Date:	Wed, 15 Jan 2014 20:24:28 -0500
From:	Matthew Wilcox <matthew.r.wilcox@...el.com>
To:	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	linux-mm@...ck.org, linux-ext4@...r.kernel.org
Cc:	Matthew Wilcox <matthew.r.wilcox@...el.com>
Subject: [PATCH v5 10/22] Remove get_xip_mem

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 the new way of writing an XIP filesystem.

Signed-off-by: Matthew Wilcox <matthew.r.wilcox@...el.com>
---
 Documentation/filesystems/Locking |   3 -
 Documentation/filesystems/xip.txt | 116 +++++++++++++++++++++-----------------
 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 +-
 10 files changed, 70 insertions(+), 106 deletions(-)

diff --git a/Documentation/filesystems/Locking b/Documentation/filesystems/Locking
index fe7afe2..75da4b6 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/xip.txt b/Documentation/filesystems/xip.txt
index b62eabf..520e73a 100644
--- a/Documentation/filesystems/xip.txt
+++ b/Documentation/filesystems/xip.txt
@@ -3,69 +3,81 @@ 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.
+
+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 XIP 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.
 
-The block device operation is optional, these block devices support it as of
-today:
+In order to support this method, the storage must be byte-accessable 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
 
-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
+Implementation Tips for Filesystem Writers
+------------------------------------------
+
+Filesystem support consists of
+- adding support to mark inodes as being XIP by setting the S_XIP flag in
+  i_flags
+- implementing the direct_IO address space operation, and calling
+  xip_do_io() instead of blockdev_direct_IO() if S_XIP is set
+- implementing an mmap file operation for XIP 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 xip_fault() and
+  xip_mkwrite(), passing the appropriate get_block() callback)
+- calling xip_truncate_page() instead of block_truncate_page() for XIP files
+- ensuring that there is sufficient locking between reads, writes,
+  truncates and page faults
+
+The get_block() callback passed to xip_do_io(), xip_fault(), xip_mkwrite()
+and xip_truncate_page() must not return uninitialised extents.  It must zero
+any blocks that it returns, and it must ensure that simultaneous calls to
+get_block() (for example by a page-fault racing with a read() or a write())
+work correctly.
 
-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
+These filesystems may be used for inspiration:
+- ext2: the second extended filesystem, see Documentation/filesystems/ext2.txt
 
-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.
+
+Even if the kernel or its modules are stored on an filesystem that supports
+XIP on a block device that supports XIP, they will still be copied into RAM.
+
+Calling get_user_pages() on a range of user memory that has been mmaped
+from an XIP 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-XIP file will fail (note
+that O_DIRECT reads/writes _of an XIP 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/fs/exofs/inode.c b/fs/exofs/inode.c
index a52a5d2..a96ce89 100644
--- a/fs/exofs/inode.c
+++ b/fs/exofs/inode.c
@@ -977,7 +977,6 @@ const struct address_space_operations exofs_aops = {
 	.direct_IO	= NULL, /* TODO: Should be trivial to do */
 
 	/* 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 c531700..57726ab 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 4b3e1ed..4b16abe 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 c77efa3..7c10319 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -372,8 +372,6 @@ 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..30d03bf 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_XIP(inode)) {
 		switch (advice) {
 		case POSIX_FADV_NORMAL:
 		case POSIX_FADV_RANDOM:
diff --git a/mm/madvise.c b/mm/madvise.c
index 539eeb9..9b487ca 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_XIP(file_inode(file))) {
 		/* no bad return value, but ignore advice */
 		return 0;
 	}
-- 
1.8.5.2

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