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

[Hugh Dickins <hughd@google.com>]

On Fri, 4 Nov 2016, akash.goel@intel.com wrote:
> From: Chris Wilson <chris@chris-wilson.co.uk>
> 
> 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 can be arbitrarily withdrawn from page migration, resulting in premature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.

Thank you for persisting with this, and sorry for all my delay.

> 
> v2:
> - Drop dev_ prefix from the members of shmem_dev_info structure. (Joonas)
> - Change the return type of shmem_set_device_op() to void and remove the
>   check for pre-existing data. (Joonas)
> - Rename shmem_set_device_op() to shmem_set_dev_info() to be consistent
>   with shmem_dev_info structure. (Joonas)
> 
> 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>

That doesn't seem quite right: the From line above implies that Chris
wrote it, and should be first Signer; but perhaps the From line is wrong.

> ---
>  include/linux/shmem_fs.h | 13 +++++++++++++
>  mm/shmem.c               | 17 ++++++++++++++++-
>  2 files changed, 29 insertions(+), 1 deletion(-)
> 
> diff --git a/include/linux/shmem_fs.h b/include/linux/shmem_fs.h
> index ff078e7..454c3ba 100644
> --- a/include/linux/shmem_fs.h
> +++ b/include/linux/shmem_fs.h
> @@ -39,11 +39,24 @@ struct shmem_sb_info {
>  	unsigned long shrinklist_len; /* Length of shrinklist */
>  };
>  
> +struct shmem_dev_info {
> +	void *private_data;
> +	int (*migratepage)(struct address_space *mapping,
> +			   struct page *newpage, struct page *page,
> +			   enum migrate_mode mode, void *dev_priv_data);

Aren't the private_data field and dev_priv_data arg a little bit
confusing and redundant?  Can't the migratepage() deduce dev_priv
for itself from mapping->private_data (perhaps wrapped by a
shmem_get_dev_info()), by using container_of()?

> +};
> +
>  static inline struct shmem_inode_info *SHMEM_I(struct inode *inode)
>  {
>  	return container_of(inode, struct shmem_inode_info, vfs_inode);
>  }
>  
> +static inline void shmem_set_dev_info(struct address_space *mapping,
> +				      struct shmem_dev_info *info)
> +{
> +	mapping->private_data = info;

Nit: if this stays as is, I'd prefer dev_info there and above,
since shmem.c uses info all over for its shmem_inode_info pointer.
But in second patch I suggest obj_info may be better than dev_info.

> +}
> +
>  /*
>   * Functions in mm/shmem.c called directly from elsewhere:
>   */
> diff --git a/mm/shmem.c b/mm/shmem.c
> index ad7813d..fce8de3 100644
> --- a/mm/shmem.c
> +++ b/mm/shmem.c
> @@ -1290,6 +1290,21 @@ static int shmem_writepage(struct page *page, struct writeback_control *wbc)
>  	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->migratepage)
> +		return dev_info->migratepage(mapping, newpage, page,
> +					     mode, dev_info->private_data);
> +
> +	return migrate_page(mapping, newpage, page, mode);
> +}
> +#endif
> +
>  #if defined(CONFIG_NUMA) && defined(CONFIG_TMPFS)
>  static void shmem_show_mpol(struct seq_file *seq, struct mempolicy *mpol)
>  {
> @@ -3654,7 +3669,7 @@ static void shmem_destroy_inodecache(void)
>  	.write_end	= shmem_write_end,
>  #endif
>  #ifdef CONFIG_MIGRATION
> -	.migratepage	= migrate_page,
> +	.migratepage	= shmem_migratepage,
>  #endif
>  	.error_remove_page = generic_error_remove_page,
>  };
> -- 
> 1.9.2

I didn't like this very much; but every time I tried to "improve" it,
found good reasons why you chose the way you did (modularity of i915,
constness of a_ops, reluctance to copy and modify a_ops, reluctance
to export those shmem methods separately).

I think perhaps later we just add a gem_ops pointer to shmem_inode_info,
for i915 or other gems to fill in as they wish (and shmem divert off to
them if set, as you've done); but for now you're trying to avoid
enlarging the shmem inode, okay.

Hugh

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