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

[Chris Wilson <chris@chris-wilson.co.uk>]

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 can be arbitrarily withdrawn from page migration, resulting in premature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

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