Re: [PATCH] mm,page_alloc,cma: configurable CMA utilization

[Chris Goldsworthy]

On Wed, Feb 01, 2023 at 03:47:58PM -0800, Minchan Kim wrote:
> Hi Chris,
> 
> On Tue, Jan 31, 2023 at 08:06:28PM -0800, Chris Goldsworthy wrote:
> > We're operating in a resource constrained environment, and we want to maximize
> > the amount of memory free / headroom for GFP_KERNEL allocations on our SoCs,
> > which are especially important for DMA allocations that use an IOMMU. We need a
> > large amount of CMA on our SoCs for various reasons (e.g. for devices not
> > upstream of an IOMMU), but whilst that CMA memory is not in use, we want to
> > route all GFP_MOVABLE allocations to the CMA regions, which will free up memory
> > for GFP_KERNEL allocations. 
> 
> I like this patch for different reason but for the specific problem you
> mentioned, How about making reclaimer/compaction aware of the problem:
> 
> IOW, when the GFP_KERNEL/DMA allocation happens but not enough memory
> in the zones, let's migrates movable pages in those zones into CMA
> area/movable zone if they are plenty of free memory.
> 
> I guess you considered but did you observe some problems?

Hi Minchan,

This is not an approach we've considered. If you have a high-level idea of the
key parts of vmscan.c you'd need to touch to implement this, could you point me
to them?

I guess one drawback with this approach is that as soon as kswapd starts,
psi_memstall_enter() is called, which can eventually lead to LMKD running in
user space, which we want to minimize. One aim of what we're doing this is to
delay the calling of psi_memstall_enter().

It would be beneficial though on top of our change: if someone called
cma_alloc() and migrated out of the CMA regions, changing kswapd to behave like
this would move things back into the CMA regions after cma_release() is called
(instead of having to kill a user space process to have the CMA re-utilized upon
further user space actions).

Thanks,

Chris.