linux-kernel - Re: zram/zsmalloc issues in very low memory conditions

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Message-ID: <CAL1ERfOSmgo=j4YHHREY8uzyh+nbRrsFdBZ86FhMXxP5GEHQkQ@mail.gmail.com>
Date:	Thu, 24 Oct 2013 18:42:48 +0800
From:	Weijie Yang <weijie.yang.kh@...il.com>
To:	Olav Haugan <ohaugan@...eaurora.org>
Cc:	minchan@...nel.org, sjenning@...ux.vnet.ibm.com,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	semenzato@...gle.com, bob.liu@...cle.com
Subject: Re: zram/zsmalloc issues in very low memory conditions

On Thu, Oct 24, 2013 at 5:51 AM, Olav Haugan <ohaugan@...eaurora.org> wrote:
> I am trying to use zram in very low memory conditions and I am having
> some issues. zram is in the reclaim path. So if the system is very low
> on memory the system is trying to reclaim pages by swapping out (in this
> case to zram). However, since we are very low on memory zram fails to
> get a page from zsmalloc and thus zram fails to store the page. We get
> into a cycle where the system is low on memory so it tries to swap out
> to get more memory but swap out fails because there is not enough memory
> in the system! The major problem I am seeing is that there does not seem
> to be a way for zram to tell the upper layers to stop swapping out
> because the swap device is essentially "full" (since there is no more
> memory available for zram pages). Has anyone thought about this issue
> already and have ideas how to solve this or am I missing something and I
> should not be seeing this issue?

I agree with Luigi and Bob.

zram's size is based on how many free memory you expect to use for zram.
In my test, the compression ratio is about 1:4, of course the working
sets may be
different with yours.

Further more, may be you can modify vm_swap_full() to let kernel free swap_entry
aggressively.


> I am also seeing a couple other issues that I was wondering whether
> folks have already thought about:
>
> 1) The size of a swap device is statically computed when the swap device
> is turned on (nr_swap_pages). The size of zram swap device is dynamic
> since we are compressing the pages and thus the swap subsystem thinks
> that the zram swap device is full when it is not really full. Any
> plans/thoughts about the possibility of being able to update the size
> and/or the # of available pages in a swap device on the fly?
>
> 2) zsmalloc fails when the page allocated is at physical address 0 (pfn
> = 0) since the handle returned from zsmalloc is encoded as (<PFN>,
> <obj_idx>) and thus the resulting handle will be 0 (since obj_idx starts
> at 0). zs_malloc returns the handle but does not distinguish between a
> valid handle of 0 and a failure to allocate. A possible solution to this
> would be to start the obj_idx at 1. Is this feasible?
>
> Thanks,
>
> Olav Haugan
>
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