On Thu, 17 Dec 2020, Alex Shi wrote: > The rcu_read_lock was used to block memcg destory, but with the detailed > calling conditions, the memcg won't gone since the page is hold. So we > don't need it now, let's remove them to save locking load in debugging. " lock_page_lruvec() and its variants used rcu_read_lock() with the intention of safeguarding against the mem_cgroup being destroyed concurrently; but so long as they are called under the specified conditions (as they are), there is no way for the page's mem_cgroup to be destroyed. Delete the unnecessary rcu_read_lock() and _unlock(). " This has little to do with a "locking load in debugging" - so what? But everything to do with deleting bogosity, the sooner the better. > > Signed-off-by: Alex Shi <alex.shi@xxxxxxxxxxxxxxxxx> > Cc: Hugh Dickins <hughd@xxxxxxxxxx> Acked-by: Hugh Dickins <hughd@xxxxxxxxxx> This really surprised me! Nice change, but how on earth did we not notice until now? The rcu_read_lock() seems to have come in, without explanation, somewhere between lru_lock v9 and v11 (I never saw v10); and I guess I was so used to needing rcu_read_lock() in my own implementation, that I was blind to its irrelevance in yours. Cc'ing Alex Duyck, since he was generally very alert to this kind of thing - be good to have his Ack too. Also Cc'ing Hui Su, who sent a similar but unexplained patch just before yours. > Cc: Johannes Weiner <hannes@xxxxxxxxxxx> > Cc: Michal Hocko <mhocko@xxxxxxxxxx> > Cc: Vladimir Davydov <vdavydov.dev@xxxxxxxxx> > Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> > Cc: cgroups@xxxxxxxxxxxxxxx > Cc: linux-mm@xxxxxxxxx > Cc: linux-kernel@xxxxxxxxxxxxxxx > --- > mm/memcontrol.c | 6 ------ > 1 file changed, 6 deletions(-) > > diff --git a/mm/memcontrol.c b/mm/memcontrol.c > index e6b50d068b2f..98bbee1d2faf 100644 > --- a/mm/memcontrol.c > +++ b/mm/memcontrol.c > @@ -1356,10 +1356,8 @@ struct lruvec *lock_page_lruvec(struct page *page) > struct lruvec *lruvec; > struct pglist_data *pgdat = page_pgdat(page); > > - rcu_read_lock(); > lruvec = mem_cgroup_page_lruvec(page, pgdat); > spin_lock(&lruvec->lru_lock); > - rcu_read_unlock(); > > lruvec_memcg_debug(lruvec, page); > > @@ -1371,10 +1369,8 @@ struct lruvec *lock_page_lruvec_irq(struct page *page) > struct lruvec *lruvec; > struct pglist_data *pgdat = page_pgdat(page); > > - rcu_read_lock(); > lruvec = mem_cgroup_page_lruvec(page, pgdat); > spin_lock_irq(&lruvec->lru_lock); > - rcu_read_unlock(); > > lruvec_memcg_debug(lruvec, page); > > @@ -1386,10 +1382,8 @@ struct lruvec *lock_page_lruvec_irqsave(struct page *page, unsigned long *flags) > struct lruvec *lruvec; > struct pglist_data *pgdat = page_pgdat(page); > > - rcu_read_lock(); > lruvec = mem_cgroup_page_lruvec(page, pgdat); > spin_lock_irqsave(&lruvec->lru_lock, *flags); > - rcu_read_unlock(); > > lruvec_memcg_debug(lruvec, page); > > -- > 2.29.GIT
