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SubjectRe: [RFC/PATCH 0/5] Contiguous Memory Allocator and get_user_pages()
2013/03/05 15:57, Marek Szyprowski 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'm interested in your idea since it seems that the idea solves my issue:

So I want to apply your idea to a memory hot plug.

Yasuaki Ishimatsu

> 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(-)

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