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

[Pavel Tatashin]

On Thu, Dec 3, 2020 at 11:03 PM Joonsoo Kim <js1304@xxxxxxxxx> wrote:
>
> 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.

Yes, this indeed could be a problem for some configurations. I will
add your comment to the commit log of one of the patches.

Thank you,
Pasha