Re: zram/zsmalloc issues in very low memory conditions

[Bob Liu <bob.liu@oracle.com>]

On 10/24/2013 05:51 AM, Olav Haugan 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?
> 

The same question as Luigi "What do you want the system to do at this
point?"

If swap fails then OOM killer will be triggered, I don't think this will
be a issue.

By the way, could you take a try with zswap? Which can write pages to
real swap device if compressed pool is full.

> 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

AFAIK, this will never happen.

> = 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
> 

-- 
Regards,
-Bob

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