist = freelist;
> >> + } else {
> >> + new.freelist = old.freelist;
> >> + }
> >> + } while (!slab_update_freelist(s, slab,
> >> + old.freelist, old.counters,
> >> + new.freelist, new.counters,
> >> + "unfreezing slab"));
> >>
> >> + /*
> >> + * Stage three: Manipulate the slab list based on the updated state.
> >> + */
> >
> > deactivate_slab() might unconsciously put empty slabs into partial list, like:
> >
> > deactivate_slab() __slab_free()
> > cmpxchg(), slab's not empty
> > cmpxchg(), slab's empty
> > and unfrozen
> > spin_lock(&n->list_lock)
> > (slab's empty but not
> > on partial list,
> >
> > spin_unlock(&n->list_lock) and return)
> > spin_lock(&n->list_lock)
> > put slab into partial list
> > spin_unlock(&n->list_lock)
> >
> > IMHO it should be fine in the real world, but just wanted to
> > mention as it doesn't seem to be intentional.
>
> I've noticed it too during review, but then realized it's not a new
> behavior, same thing could happen with deactivate_slab() already before the
> series.
Ah, you are right.
> Free slabs on partial list are supported, we even keep some
> intentionally as long as "n->nr_partial < s->min_partial" (and that check is
> racy too) so no need to try making this more strict.
Agreed.
> > Otherwise it looks good to me!
>
> Good enough for a reviewed-by? :)
Yes,
Reviewed-by: Hyeonggon Yoo <42.hyeyoo@xxxxxxxxx>
Thanks!
--
Hyeonggon
