[RFC] mm: Allow ZONE_DMA32 to be disabled via kernel command line

[Chris Goldsworthy]

On Thu, Jan 26, 2023 at 07:15:26PM +0000, Robin Murphy wrote:
> On 2023-01-26 16:43, Georgi Djakov wrote:
> >From: Chris Goldsworthy <quic_cgoldswo at quicinc.com>
> >
> >It's useful to have an option to disable the ZONE_DMA32 during boot as
> >CONFIG_ZONE_DMA32 is by default enabled (on multiplatform kernels for
> >example). There are platforms that do not use this zone and in some high
> >memory pressure scenarios this would help on easing kswapd (to leave file
> >backed memory intact / unreclaimed). When the ZONE_DMA32 is enabled on
> >these platforms - kswapd is woken up more easily and drains the file cache
> >which leads to some performance issues.
> >
> >Signed-off-by: Chris Goldsworthy <quic_cgoldswo at quicinc.com>
> >[georgi: updated commit text]
> >Signed-off-by: Georgi Djakov <quic_c_gdjako at quicinc.com>
> >---
> >The main question here is whether we can have a kernel command line
> >option to disable CONFIG_ZONE_DMA32 during boot (at least on arm64).
> >I can imagine this being useful also for Linux distros.
> 
> FWIW I'd say that "disabled" and "left empty then awkwardly tiptoed around
> in a few places" are very different notions...
> 
> However, I'm just going to take a step back and read the commit message a
> few more times... Given what it claims, I can't help but ask why wouldn't we
> want a parameter to control kswapd's behaviour and address that issue
> directly, rather than a massive hammer that breaks everyone allocating
> explicitly or implicitly with __GFP_DMA32 (especially on systems where it
> doesn't normally matter because all memory is below 4GB anyway), just to
> achieve one rather niche side-effect?
> 
> Thanks,
> Robin.

Hi Robin,

The commit text doesn't spell out the scenario we want to avoid, so I
will do that for clarity. We use a kernel binary compiled for us, and
by default has CONFIG_ZONE_DMA32 (and it can't be disabled for now as
another party needs it). Our higher-end SoCs are usually used with
8-12 GB of DDR, so using a 12 GB device as an example, we would have 8
GB of ZONE_NORMAL memory and 4 GB of ZONE_MOVABLE memory with the
feature, and 4 GB of ZONE_DMA32, 4 GB of ZONE_NORMAL and 4 GB of
ZONE_MOVABLE otherwise.

Without the feature enabled, consider a GFP_KERNEL allocation that
causes a low watermark beach in ZONE_NORMAL, such that such that
ZONE_DMA32 is almost full. This will cause kswapd to start reclaiming
memory, despite the fact that that we might have gigabytes of free
memory in ZONE_DMA32 that can be used by anyone (since GFP_MOVABLE and
GFP_NORMAL can fall back to using ZONE_DMA32).

So, fleshing out your suggestion to make it concrete for our case, we
would want to stop kswapd from doing reclaim on ZONE_NORMAL watermark
breaches when ZONE_DMA32 is present (since anything targeting
ZONE_NORMAL can fall back to ZONE_DMA32).

Thanks,

Chris.