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?
Best regards Marek Szyprowski Samsung Poland R&D Center
Patch summary:
Marek Szyprowski (5): mm: introduce migrate_replace_page() for migrating page to the given target mm: get_user_pages: use static inline mm: get_user_pages: use NON-MOVABLE pages when FOLL_DURABLE flag is set mm: get_user_pages: migrate out CMA pages when FOLL_DURABLE flag is set media: vb2: use FOLL_DURABLE and __get_user_pages() to avoid CMA migration issues
drivers/media/v4l2-core/videobuf2-dma-contig.c | 8 +- include/linux/highmem.h | 12 ++- include/linux/migrate.h | 5 + include/linux/mm.h | 76 ++++++++++++- mm/internal.h | 12 +++ mm/memory.c | 136 +++++++++++------------- mm/migrate.c | 59 ++++++++++ 7 files changed, 225 insertions(+), 83 deletions(-)