Date:	Tue, 8 Apr 2014 20:20:59 +0200
From:	Jan Kara <jack@...e.cz>
To:	Matthew Wilcox <matthew.r.wilcox@...el.com>
Cc:	linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, willy@...ux.intel.com
Subject: Re: [PATCH v7 10/22] Remove get_xip_mem

On Sun 23-03-14 15:08:36, Matthew Wilcox wrote:
> All callers of get_xip_mem() are now gone.  Remove checks for it,
> initialisers of it, documentation of it and the only implementation of it.
> 
> Add documentation for writing a filesystem that supports DAX.
> 
> Signed-off-by: Matthew Wilcox <matthew.r.wilcox@...el.com>
> Reviewed-by: Randy Dunlap <rdunlap@...radead.org>
  The patch looks good. You can add:
Reviewed-by: Jan Kara <jack@...e.cz>

								Honza
> ---
>  Documentation/filesystems/Locking |  3 --
>  Documentation/filesystems/dax.txt | 82 +++++++++++++++++++++++++++++++++++++++
>  Documentation/filesystems/xip.txt | 71 ---------------------------------
>  fs/exofs/inode.c                  |  1 -
>  fs/ext2/inode.c                   |  1 -
>  fs/ext2/xip.c                     | 37 ------------------
>  fs/ext2/xip.h                     |  3 --
>  fs/open.c                         |  5 +--
>  include/linux/fs.h                |  2 -
>  mm/fadvise.c                      |  6 ++-
>  mm/madvise.c                      |  2 +-
>  11 files changed, 88 insertions(+), 125 deletions(-)
>  create mode 100644 Documentation/filesystems/dax.txt
>  delete mode 100644 Documentation/filesystems/xip.txt
> 
> diff --git a/Documentation/filesystems/Locking b/Documentation/filesystems/Locking
> index 5b0c083..2780d47 100644
> --- a/Documentation/filesystems/Locking
> +++ b/Documentation/filesystems/Locking
> @@ -194,8 +194,6 @@ prototypes:
>  	void (*freepage)(struct page *);
>  	int (*direct_IO)(int, struct kiocb *, const struct iovec *iov,
>  			loff_t offset, unsigned long nr_segs);
> -	int (*get_xip_mem)(struct address_space *, pgoff_t, int, void **,
> -				unsigned long *);
>  	int (*migratepage)(struct address_space *, struct page *, struct page *);
>  	int (*launder_page)(struct page *);
>  	int (*is_partially_uptodate)(struct page *, read_descriptor_t *, unsigned long);
> @@ -220,7 +218,6 @@ invalidatepage:		yes
>  releasepage:		yes
>  freepage:		yes
>  direct_IO:
> -get_xip_mem:					maybe
>  migratepage:		yes (both)
>  launder_page:		yes
>  is_partially_uptodate:	yes
> diff --git a/Documentation/filesystems/dax.txt b/Documentation/filesystems/dax.txt
> new file mode 100644
> index 0000000..06f84e5
> --- /dev/null
> +++ b/Documentation/filesystems/dax.txt
> @@ -0,0 +1,82 @@
> +Execute-in-place for file mappings
> +----------------------------------
> +
> +Motivation
> +----------
> +
> +File mappings are usually performed by mapping page cache pages to
> +userspace.  In addition, read & write file operations also transfer data
> +between the page cache and storage.
> +
> +For memory backed storage devices that use the block device interface,
> +the page cache pages are just copies of the original storage.  The
> +execute-in-place code removes the extra copy by performing reads and
> +writes directly on the memory backed storage device.  For file mappings,
> +the storage device itself is mapped directly into userspace.
> +
> +
> +Implementation Tips for Block Driver Writers
> +--------------------------------------------
> +
> +To support DAX in your block driver, implement the 'direct_access'
> +block device operation.  It is used to translate the sector number
> +(expressed in units of 512-byte sectors) 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 it can provide, although it must not exceed the number of
> +bytes requested.  It may also return a negative errno if an error occurs.
> +
> +In order to support this method, the storage must be byte-accessible by
> +the CPU at all times.  If your device uses paging techniques to expose
> +a large amount of memory through a smaller window, then you cannot
> +implement direct_access.  Equally, if your device can occasionally
> +stall the CPU for an extended period, you should also not attempt to
> +implement direct_access.
> +
> +These block devices may be used for inspiration:
> +- axonram: Axon DDR2 device driver
> +- brd: RAM backed block device driver
> +- dcssblk: s390 dcss block device driver
> +
> +
> +Implementation Tips for Filesystem Writers
> +------------------------------------------
> +
> +Filesystem support consists of
> +- adding support to mark inodes as being DAX by setting the S_DAX flag in
> +  i_flags
> +- implementing the direct_IO address space operation, and calling
> +  dax_do_io() instead of blockdev_direct_IO() if S_DAX is set
> +- implementing an mmap file operation for DAX files which sets the
> +  VM_MIXEDMAP flag on the VMA, and setting the vm_ops to include handlers
> +  for fault and page_mkwrite (which should probably call dax_fault() and
> +  dax_mkwrite(), passing the appropriate get_block() callback)
> +- calling dax_truncate_page() instead of block_truncate_page() for DAX files
> +- ensuring that there is sufficient locking between reads, writes,
> +  truncates and page faults
> +
> +The get_block() callback passed to the DAX functions may return
> +uninitialised extents.  If it does, it must ensure that simultaneous
> +calls to get_block() (for example by a page-fault racing with a read()
> +or a write()) work correctly.
> +
> +These filesystems may be used for inspiration:
> +- ext2: the second extended filesystem, see Documentation/filesystems/ext2.txt
> +
> +
> +Shortcomings
> +------------
> +
> +Even if the kernel or its modules are stored on a filesystem that supports
> +DAX on a block device that supports DAX, they will still be copied into RAM.
> +
> +Calling get_user_pages() on a range of user memory that has been mmaped
> +from a DAX file will fail as there are no 'struct page' to describe
> +those pages.  This problem is being worked on.  That means that O_DIRECT
> +reads/writes to those memory ranges from a non-DAX file will fail (note
> +that O_DIRECT reads/writes _of a DAX file_ do work, it is the memory
> +that is being accessed that is key here).  Other things that will not
> +work include RDMA, sendfile() and splice().
> diff --git a/Documentation/filesystems/xip.txt b/Documentation/filesystems/xip.txt
> deleted file mode 100644
> index b62eabf..0000000
> --- a/Documentation/filesystems/xip.txt
> +++ /dev/null
> @@ -1,71 +0,0 @@
> -Execute-in-place for file mappings
> -----------------------------------
> -
> -Motivation
> -----------
> -File mappings are performed by mapping page cache pages to userspace. In
> -addition, read&write type file operations also transfer data from/to the page
> -cache.
> -
> -For memory backed storage devices that use the block device interface, the page
> -cache pages are in fact copies of the original storage. Various approaches
> -exist to work around the need for an extra copy. The ramdisk driver for example
> -does read the data into the page cache, keeps a reference, and discards the
> -original data behind later on.
> -
> -Execute-in-place solves this issue the other way around: instead of keeping
> -data in the page cache, the need to have a page cache copy is eliminated
> -completely. With execute-in-place, read&write type operations are performed
> -directly from/to the memory backed storage device. For file mappings, the
> -storage device itself is mapped directly into userspace.
> -
> -This implementation was initially written for shared memory segments between
> -different virtual machines on s390 hardware to allow multiple machines to
> -share the same binaries and libraries.
> -
> -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 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 it can provide, although it must not exceed the number of
> -bytes requested.  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:
> -- dcssblk: s390 dcss block device driver
> -
> -An address space operation named get_xip_mem is used to retrieve references
> -to a page frame number and a kernel address. To obtain these values a reference
> -to an address_space is provided. This function assigns values to the kmem and
> -pfn parameters. The third argument indicates whether the function should allocate
> -blocks if needed.
> -
> -This address space operation is mutually exclusive with readpage&writepage that
> -do page cache read/write operations.
> -The following filesystems support it as of today:
> -- ext2: the second extended filesystem, see Documentation/filesystems/ext2.txt
> -
> -A set of file operations that do utilize get_xip_page can be found in
> -mm/filemap_xip.c . The following file operation implementations are provided:
> -- aio_read/aio_write
> -- readv/writev
> -- sendfile
> -
> -The generic file operations do_sync_read/do_sync_write can be used to implement
> -classic synchronous IO calls.
> -
> -Shortcomings
> -------------
> -This implementation is limited to storage devices that are cpu addressable at
> -all times (no highmem or such). It works well on rom/ram, but enhancements are
> -needed to make it work with flash in read+write mode.
> -Putting the Linux kernel and/or its modules on a xip filesystem does not mean
> -they are not copied.
> diff --git a/fs/exofs/inode.c b/fs/exofs/inode.c
> index ee4317fa..f9a5bf6 100644
> --- a/fs/exofs/inode.c
> +++ b/fs/exofs/inode.c
> @@ -985,7 +985,6 @@ const struct address_space_operations exofs_aops = {
>  	.direct_IO	= exofs_direct_IO,
>  
>  	/* With these NULL has special meaning or default is not exported */
> -	.get_xip_mem	= NULL,
>  	.migratepage	= NULL,
>  	.launder_page	= NULL,
>  	.is_partially_uptodate = NULL,
> diff --git a/fs/ext2/inode.c b/fs/ext2/inode.c
> index 252481f..b156fe8 100644
> --- a/fs/ext2/inode.c
> +++ b/fs/ext2/inode.c
> @@ -891,7 +891,6 @@ const struct address_space_operations ext2_aops = {
>  
>  const struct address_space_operations ext2_aops_xip = {
>  	.bmap			= ext2_bmap,
> -	.get_xip_mem		= ext2_get_xip_mem,
>  	.direct_IO		= ext2_direct_IO,
>  };
>  
> diff --git a/fs/ext2/xip.c b/fs/ext2/xip.c
> index fa40091..ca745ff 100644
> --- a/fs/ext2/xip.c
> +++ b/fs/ext2/xip.c
> @@ -22,27 +22,6 @@ static inline long __inode_direct_access(struct inode *inode, sector_t block,
>  	return ops->direct_access(bdev, sector, kaddr, pfn, size);
>  }
>  
> -static inline int
> -__ext2_get_block(struct inode *inode, pgoff_t pgoff, int create,
> -		   sector_t *result)
> -{
> -	struct buffer_head tmp;
> -	int rc;
> -
> -	memset(&tmp, 0, sizeof(struct buffer_head));
> -	tmp.b_size = 1 << inode->i_blkbits;
> -	rc = ext2_get_block(inode, pgoff, &tmp, create);
> -	*result = tmp.b_blocknr;
> -
> -	/* did we get a sparse block (hole in the file)? */
> -	if (!tmp.b_blocknr && !rc) {
> -		BUG_ON(create);
> -		rc = -ENODATA;
> -	}
> -
> -	return rc;
> -}
> -
>  int
>  ext2_clear_xip_target(struct inode *inode, sector_t block)
>  {
> @@ -69,19 +48,3 @@ void ext2_xip_verify_sb(struct super_block *sb)
>  			     "not supported by bdev");
>  	}
>  }
> -
> -int ext2_get_xip_mem(struct address_space *mapping, pgoff_t pgoff, int create,
> -				void **kmem, unsigned long *pfn)
> -{
> -	long rc;
> -	sector_t block;
> -
> -	/* first, retrieve the sector number */
> -	rc = __ext2_get_block(mapping->host, pgoff, create, &block);
> -	if (rc)
> -		return rc;
> -
> -	/* retrieve address of the target data */
> -	rc = __inode_direct_access(mapping->host, block, kmem, pfn, PAGE_SIZE);
> -	return (rc < 0) ? rc : 0;
> -}
> diff --git a/fs/ext2/xip.h b/fs/ext2/xip.h
> index 29be737..0fa8b7f 100644
> --- a/fs/ext2/xip.h
> +++ b/fs/ext2/xip.h
> @@ -14,11 +14,8 @@ static inline int ext2_use_xip (struct super_block *sb)
>  	struct ext2_sb_info *sbi = EXT2_SB(sb);
>  	return (sbi->s_mount_opt & EXT2_MOUNT_XIP);
>  }
> -int ext2_get_xip_mem(struct address_space *, pgoff_t, int,
> -				void **, unsigned long *);
>  #else
>  #define ext2_xip_verify_sb(sb)			do { } while (0)
>  #define ext2_use_xip(sb)			0
>  #define ext2_clear_xip_target(inode, chain)	0
> -#define ext2_get_xip_mem			NULL
>  #endif
> diff --git a/fs/open.c b/fs/open.c
> index b9ed8b2..bc9f002 100644
> --- a/fs/open.c
> +++ b/fs/open.c
> @@ -665,11 +665,8 @@ int open_check_o_direct(struct file *f)
>  {
>  	/* NB: we're sure to have correct a_ops only after f_op->open */
>  	if (f->f_flags & O_DIRECT) {
> -		if (!f->f_mapping->a_ops ||
> -		    ((!f->f_mapping->a_ops->direct_IO) &&
> -		    (!f->f_mapping->a_ops->get_xip_mem))) {
> +		if (!f->f_mapping->a_ops || !f->f_mapping->a_ops->direct_IO)
>  			return -EINVAL;
> -		}
>  	}
>  	return 0;
>  }
> diff --git a/include/linux/fs.h b/include/linux/fs.h
> index 9752ae5..c777056 100644
> --- a/include/linux/fs.h
> +++ b/include/linux/fs.h
> @@ -375,8 +375,6 @@ struct address_space_operations {
>  	void (*freepage)(struct page *);
>  	ssize_t (*direct_IO)(int, struct kiocb *, const struct iovec *iov,
>  			loff_t offset, unsigned long nr_segs);
> -	int (*get_xip_mem)(struct address_space *, pgoff_t, int,
> -						void **, unsigned long *);
>  	/*
>  	 * migrate the contents of a page to the specified target. If
>  	 * migrate_mode is MIGRATE_ASYNC, it must not block.
> diff --git a/mm/fadvise.c b/mm/fadvise.c
> index 3bcfd81..1f1925f 100644
> --- a/mm/fadvise.c
> +++ b/mm/fadvise.c
> @@ -28,6 +28,7 @@
>  SYSCALL_DEFINE4(fadvise64_64, int, fd, loff_t, offset, loff_t, len, int, advice)
>  {
>  	struct fd f = fdget(fd);
> +	struct inode *inode;
>  	struct address_space *mapping;
>  	struct backing_dev_info *bdi;
>  	loff_t endbyte;			/* inclusive */
> @@ -39,7 +40,8 @@ SYSCALL_DEFINE4(fadvise64_64, int, fd, loff_t, offset, loff_t, len, int, advice)
>  	if (!f.file)
>  		return -EBADF;
>  
> -	if (S_ISFIFO(file_inode(f.file)->i_mode)) {
> +	inode = file_inode(f.file);
> +	if (S_ISFIFO(inode->i_mode)) {
>  		ret = -ESPIPE;
>  		goto out;
>  	}
> @@ -50,7 +52,7 @@ SYSCALL_DEFINE4(fadvise64_64, int, fd, loff_t, offset, loff_t, len, int, advice)
>  		goto out;
>  	}
>  
> -	if (mapping->a_ops->get_xip_mem) {
> +	if (IS_DAX(inode)) {
>  		switch (advice) {
>  		case POSIX_FADV_NORMAL:
>  		case POSIX_FADV_RANDOM:
> diff --git a/mm/madvise.c b/mm/madvise.c
> index 539eeb9..b6a2f52 100644
> --- a/mm/madvise.c
> +++ b/mm/madvise.c
> @@ -236,7 +236,7 @@ static long madvise_willneed(struct vm_area_struct *vma,
>  	if (!file)
>  		return -EBADF;
>  
> -	if (file->f_mapping->a_ops->get_xip_mem) {
> +	if (IS_DAX(file_inode(file))) {
>  		/* no bad return value, but ignore advice */
>  		return 0;
>  	}
> -- 
> 1.9.0
> 
> --
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-- 
Jan Kara <jack@...e.cz>
SUSE Labs, CR
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