Re: [PATCH v3] rcu/kvfree: Prevents cache growing when the backoff_page_cache_fill is set

[Paul E. McKenney]

On Sat, Apr 08, 2023 at 10:25:17PM +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 and disable allocated page in fill_page_cache_func().
> 
> Signed-off-by: Zqiang <qiang1.zhang@xxxxxxxxx>

Much improved!  But still some questions below...

							Thanx, Paul

> ---
>  kernel/rcu/tree.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index cc34d13be181..9d9d3772cc45 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -2908,6 +2908,8 @@ 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;

This will mean that under low-memory conditions, we will keep zero
pages in ->bkvcache.  All attempts to put something there will fail.

This is probably not an issue for structures containing an rcu_head
that are passed to kfree_rcu(p, field), but doesn't this mean that
kfree_rcu_mightsleep() unconditionally invokes synchronize_rcu()?
This could seriously slow up freeing under low-memory conditions,
which might exacerbate the low-memory conditions.

Is this really what we want?  Zero cached rather than just fewer cached?

>  	// Check the limit.
>  	if (krcp->nr_bkv_objs >= rcu_min_cached_objs)
>  		return false;
> @@ -3221,7 +3223,7 @@ static void fill_page_cache_func(struct work_struct *work)
>  	int i;
>  
>  	nr_pages = atomic_read(&krcp->backoff_page_cache_fill) ?
> -		1 : rcu_min_cached_objs;
> +		0 : rcu_min_cached_objs;
>  
>  	for (i = 0; i < nr_pages; i++) {

I am still confused as to why we start "i" at zero rather than at
->nr_bkv_objs.  What am I missing here?

>  		bnode = (struct kvfree_rcu_bulk_data *)
> -- 
> 2.32.0
>