Date:	Thu, 16 Oct 2014 10:45:50 +0200
From:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:	Matthew Wilcox <matthew.r.wilcox@...el.com>
Cc:	linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v11 02/21] block: Change direct_access calling convention

On 25-Sep-2014 04:33:19 PM, Matthew Wilcox wrote:
> In order to support accesses to larger chunks of memory, pass in a
> 'size' parameter (counted in bytes), and return the amount available at
> that address.
> 
> Add a new helper function, bdev_direct_access(), to handle common
> functionality including partition handling, checking the length requested
> is positive, checking for the sector being page-aligned, and checking
> the length of the request does not pass the end of the partition.
> 
> Signed-off-by: Matthew Wilcox <matthew.r.wilcox@...el.com>
> Reviewed-by: Jan Kara <jack@...e.cz>
> Reviewed-by: Boaz Harrosh <boaz@...xistor.com>
> ---
>  Documentation/filesystems/xip.txt | 15 +++++++++------
>  arch/powerpc/sysdev/axonram.c     | 17 ++++-------------
>  drivers/block/brd.c               | 12 +++++-------
>  drivers/s390/block/dcssblk.c      | 21 +++++++++-----------
>  fs/block_dev.c                    | 40 +++++++++++++++++++++++++++++++++++++++
>  fs/ext2/xip.c                     | 31 +++++++++++++-----------------
>  include/linux/blkdev.h            |  6 ++++--
>  7 files changed, 84 insertions(+), 58 deletions(-)
> 
> diff --git a/Documentation/filesystems/xip.txt b/Documentation/filesystems/xip.txt
> index 0466ee5..b774729 100644
> --- a/Documentation/filesystems/xip.txt
> +++ b/Documentation/filesystems/xip.txt
> @@ -28,12 +28,15 @@ Implementation
>  Execute-in-place is implemented in three steps: block device operation,
>  address space operation, and file operations.
>  
> -A block device operation named direct_access is used to retrieve a
> -reference (pointer) to a block on-disk. The reference is supposed to be
> -cpu-addressable, physical address and remain valid until the release operation
> -is performed. A struct block_device reference is used to address the device,
> -and a sector_t argument is used to identify the individual block. As an
> -alternative, memory technology devices can be used for this.
> +A block device operation named direct_access is used to translate the
> +block device sector number to a page frame number (pfn) that identifies
> +the physical page for the memory.  It also returns a kernel virtual
> +address that can be used to access the memory.
> +
> +The direct_access method takes a 'size' parameter that indicates the
> +number of bytes being requested.  The function should return the number
> +of bytes that can be contiguously accessed at that offset.  It may also
> +return a negative errno if an error occurs.
>  
>  The block device operation is optional, these block devices support it as of
>  today:
> diff --git a/arch/powerpc/sysdev/axonram.c b/arch/powerpc/sysdev/axonram.c
> index 830edc8..8709b9f 100644
> --- a/arch/powerpc/sysdev/axonram.c
> +++ b/arch/powerpc/sysdev/axonram.c
> @@ -139,26 +139,17 @@ axon_ram_make_request(struct request_queue *queue, struct bio *bio)
>   * axon_ram_direct_access - direct_access() method for block device
>   * @device, @sector, @data: see block_device_operations method
>   */
> -static int
> +static long
>  axon_ram_direct_access(struct block_device *device, sector_t sector,
> -		       void **kaddr, unsigned long *pfn)
> +		       void **kaddr, unsigned long *pfn, long size)

Why "long" as type for size ? What is the intent to have it signed, and
why using a 32-bit type on 32-bit architectures rather than 64-bit ?
Can we run into issues if we try to map a >2GB file on 32-bit
architectures ?

>  {
>  	struct axon_ram_bank *bank = device->bd_disk->private_data;
> -	loff_t offset;
> -
> -	offset = sector;
> -	if (device->bd_part != NULL)
> -		offset += device->bd_part->start_sect;
> -	offset <<= AXON_RAM_SECTOR_SHIFT;
> -	if (offset >= bank->size) {
> -		dev_err(&bank->device->dev, "Access outside of address space\n");
> -		return -ERANGE;
> -	}
> +	loff_t offset = (loff_t)sector << AXON_RAM_SECTOR_SHIFT;
>  
>  	*kaddr = (void *)(bank->ph_addr + offset);
>  	*pfn = virt_to_phys(*kaddr) >> PAGE_SHIFT;
>  
> -	return 0;
> +	return bank->size - offset;


>  }
>  
>  static const struct block_device_operations axon_ram_devops = {
> diff --git a/drivers/block/brd.c b/drivers/block/brd.c
> index 3598110..78fe510 100644
> --- a/drivers/block/brd.c
> +++ b/drivers/block/brd.c
> @@ -370,25 +370,23 @@ static int brd_rw_page(struct block_device *bdev, sector_t sector,
>  }
>  
>  #ifdef CONFIG_BLK_DEV_XIP
> -static int brd_direct_access(struct block_device *bdev, sector_t sector,
> -			void **kaddr, unsigned long *pfn)
> +static long brd_direct_access(struct block_device *bdev, sector_t sector,
> +			void **kaddr, unsigned long *pfn, long size)
>  {
>  	struct brd_device *brd = bdev->bd_disk->private_data;
>  	struct page *page;
>  
>  	if (!brd)
>  		return -ENODEV;
> -	if (sector & (PAGE_SECTORS-1))
> -		return -EINVAL;
> -	if (sector + PAGE_SECTORS > get_capacity(bdev->bd_disk))
> -		return -ERANGE;
>  	page = brd_insert_page(brd, sector);
>  	if (!page)
>  		return -ENOSPC;
>  	*kaddr = page_address(page);
>  	*pfn = page_to_pfn(page);
>  
> -	return 0;
> +	/* If size > PAGE_SIZE, we could look to see if the next page in the
> +	 * file happens to be mapped to the next page of physical RAM */

The style for this comment should be:

/*
 * ....
 */

Perhaps with a "TODO" ?

> +	return PAGE_SIZE;
>  }
>  #endif
>  
> diff --git a/drivers/s390/block/dcssblk.c b/drivers/s390/block/dcssblk.c
> index 0f47175..96bc411 100644
> --- a/drivers/s390/block/dcssblk.c
> +++ b/drivers/s390/block/dcssblk.c
> @@ -28,8 +28,8 @@
>  static int dcssblk_open(struct block_device *bdev, fmode_t mode);
>  static void dcssblk_release(struct gendisk *disk, fmode_t mode);
>  static void dcssblk_make_request(struct request_queue *q, struct bio *bio);
> -static int dcssblk_direct_access(struct block_device *bdev, sector_t secnum,
> -				 void **kaddr, unsigned long *pfn);
> +static long dcssblk_direct_access(struct block_device *bdev, sector_t secnum,
> +				 void **kaddr, unsigned long *pfn, long size);
>  
>  static char dcssblk_segments[DCSSBLK_PARM_LEN] = "\0";
>  
> @@ -866,25 +866,22 @@ fail:
>  	bio_io_error(bio);
>  }
>  
> -static int
> +static long
>  dcssblk_direct_access (struct block_device *bdev, sector_t secnum,
> -			void **kaddr, unsigned long *pfn)
> +			void **kaddr, unsigned long *pfn, long size)
>  {
>  	struct dcssblk_dev_info *dev_info;
> -	unsigned long pgoff;
> +	unsigned long offset, dev_sz;
>  
>  	dev_info = bdev->bd_disk->private_data;
>  	if (!dev_info)
>  		return -ENODEV;
> -	if (secnum % (PAGE_SIZE/512))
> -		return -EINVAL;
> -	pgoff = secnum / (PAGE_SIZE / 512);
> -	if ((pgoff+1)*PAGE_SIZE-1 > dev_info->end - dev_info->start)
> -		return -ERANGE;
> -	*kaddr = (void *) (dev_info->start+pgoff*PAGE_SIZE);
> +	dev_sz = dev_info->end - dev_info->start;
> +	offset = secnum * 512;
> +	*kaddr = (void *) (dev_info->start + offset);
>  	*pfn = virt_to_phys(*kaddr) >> PAGE_SHIFT;
>  
> -	return 0;
> +	return dev_sz - offset;
>  }
>  
>  static void
> diff --git a/fs/block_dev.c b/fs/block_dev.c
> index 6d72746..ffe0761 100644
> --- a/fs/block_dev.c
> +++ b/fs/block_dev.c
> @@ -427,6 +427,46 @@ int bdev_write_page(struct block_device *bdev, sector_t sector,
>  }
>  EXPORT_SYMBOL_GPL(bdev_write_page);
>  
> +/**
> + * bdev_direct_access() - Get the address for directly-accessibly memory
> + * @bdev: The device containing the memory
> + * @sector: The offset within the device
> + * @addr: Where to put the address of the memory
> + * @pfn: The Page Frame Number for the memory
> + * @size: The number of bytes requested
> + *
> + * If a block device is made up of directly addressable memory, this function
> + * will tell the caller the PFN and the address of the memory.  The address
> + * may be directly dereferenced within the kernel without the need to call
> + * ioremap(), kmap() or similar.  The PFN is suitable for inserting into
> + * page tables.
> + *
> + * Return: negative errno if an error occurs, otherwise the number of bytes
> + * accessible at this address.
> + */
> +long bdev_direct_access(struct block_device *bdev, sector_t sector,
> +			void **addr, unsigned long *pfn, long size)
> +{
> +	long avail;
> +	const struct block_device_operations *ops = bdev->bd_disk->fops;
> +
> +	if (size < 0)
> +		return size;

I'm wondering how we should handle size == 0 here. Should it be accepted
or refused ?

Thanks,

Mathieu

> +	if (!ops->direct_access)
> +		return -EOPNOTSUPP;
> +	if ((sector + DIV_ROUND_UP(size, 512)) >
> +					part_nr_sects_read(bdev->bd_part))
> +		return -ERANGE;
> +	sector += get_start_sect(bdev);
> +	if (sector % (PAGE_SIZE / 512))
> +		return -EINVAL;
> +	avail = ops->direct_access(bdev, sector, addr, pfn, size);
> +	if (!avail)
> +		return -ERANGE;
> +	return min(avail, size);
> +}
> +EXPORT_SYMBOL_GPL(bdev_direct_access);
> +
>  /*
>   * pseudo-fs
>   */
> diff --git a/fs/ext2/xip.c b/fs/ext2/xip.c
> index e98171a..bbc5fec 100644
> --- a/fs/ext2/xip.c
> +++ b/fs/ext2/xip.c
> @@ -13,18 +13,12 @@
>  #include "ext2.h"
>  #include "xip.h"
>  
> -static inline int
> -__inode_direct_access(struct inode *inode, sector_t block,
> -		      void **kaddr, unsigned long *pfn)
> +static inline long __inode_direct_access(struct inode *inode, sector_t block,
> +				void **kaddr, unsigned long *pfn, long size)
>  {
>  	struct block_device *bdev = inode->i_sb->s_bdev;
> -	const struct block_device_operations *ops = bdev->bd_disk->fops;
> -	sector_t sector;
> -
> -	sector = block * (PAGE_SIZE / 512); /* ext2 block to bdev sector */
> -
> -	BUG_ON(!ops->direct_access);
> -	return ops->direct_access(bdev, sector, kaddr, pfn);
> +	sector_t sector = block * (PAGE_SIZE / 512);
> +	return bdev_direct_access(bdev, sector, kaddr, pfn, size);
>  }
>  
>  static inline int
> @@ -53,12 +47,13 @@ ext2_clear_xip_target(struct inode *inode, sector_t block)
>  {
>  	void *kaddr;
>  	unsigned long pfn;
> -	int rc;
> +	long size;
>  
> -	rc = __inode_direct_access(inode, block, &kaddr, &pfn);
> -	if (!rc)
> -		clear_page(kaddr);
> -	return rc;
> +	size = __inode_direct_access(inode, block, &kaddr, &pfn, PAGE_SIZE);
> +	if (size < 0)
> +		return size;
> +	clear_page(kaddr);
> +	return 0;
>  }
>  
>  void ext2_xip_verify_sb(struct super_block *sb)
> @@ -77,7 +72,7 @@ void ext2_xip_verify_sb(struct super_block *sb)
>  int ext2_get_xip_mem(struct address_space *mapping, pgoff_t pgoff, int create,
>  				void **kmem, unsigned long *pfn)
>  {
> -	int rc;
> +	long rc;
>  	sector_t block;
>  
>  	/* first, retrieve the sector number */
> @@ -86,6 +81,6 @@ int ext2_get_xip_mem(struct address_space *mapping, pgoff_t pgoff, int create,
>  		return rc;
>  
>  	/* retrieve address of the target data */
> -	rc = __inode_direct_access(mapping->host, block, kmem, pfn);
> -	return rc;
> +	rc = __inode_direct_access(mapping->host, block, kmem, pfn, PAGE_SIZE);
> +	return (rc < 0) ? rc : 0;
>  }
> diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
> index 518b465..ac25166 100644
> --- a/include/linux/blkdev.h
> +++ b/include/linux/blkdev.h
> @@ -1615,8 +1615,8 @@ struct block_device_operations {
>  	int (*rw_page)(struct block_device *, sector_t, struct page *, int rw);
>  	int (*ioctl) (struct block_device *, fmode_t, unsigned, unsigned long);
>  	int (*compat_ioctl) (struct block_device *, fmode_t, unsigned, unsigned long);
> -	int (*direct_access) (struct block_device *, sector_t,
> -						void **, unsigned long *);
> +	long (*direct_access)(struct block_device *, sector_t,
> +					void **, unsigned long *pfn, long size);
>  	unsigned int (*check_events) (struct gendisk *disk,
>  				      unsigned int clearing);
>  	/* ->media_changed() is DEPRECATED, use ->check_events() instead */
> @@ -1634,6 +1634,8 @@ extern int __blkdev_driver_ioctl(struct block_device *, fmode_t, unsigned int,
>  extern int bdev_read_page(struct block_device *, sector_t, struct page *);
>  extern int bdev_write_page(struct block_device *, sector_t, struct page *,
>  						struct writeback_control *);
> +extern long bdev_direct_access(struct block_device *, sector_t, void **addr,
> +						unsigned long *pfn, long size);
>  #else /* CONFIG_BLOCK */
>  
>  struct block_device;
> -- 
> 2.1.0
> 
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> 

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com
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