Re: [PATCH v4 1/2] shmem: Support for registration of driver/file owner specific ops

Chris Wilson
2016-04-15T16:09:41Z

On Mon, Apr 04, 2016 at 02:18:10PM +0100, Chris Wilson wrote:
> From: Akash Goel <akash.goel@intel.com>
>
> This provides support for the drivers or shmem file owners to register
> a set of callbacks, which can be invoked from the address space
> operations methods implemented by shmem. This allow the file owners to
> hook into the shmem address space operations to do some extra/custom
> operations in addition to the default ones.
>
> The private_data field of address_space struct is used to store the
> pointer to driver specific ops. Currently only one ops field is defined,
> which is migratepage, but can be extended on an as-needed basis.
>
> The need for driver specific operations arises since some of the
> operations (like migratepage) may not be handled completely within shmem,
> so as to be effective, and would need some driver specific handling also.
> Specifically, i915.ko would like to participate in migratepage().
> i915.ko uses shmemfs to provide swappable backing storage for its user
> objects, but when those objects are in use by the GPU it must pin the
> entire object until the GPU is idle. As a result, large chunks of memory
> can be arbitrarily withdrawn from page migration, resulting in premature
> out-of-memory due to fragmentation. However, if i915.ko can receive the
> migratepage() request, it can then flush the object from the GPU, remove
> its pin and thus enable the migration.
>
> Since gfx allocations are one of the major consumer of system memory, its
> imperative to have such a mechanism to effectively deal with
> fragmentation. And therefore the need for such a provision for initiating
> driver specific actions during address space operations.
>
> Cc: Hugh Dickins <hughd@google.com>
> Cc: linux-mm@kvack.org
> Cc: linux-kernel@vger.linux.org
> Signed-off-by: Sourab Gupta <sourab.gupta@intel.com>
> Signed-off-by: Akash Goel <akash.goel@intel.com>
> Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>

Ping?

> ---
> include/linux/shmem_fs.h | 17 +++++++++++++++++
> mm/shmem.c | 17 ++++++++++++++++-
> 2 files changed, 33 insertions(+), 1 deletion(-)
>
> diff --git a/include/linux/shmem_fs.h b/include/linux/shmem_fs.h
> index 4d4780c00d34..d7925b66c240 100644
> --- a/include/linux/shmem_fs.h
> +++ b/include/linux/shmem_fs.h
> @@ -34,11 +34,28 @@ struct shmem_sb_info {
> struct mempolicy *mpol; /* default memory policy for mappings */
> };
>
> +struct shmem_dev_info {
> + void *dev_private_data;
> + int (*dev_migratepage)(struct address_space *mapping,
> + struct page *newpage, struct page *page,
> + enum migrate_mode mode, void *dev_priv_data);
> +};
> +
> static inline struct shmem_inode_info *SHMEM_I(struct inode *inode)
> {
> return container_of(inode, struct shmem_inode_info, vfs_inode);
> }
>
> +static inline int shmem_set_device_ops(struct address_space *mapping,
> + struct shmem_dev_info *info)
> +{
> + if (mapping->private_data != NULL)
> + return -EEXIST;
> +
> + mapping->private_data = info;
> + return 0;
> +}
> +
> /*
> * Functions in mm/shmem.c called directly from elsewhere:
> */
> diff --git a/mm/shmem.c b/mm/shmem.c
> index 9428c51ab2d6..6ed953193883 100644
> --- a/mm/shmem.c
> +++ b/mm/shmem.c
> @@ -947,6 +947,21 @@ redirty:
> return 0;
> }
>
> +#ifdef CONFIG_MIGRATION
> +static int shmem_migratepage(struct address_space *mapping,
> + struct page *newpage, struct page *page,
> + enum migrate_mode mode)
> +{
> + struct shmem_dev_info *dev_info = mapping->private_data;
> +
> + if (dev_info && dev_info->dev_migratepage)
> + return dev_info->dev_migratepage(mapping, newpage, page,
> + mode, dev_info->dev_private_data);
> +
> + return migrate_page(mapping, newpage, page, mode);
> +}
> +#endif
> +
> #ifdef CONFIG_NUMA
> #ifdef CONFIG_TMPFS
> static void shmem_show_mpol(struct seq_file *seq, struct mempolicy *mpol)
> @@ -3161,7 +3176,7 @@ static const struct address_space_operations shmem_aops = {
> .write_end = shmem_write_end,
> #endif
> #ifdef CONFIG_MIGRATION
> - .migratepage = migrate_page,
> + .migratepage = shmem_migratepage,
> #endif
> .error_remove_page = generic_error_remove_page,
> };

--
Chris Wilson, Intel Open Source Technology Centre