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Date:   Thu, 6 Apr 2023 10:50:25 -0700
From:   "Paul E. McKenney" <paulmck@kernel.org>
To:     Uladzislau Rezki <urezki@gmail.com>
Cc:     Zqiang <qiang1.zhang@intel.com>, frederic@kernel.org,
        joel@joelfernandes.org, qiang.zhang1211@gmail.com,
        rcu@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2] rcu/kvfree: Prevents cache growing when the
 backoff_page_cache_fill is set
Message-ID: <8da4b48a-820c-461f-9dc0-a5afaf335177@paulmck-laptop>
Reply-To: paulmck@kernel.org
References: <20230406001238.647536-1-qiang1.zhang@intel.com>
 <ZC5MoREk30MIt79u@pc636>
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On Thu, Apr 06, 2023 at 06:37:53AM +0200, Uladzislau Rezki wrote:
> On Thu, Apr 06, 2023 at 08:12:38AM +0800, Zqiang wrote:
> > Currently, in kfree_rcu_shrink_scan(), the drain_page_cache() is
> > executed before kfree_rcu_monitor() to drain page cache, if the bnode
> > structure's->gp_snap has done, the kvfree_rcu_bulk() will fill the
> > page cache again in kfree_rcu_monitor(), this commit add a check
> > for krcp structure's->backoff_page_cache_fill in put_cached_bnode(),
> > if the krcp structure's->backoff_page_cache_fill is set, prevent page
> > cache growing.
> > 
> > Signed-off-by: Zqiang <qiang1.zhang@intel.com>
> > ---
> >  kernel/rcu/tree.c | 2 ++
> >  1 file changed, 2 insertions(+)
> > 
> > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> > index 9cc0a7766fd2..f25430ae1936 100644
> > --- a/kernel/rcu/tree.c
> > +++ b/kernel/rcu/tree.c
> > @@ -2907,6 +2907,8 @@ static inline bool
> >  put_cached_bnode(struct kfree_rcu_cpu *krcp,
> >  	struct kvfree_rcu_bulk_data *bnode)
> >  {
> > +	if (atomic_read(&krcp->backoff_page_cache_fill))
> > +		return false;
> >  	// Check the limit.
> >  	if (krcp->nr_bkv_objs >= rcu_min_cached_objs)
> >  		return false;
> > -- 
> > 2.32.0
> >
> Reviewed-by: Uladzislau Rezki (Sony) <urezki@gmail.com>

Thank you both!

One question, though.  Might it be better to instead modify the "for"
loop in fill_page_cache_func() to start at krcp->nr_bkv_objs instead
of starting at zero?  That way, we still provide a single page under
low-memory conditions, but provide rcu_min_cached_objs of them if memory
is plentiful.

Alternatively, if we really don't want to allow any pages at all under
low-memory conditions, shouldn't the fill_page_cache_func() set nr_pages
to zero (instead of the current 1) when the krcp->backoff_page_cache_fill
flag is set?  This would likely mean also breaking out of that loop if
krcp->backoff_page_cache_fill was set in the meantime (which happens
implicitly with Zqiang's patch).

Or am I missing something subtle here?

							Thanx, Paul