Re: [PATCH] Should GFP_ATOMIC fail when we're below low watermark?

[Peter Zijlstra <peterz@infradead.org>]

On Mon, 2007-08-20 at 11:38 +1000, Nigel Cunningham wrote:
> Hi all.
> 
> In current git (and for a while now), an attempt to allocate memory with 
> GFP_ATOMIC will fail if we're below the low watermark level. The only way to 
> access that memory that I can see (not that I've looked that hard) is to have 
> PF_MEMALLOC set (ie from kswapd). I'm wondering if this behaviour is correct. 
> Shouldn't GFP_ATOMIC allocations ignore watermarks too? How about GFP_KERNEL?
> 
> The following patch is a potential fix for GFP_ATOMIC.

Sorry, no.

GFP_ATOMIC must fail when below the watermark. GFP_KERNEL has __GFP_WAIT
and hence can sleep and wait for reclaim so that should not be a problem
(usually).

GFP_ATOMIC may not access the reserves because the reserves are needed
to get out of OOM deadlocks within the VM. Consider the fact that
freeing memory needs memory - if there is no memory free, you cannot
free memory and you're pretty much stuck.