Re: [PATCH 0/6] prohibit pinning pages in ZONE_MOVABLE

[Joonsoo Kim]

Hello,

On Wed, Dec 02, 2020 at 12:23:24AM -0500, Pavel Tatashin wrote:
> When page is pinned it cannot be moved and its physical address stays
> the same until pages is unpinned.
> 
> This is useful functionality to allows userland to implementation DMA
> access. For example, it is used by vfio in vfio_pin_pages().
> 
> However, this functionality breaks memory hotplug/hotremove assumptions
> that pages in ZONE_MOVABLE can always be migrated.
> 
> This patch series fixes this issue by forcing new allocations during
> page pinning to omit ZONE_MOVABLE, and also to migrate any existing
> pages from ZONE_MOVABLE during pinning.

I love what this patchset does, but, at least, it's better to consider
the side-effect of this patchset and inform it in somewhere. IIUC,
ZONE_MOVABLE exists for two purposes.

1) increasing availability of THP
2) memory hot-unplug

Potential issue would come from the case 1). They uses ZONE_MOVABLE
for THP availability and hard guarantee for migration isn't required
until now. So, there would be a system with following congifuration.

- memory layout: ZONE_NORMAL-512MB, ZONE_MOVABLE-512MB
- memory usage: unmovable-256MB, movable pinned-256MB, movable
  unpinned-512MB

With this patchset, movable pinned should be placed in ZONE_NORMAL so
512MB is required for ZONE_NORMAL. ZONE_NORMAL would be exhausted and
system performance would be highly afftect according to memory usage
pattern.

I'm not sure whether such configuration exists or not, but, at least,
it's better to write down this risk on commit message or something
else.

Thanks.