Re: [PATCH v3 5/7] zram: support idle/huge page writeback

[Minchan Kim <minchan@kernel.org>]

Hi Andrew,

On Wed, Nov 28, 2018 at 03:35:59PM -0800, Andrew Morton wrote:
> On Tue, 27 Nov 2018 14:54:27 +0900 Minchan Kim <minchan@kernel.org> wrote:
> 
> > This patch supports new feature "zram idle/huge page writeback".
> > On zram-swap usecase, zram has usually many idle/huge swap pages.
> > It's pointless to keep in memory(ie, zram).
> > 
> > To solve the problem, this feature introduces idle/huge page
> > writeback to backing device so the goal is to save more memory
> > space on embedded system.
> > 
> > Normal sequence to use idle/huge page writeback feature is as follows,
> > 
> > while (1) {
> >         # mark allocated zram slot to idle
> >         echo all > /sys/block/zram0/idle
> >         # leave system working for several hours
> >         # Unless there is no access for some blocks on zram,
> > 	# they are still IDLE marked pages.
> > 
> >         echo "idle" > /sys/block/zram0/writeback
> > 	or/and
> > 	echo "huge" > /sys/block/zram0/writeback
> >         # write the IDLE or/and huge marked slot into backing device
> > 	# and free the memory.
> > }
> > 
> > By per discussion:
> > https://lore.kernel.org/lkml/20181122065926.GG3441@jagdpanzerIV/T/#u,
> > 
> > This patch removes direct incommpressibe page writeback feature
> > (d2afd25114f4, zram: write incompressible pages to backing device)
> > so we could regard it as regression because incompressible pages
> > doesn't go to backing storage automatically. Instead, usre should
> > do it via "echo huge" > /sys/block/zram/writeback" manually.
> 
> I'm not in any position to determine the regression risk here.
> 
> Why is that feature being removed, anyway?

Below concerns from Sergey:
https://lore.kernel.org/lkml/20181122065926.GG3441@jagdpanzerIV/T/#u

== &< ==
"IDLE writeback" is superior to "incompressible writeback".

"incompressible writeback" is completely unpredictable and
uncontrollable; it depens on data patterns and compression algorithms.
While "IDLE writeback" is predictable.

I even suspect, that, *ideally*, we can remove "incompressible
writeback". "IDLE pages" is a super set which also includes
"incompressible" pages. So, technically, we still can do
"incompressible writeback" from "IDLE writeback" path; but a much
more reasonable one, based on a page idling period.

I understand that you want to keep "direct incompressible writeback"
around. ZRAM is especially popular on devices which do suffer from
flash wearout, so I can see "incompressible writeback" path becoming
a dead code, long term.
== &< ==

My concern is if we enable CONFIG_ZRAM_WRITEBACK in this implementation,
both hugepage/idlepage writeck will turn on. However someuser want
to enable only idlepage writeback so we need to introduce turn on/off
knob for hugepage or new CONFIG_ZRAM_IDLEPAGE_WRITEBACK for those usecase.
I don't want to make it complicated *if possible*.

Long term, I imagine we need to make VM aware of new swap hierarchy
a little bit different with as-is.
For example, first high priority swap can return -EIO or -ENOCOMP,
swap try to fallback to next lower priority swap device. With that,
hugepage writeback will work tranparently.
	
> 
> > If we hear some regression, we could restore the function.
> 
> Why not do that now?
> 

We want to remove it at this moment.