Re: [PATCH] page_alloc: skip cpuset enforcement for lower zone allocations

[Marcelo Tosatti <mtosatti@redhat.com>]

On Tue, May 27, 2014 at 11:53:52AM -0300, Marcelo Tosatti wrote:
> On Tue, May 27, 2014 at 09:21:32AM -0500, Christoph Lameter wrote:
> > On Fri, 23 May 2014, Marcelo Tosatti wrote:
> > 
> > > Zone specific allocations, such as GFP_DMA32, should not be restricted
> > > to cpusets allowed node list: the zones which such allocations demand
> > > might be contained in particular nodes outside the cpuset node list.
> > >
> > > The alternative would be to not perform such allocations from
> > > applications which are cpuset restricted, which is unrealistic.
> > >
> > > Fixes KVM's alloc_page(gfp_mask=GFP_DMA32) with cpuset as explained.
> > 
> > Memory policies are only applied to a specific zone so this is not
> > unprecedented. However, if a user wants to limit allocation to a specific
> > node and there is no DMA memory there then may be that is a operator
> > error? After all the application will be using memory from a node that the
> > operator explicitly wanted not to be used.
> 
> Ok here is the use-case:
> 
> - machine contains driver which requires zone specific memory (such as
> KVM, which requires root pagetable at paddr < 4GB).
> 
> - user wants to limit allocation of application to nodeX, and nodeX has
> no memory < 4GB.
> 
> How would you solve that? Options:
> 
> 1) force admin to allow allocation from node(s) which contain 0-4GB
>   range, which unfortunately would allow every allocation, including
>   ones which are not restricted to particular nodes, to be performed
>   there.
> 
> or
> 
> 2) allow zone specific allocations to bypass memory policies.
> 
> It seems 2) is the best option (and there is precedent for it).
> 
> > There is also the hardwall flag. I think its ok to allocate outside of the
> > cpuset if that flag is not set. However, if it is set then any attempt to
> > alloc outside of the cpuset should fail.
> 
> GFP_ATOMIC bypasses hardwall:
> 
>  * The second pass through get_page_from_freelist() doesn't even call
>  * here for GFP_ATOMIC calls.  For those calls, the __alloc_pages()
>  * variable 'wait' is not set, and the bit ALLOC_CPUSET is not set
>  * in alloc_flags.  That logic and the checks below have the combined
>  * affect that:
>  *      in_interrupt - any node ok (current task context irrelevant)
>  *      GFP_ATOMIC   - any node ok
>  *      TIF_MEMDIE   - any node ok
>  *      GFP_KERNEL   - any node in enclosing hardwalled cpuset ok
>  *      GFP_USER     - only nodes in current tasks mems allowed ok.

Thats softwall nevermind. 

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