On Fri, Dec 04, 2020 at 10:55:30AM -0500, Pavel Tatashin wrote: > 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. It sounds like there is some inherent tension here, breaking THP's when doing pin_user_pages() is a really nasty thing to do. DMA benefits greatly from THP. I know nothing about ZONE_MOVABLE, is this auto-setup or an admin option? If the result of this patch is standard systems can no longer pin > 80% of their memory I have some regression concerns.. Jason
