linux-kernel - Re: [PATCH v2] zram: Prevent page allocation failure during zcomp_strm

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Message-ID: <20151123074337.GD7449@swordfish>
Date:	Mon, 23 Nov 2015 16:43:37 +0900
From:	Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:	Minchan Kim <minchan@...nel.org>
Cc:	Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
	Kyeongdon Kim <kyeongdon.kim@....com>,
	Andrew Morton <akpm@...ux-foundation.org>, ngupta@...are.org,
	linux-kernel@...r.kernel.org, sergey.senozhatsky@...il.com
Subject: Re: [PATCH v2] zram: Prevent page allocation failure during
 zcomp_strm_alloc

On (11/23/15 13:18), Minchan Kim wrote:
[..]
> > https://lkml.org/lkml/2015/6/16/465
> 
> Sorry, I have missed that.
> It's worth to fix that you proved it that could happen.
> But when I read your patch, GFP_NOIO instead GFP_NOFS would
> better way. Could you resend it?

no problem.

agree. we also would want to switch from vzalloc() to
	__vmalloc_node_flags(size, NUMA_NO_NODE,
			GFP_NOIO | __GFP_HIGHMEM |  __GFP_ZERO)

in fallbacks. I'll send the patch later today.

> > 
> > > If it is true, we should fix several allocation flags in
> > > zcomp_strm_alloc. I just want to record this warning for the future
> > > in this thread so someone who is finding for the contribution
> > > material will prove and fix it. :)
> > 
> > I can re-send the patch.
> > 
> > And, in case if you missed it, what's your opinion on the idea of
> > reducing ->max_strm if we can't allocate new streams. Here:
> > http://marc.info/?l=linux-kernel&m=144798049429861
> 
> Hmm, auto backoff max_comp_streams with warn to user, I'm not sure
> we really need it. Alloc failure is depenent on the workload and
> timing so although there are a fail in t1, it could be successful
> in t2 so if we *really* want to introduce auto backoff, the logic
> should include advance routine as well as backoff. With that,
> I want to handle it traparently without notice to user.

yes. auto roll-back is important (that's why I mentioned it). the idea
is to avoid stealing of pages for streams. for example, in case of low
memory decrease ->max_strm to
	MAX(->avail_strm, ->max_strm, online cpus() / 2)

and even probably release some idle streams (um... as a reaction
to shrinker call???). or at least prevent setting of ->max_sgtrm to
some unreasonably huge values... "> 4 * NR_CPUS", for instance. but
this is not so critical.

> So, Kyeongdon's patch will remove warning overhead and likely to
> make zcomp_stram_alloc successful with vmalloc so I want to
> roll it out first. And let's add a WARN_ON_ONCE to detect of
> failure and rethink it when we receive such report.

	-ss
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