On Tue, Sep 12, 2017 at 02:44:36PM +0800, Huang, Ying wrote:
> 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.
My concern is users have been disabled swap readahead by page-cluster would
be regressed. Please take care of them.
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