Re: [PATCH 2/3] zram: try vmalloc() after kmalloc()

[Minchan Kim <minchan@kernel.org>]

On Tue, Nov 24, 2015 at 04:36:36PM +0900, Sergey Senozhatsky wrote:
> On (11/24/15 16:03), Minchan Kim wrote:
> > 
> > Good question.
> > 
> > Actually, failure of allocation came from backend->create as Kyeongdon's
> > comment because it requires order-3 allocation which is very fragile
> > in embedded system recenlty(Android, webOS. That's why Joonsoo are solving
> > the trouble by fixing compaction part). Otherwise, other kmalloc/vmalloc
> > stuff in our example would be almost no trouble in real practice(Of course,
> > if you says it might, you're absolutely right. It could fail but I think
> > it's *really* rare in real practice).
> > 
> > More concern is order-1 allocation rather than kmalloc/vmalloc.
> > I've got lots of allocation failure reports from product team until now
> > and frankly speaking, I don't get such order-1 fail report until now.
> > I guess the reason is that system is almost trobule due to heavy fragmentation
> > before the notice such failure.
> > 
> > So, I think if we solve order-3 allocation in backend->create,
> > above problem will be almost solved.
> 
> hm, ok, may be.
> but the question whether we want to waste pages on additional streams
> (especially, e.g. if we already have, say, 10 streams) is still valid.
> a more intuitive here is to release some unneeded streams, not to increase
> our pressure allocating new ones. well, at least it seems to be so.
> those pages can be used by zsmalloc, for example.

I think your claim make sense if the failure comes from high memory
pressure but order-3 alloc failure even if there are lots of order-0
free pages in my experience is easy to encouter so I think it doesn't
mean memory pressure but just memory fragmentation.

> 
> > > and I'd prefer it to be a bit different -- use likely path first and
> > > avoid an assignment in unlikely path.
> > 
> > Personally, I like one return case unless other is really better for
> > performance. I have trained it for Andrew, I belive. :)
> > But if you don't like this by performance reason, I will add unlikely
> > for vmalloc path. If you hate it just by personal preferenece, please
> > I want to stick my code.
> 
> no, I don't hate it.

Thanks!

> 
> > > ... and add GFP_NOIO to both kzalloc() and __vmalloc().
> > 
> > I can add it. The harmness is really ignorable but as I mentioned
> > at reply of Andrew, what's the benefit with GFP_NOIO?
> > We couldn't make forward progress with __GFP_RECLAIM in reclaim
> > context.
> 
> aha, I probably missed that out.
> (well, and, technically, things can change).

My speaking came from MM internal knowledge so I accept your concern.
if you prefer like GFP_NOIO, I will use it in next spin which
makes reader less confused.
Thanks!

> 
> 	-ss

-- 
Kind regards,
Minchan Kim