Re: mm allocation failure and hang when running xfstests generic/269 on xfs

[Brian Foster]

On Thu, Mar 02, 2017 at 02:50:01PM +0100, Michal Hocko wrote:
> On Thu 02-03-17 08:41:58, Brian Foster wrote:
> > On Thu, Mar 02, 2017 at 02:27:55PM +0100, Michal Hocko wrote:
> [...]
> > > I see your argument about being in sync with other kmem helpers but
> > > those are bit different because regular page/slab allocators allow never
> > > fail semantic (even though this is mostly ignored by those helpers which
> > > implement their own retries but that is a different topic).
> > > 
> > 
> > ... but what I'm trying to understand here is whether this failure
> > scenario is specific to vmalloc() or whether the other kmem_*()
> > functions are susceptible to the same problem. For example, suppose we
> > replaced this kmem_zalloc_greedy() call with a kmem_zalloc(PAGE_SIZE,
> > KM_SLEEP) call. Could we hit the same problem if the process is killed?
> 
> Well, kmem_zalloc uses kmalloc which can also fail when we are out of
> memory but in that case we can expect the OOM killer releasing some
> memory which would allow us to make a forward progress on the next
> retry. So essentially retrying around kmalloc is much more safe in this
> regard. Failing vmalloc might be permanent because there is no vmalloc
> space to allocate from or much more likely due to already mentioned
> patch. So vmalloc is different, really.

Right.. that's why I'm asking. So it's technically possible but highly
unlikely due to the different failure characteristics. That seems
reasonable to me, then. 

To be clear, do we understand what causes the vzalloc() failure to be
effectively permanent in this specific reproducer? I know you mention
above that we could be out of vmalloc space, but that doesn't clarify
whether there are other potential failure paths or then what this has to
do with the fact that the process was killed. Does the pending signal
cause the subsequent failures or are you saying that there is some other
root cause of the failure, this process would effectively be spinning
here anyways, and we're just noticing it because it's trying to exit?

Brian

> -- 
> Michal Hocko
> SUSE Labs
> 
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