Re: [RFC][PATCH 0/8] Critical Page Pool

[Avi Kivity <avi@argo.co.il>]

Matthew Dobson wrote:

> Avi Kivity wrote:
>  
>
> > 1. If you have two subsystems which allocate critical pages, how do you
> > protect against the condition where one subsystem allocates all the
> > critical memory, causing the second to oom?
> >    
>
> You don't.  You make sure that you size the critical pool appropriately for
> your workload.
>
>  
This may not be possible. What if subsystem A depends on subsystem B to 
do its work, both are critical, and subsystem A allocated all the memory 
reserve?
If A and B have different allocation thresholds, the deadlock is avoided.

At the very least you need a critical pool per subsystem.

>  
>
> > 2. There already exists a critical pool: ordinary allocations fail if
> > free memory is below some limit, but special processes (kswapd) can
> > allocate that memory by setting PF_MEMALLOC. Perhaps this should be
> > extended, possibly with a per-process threshold.
> >    
>
> The exception for threads with PF_MEMALLOC set is there because those
> threads are essentially promising that if the kernel gives them memory,
> they will use that memory to free up MORE memory.  If we ignore that
> promise, and (ab)use the PF_MEMALLOC flag to simply bypass the
> zone_watermarks, we'll simply OOM faster, and potentially in situations
> that could be avoided (ie: we steal memory that kswapd could have used to
> free up more memory).
>  
Sure, but that's just an example of a critical subsystem.

If we introduce yet another mechanism for critical memory allocation, 
we'll have a hard time making different subsystems, which use different 
critical allocation mechanisms, play well together.

I propose that instead of a single watermark, there should be a 
watermark per critical subsystem. The watermarks would be arranged 
according to the dependency graph, with the depended-on services allowed 
to go the deepest into the reserves.

(instead of PF_MEMALLOC have a tsk->memory_allocation_threshold, or 
similar. set it to 0 for kswapd, and for other systems according to taste)

--
Do not meddle in the internals of kernels, for they are subtle and quick to panic.

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