Re: [PATCH v2] rcu/kvfree: Prevents cache growing when the backoff_page_cache_fill is set

[Paul E. McKenney]

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@xxxxxxxxx>
> > ---
> >  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@xxxxxxxxx>

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