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

[Huang\, Ying]

Minchan Kim <minchan@xxxxxxxxxx> writes:

> 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.

The document is in

Documentation/ABI/testing/sysfs-kernel-mm-swap

The concern of unifying it with page-cluster is as following.

Original swap readahead on tmpfs may not work well because the combined
workload is running, so we want to disable or constrain it.  But at the
same time, the VMA based swap readahead may work better.  So I think it
may be necessary to control them separately.

Best Regards,
Huang, Ying