Re: [PATCH 4/5] mm:swap: respect page_cluster for readahead

[Minchan Kim]

On Tue, Sep 12, 2017 at 01:23:01PM +0800, Huang, Ying wrote:
> Minchan Kim <minchan@xxxxxxxxxx> writes:
> 
> > page_cluster 0 means "we don't want readahead" so in the case,
> > let's skip the readahead detection logic.
> >
> > Cc: "Huang, Ying" <ying.huang@xxxxxxxxx>
> > Signed-off-by: Minchan Kim <minchan@xxxxxxxxxx>
> > ---
> >  include/linux/swap.h | 3 ++-
> >  1 file changed, 2 insertions(+), 1 deletion(-)
> >
> > diff --git a/include/linux/swap.h b/include/linux/swap.h
> > index 0f54b491e118..739d94397c47 100644
> > --- a/include/linux/swap.h
> > +++ b/include/linux/swap.h
> > @@ -427,7 +427,8 @@ extern bool has_usable_swap(void);
> >  
> >  static inline bool swap_use_vma_readahead(void)
> >  {
> > -	return READ_ONCE(swap_vma_readahead) && !atomic_read(&nr_rotate_swap);
> > +	return page_cluster > 0 && READ_ONCE(swap_vma_readahead)
> > +				&& !atomic_read(&nr_rotate_swap);
> >  }
> >  
> >  /* Swap 50% full? Release swapcache more aggressively.. */
> 
> Now the readahead window size of the VMA based swap readahead is
> controlled by /sys/kernel/mm/swap/vma_ra_max_order, while that of the
> original swap readahead is controlled by sysctl page_cluster.  It is
> possible for anonymous memory to use VMA based swap readahead and tmpfs
> to use original swap readahead algorithm at the same time.  So that, I
> think it is necessary to use different control knob to control these two
> algorithm.  So if we want to disable readahead for tmpfs, but keep it
> for VMA based readahead, we can set 0 to page_cluster but non-zero to
> /sys/kernel/mm/swap/vma_ra_max_order.  With your change, this will be
> impossible.

For a long time, page-cluster have been used as controlling swap readahead.
One of example, zram users have been disabled readahead via 0 page-cluster.
However, with your change, it would be regressed if it doesn't disable
vma_ra_max_order.

As well, all of swap users should be aware of vma_ra_max_order as well as
page-cluster to control swap readahead but I didn't see any document about
that. Acutaully, I don't like it but want to unify it with page-cluster.