Re: [PATCH 10/17] prmem: documentation

[Peter Zijlstra <peterz@infradead.org>]

On Tue, Oct 30, 2018 at 02:25:51PM -0700, Matthew Wilcox wrote:
> On Tue, Oct 30, 2018 at 11:51:17AM -0700, Andy Lutomirski wrote:
> > Finally, one issue: rare_alloc() is going to utterly suck
> > performance-wise due to the global IPI when the region gets zapped out
> > of the direct map or otherwise made RO.  This is the same issue that
> > makes all existing XPO efforts so painful. We need to either optimize
> > the crap out of it somehow or we need to make sure it’s not called
> > except during rare events like device enumeration.
> 
> Batching operations is kind of the whole point of the VM ;-)  Either
> this rare memory gets used a lot, in which case we'll want to create slab
> caches for it, make it a MM zone and the whole nine yeards, or it's not
> used very much in which case it doesn't matter that performance sucks.

Yes, for the dynamic case something along those lines would be needed.
If we have a single rare zone, we could even have __GFP_RARE or whatever
that manages this.

The page allocator would have to grow a rare memblock type, and every
rare alloc would allocate from a rare memblock, when none is available,
creation of a rare block would set up the mappings etc..

> For now, I'd suggest allocating 2MB chunks as needed, and having a
> shrinker to hand back any unused pieces.

Something like the percpu allocator?