Hello, On 3/6/2013 9:47 AM, Minchan Kim wrote:
Hello, On Tue, Mar 5, 2013 at 3:57 PM, Marek Szyprowski <m.szyprowski@xxxxxxxxxxx> wrote: > Hello, > > Contiguous Memory Allocator is very sensitive about migration failures > of the individual pages. A single page, which causes permanent migration > failure can break large conitguous allocations and cause the failure of > a multimedia device driver. > > One of the known issues with migration of CMA pages are the problems of > migrating the anonymous user pages, for which the others called > get_user_pages(). This takes a reference to the given user pages to let > kernel to operate directly on the page content. This is usually used for > preventing swaping out the page contents and doing direct DMA to/from > userspace. > > To solving this issue requires preventing locking of the pages, which > are placed in CMA regions, for a long time. Our idea is to migrate > anonymous page content before locking the page in get_user_pages(). This > cannot be done automatically, as get_user_pages() interface is used very > often for various operations, which usually last for a short period of > time (like for example exec syscall). We have added a new flag > indicating that the given get_user_space() call will grab pages for a > long time, thus it is suitable to use the migration workaround in such > cases. > > The proposed extensions is used by V4L2/VideoBuf2 > (drivers/media/v4l2-core/videobuf2-dma-contig.c), but that is not the > only place which might benefit from it, like any driver which use DMA to > userspace with get_user_pages(). This one is provided to demonstrate the > use case. > > I would like to hear some comments on the presented approach. What do > you think about it? Is there a chance to get such workaround merged at > some point to mainline? > I discussed similar patch from memory-hotplug guys with Mel. Look at http://marc.info/?l=linux-mm&m=136014458829566&w=2 The conern is that we ends up forcing using FOLL_DURABLE/GUP_NM for all drivers and subsystems for making sure CMA/memory-hotplug works well. You mentioned driver grab a page for a long time should use FOLL_DURABLE flag but "for a long time" is very ambiguous. For example, there is a driver get_user_pages() some operation. put_pages You can make sure some operation is really fast always?
Well, in our case (judging from the logs) we observed 2 usage patterns for get_user_pages() calls. One group was lots of short time locks, whose call stacks originated in various kernel places, the second group was device drivers which used get_user_pages() to create a buffer for the DMA. Such buffers were used for the whole lifetime of the session to the given device, what was equivalent to infinity from the migration/CMA point of view. This was however based on the specific use case at out target system, that's why I wanted to start the discussion and find some generic approach.
For example, what if it depends on other event which is normally very fast but quite slow once a week or try to do dynamic memory allocation but memory pressure is severe? For 100% working well, at last we need to change all GUP user with GUP_NM or your FOLL_DURABLE whatever but the concern Mel pointed out is it could cause lowmem exhaustion problem.
This way we sooner or later end up without any movable pages at all. I assume that keeping some temporary references on movable/cma pages must be allowed, because otherwise we limit the functionality too much.
At the moment, there is other problem against migratoin, which are not related with your patch. ex, zcache, zram, zswap. Their pages couldn't be migrated out so I think below Mel's suggestion or some generic infrastructure can move pinned page is more proper way to go.
zcache/zram/zswap (vsmalloc based code) can be also extended to support migration. It requires some significant amount of work, but it is really doable.
"To guarantee CMA can migrate pages pinned by drivers I think you need migrate-related callsbacks to unpin, barrier the driver until migration completes and repin."
Right, this might improve the migration reliability. Are there any works being done in this direction? Best regards -- Marek Szyprowski Samsung Poland R&D Center -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>