Re: [RFC/PATCH 0/5] Contiguous Memory Allocator and get_user_pages()

[Yasuaki Ishimatsu]

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:
https://lkml.org/lkml/2012/11/29/69

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

Thanks,
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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