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

["Huang\, Ying" <ying.huang@intel.com>]

Minchan Kim <minchan@kernel.org> writes:

> On Tue, Sep 12, 2017 at 03:29:45PM +0800, Huang, Ying wrote:
>> Minchan Kim <minchan@kernel.org> writes:
>> 
>> > On Tue, Sep 12, 2017 at 02:44:36PM +0800, Huang, Ying wrote:
>> >> Minchan Kim <minchan@kernel.org> writes:
>> >> 
>> >> > On Tue, Sep 12, 2017 at 01:23:01PM +0800, Huang, Ying wrote:
>> >> >> Minchan Kim <minchan@kernel.org> 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@intel.com>
>> >> >> > Signed-off-by: Minchan Kim <minchan@kernel.org>
>> >> >> > ---
>> >> >> >  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.
>> >
>> > My concern is users have been disabled swap readahead by page-cluster would
>> > be regressed. Please take care of them.
>> 
>> How about disable VMA based swap readahead if zram used as swap?  Like
>> we have done for hard disk?
>
> It could be with SWP_SYNCHRONOUS_IO flag which indicates super-fast,
> no seek cost swap devices if this patchset is merged so VM automatically
> disables readahead. It is in my TODO but it's orthogonal work.
>
> The problem I raised is "Why shouldn't we obey user's decision?",
> not zram sepcific issue.
>
> A user has used SSD as swap devices decided to disable swap readahead
> by some reason(e.g., small memory system). Anyway, it has worked
> via page-cluster for a several years but with vma-based swap devices,
> it doesn't work any more.

Can they add one more line to their configuration scripts?

echo 0 > /sys/kernel/mm/swap/vma_ra_max_order

Best Regards,
Huang, Ying

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