Re: [patch 0/9] Critical Mempools

[Pekka Enberg <penberg@cs.helsinki.fi>]

Hi,

Pekka wrote:
> > As as side note, we already have __GFP_NOFAIL. How is it different
> > from GFP_CRITICAL and why aren't we improving that?

On 1/27/06, Paul Jackson <pj@sgi.com> wrote:
> Don't these two flags invoke two different mechanisms.
>   __GFP_NOFAIL can sleep for HZ/50 then retry, rather than return failure.
>   __GFP_CRITICAL can steal from the emergency pool rather than fail.
>
> I would favor renaming at least the __GFP_CRITICAL to something
> like __GFP_EMERGPOOL, to highlight the relevant distinction.

Yeah you're right. __GFP_NOFAIL guarantees to never fail but it
doesn't guarantee to actually succeed either. I think the suggested
semantics for __GFP_EMERGPOOL are that while it can fail, it tries to
avoid that by dipping into page reserves. However, I do still think
it's a bad idea to allow the slab allocator to steal whole pages for
critical allocations because in low-memory condition, it should be
fairly easy to exhaust the reserves and waste most of that memory at
the same time.

                            Pekka

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