Date:	Thu, 16 Oct 2014 11:50:27 +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 07/21] dax,ext2: Replace XIP read and write with DAX
 I/O

On 25-Sep-2014 04:33:24 PM, Matthew Wilcox wrote:
> Use the generic AIO infrastructure instead of custom read and write
> methods.  In addition to giving us support for AIO, this adds the missing
> locking between read() and truncate().
> 
> Signed-off-by: Matthew Wilcox <matthew.r.wilcox@...el.com>
> Reviewed-by: Ross Zwisler <ross.zwisler@...ux.intel.com>
> Reviewed-by: Jan Kara <jack@...e.cz>
> ---
>  MAINTAINERS        |   6 ++
>  fs/Makefile        |   1 +
>  fs/dax.c           | 195 ++++++++++++++++++++++++++++++++++++++++++++
>  fs/ext2/file.c     |   6 +-
>  fs/ext2/inode.c    |   8 +-
>  include/linux/fs.h |  18 ++++-
>  mm/filemap.c       |   6 +-
>  mm/filemap_xip.c   | 234 -----------------------------------------------------
>  8 files changed, 229 insertions(+), 245 deletions(-)
>  create mode 100644 fs/dax.c
> 
> diff --git a/MAINTAINERS b/MAINTAINERS
> index 670b3dc..c96056b 100644
> --- a/MAINTAINERS
> +++ b/MAINTAINERS
> @@ -2934,6 +2934,12 @@ L:	linux-i2c@...r.kernel.org
>  S:	Maintained
>  F:	drivers/i2c/busses/i2c-diolan-u2c.c
>  
> +DIRECT ACCESS (DAX)
> +M:	Matthew Wilcox <willy@...ux.intel.com>
> +L:	linux-fsdevel@...r.kernel.org
> +S:	Supported
> +F:	fs/dax.c
> +
>  DIRECTORY NOTIFICATION (DNOTIFY)
>  M:	Eric Paris <eparis@...isplace.org>
>  S:	Maintained
> diff --git a/fs/Makefile b/fs/Makefile
> index 90c8852..0325ec3 100644
> --- a/fs/Makefile
> +++ b/fs/Makefile
> @@ -28,6 +28,7 @@ obj-$(CONFIG_SIGNALFD)		+= signalfd.o
>  obj-$(CONFIG_TIMERFD)		+= timerfd.o
>  obj-$(CONFIG_EVENTFD)		+= eventfd.o
>  obj-$(CONFIG_AIO)               += aio.o
> +obj-$(CONFIG_FS_XIP)		+= dax.o
>  obj-$(CONFIG_FILE_LOCKING)      += locks.o
>  obj-$(CONFIG_COMPAT)		+= compat.o compat_ioctl.o
>  obj-$(CONFIG_BINFMT_AOUT)	+= binfmt_aout.o
> diff --git a/fs/dax.c b/fs/dax.c
> new file mode 100644
> index 0000000..108c68e
> --- /dev/null
> +++ b/fs/dax.c
> @@ -0,0 +1,195 @@
> +/*
> + * fs/dax.c - Direct Access filesystem code
> + * Copyright (c) 2013-2014 Intel Corporation
> + * Author: Matthew Wilcox <matthew.r.wilcox@...el.com>
> + * Author: Ross Zwisler <ross.zwisler@...ux.intel.com>
> + *
> + * This program is free software; you can redistribute it and/or modify it
> + * under the terms and conditions of the GNU General Public License,
> + * version 2, as published by the Free Software Foundation.
> + *
> + * This program is distributed in the hope it will be useful, but WITHOUT
> + * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
> + * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License for
> + * more details.
> + */
> +
> +#include <linux/atomic.h>
> +#include <linux/blkdev.h>
> +#include <linux/buffer_head.h>
> +#include <linux/fs.h>
> +#include <linux/genhd.h>
> +#include <linux/mutex.h>
> +#include <linux/uio.h>
> +
> +static long dax_get_addr(struct buffer_head *bh, void **addr, unsigned blkbits)
> +{
> +	unsigned long pfn;
> +	sector_t sector = bh->b_blocknr << (blkbits - 9);
> +	return bdev_direct_access(bh->b_bdev, sector, addr, &pfn, bh->b_size);
> +}
> +
> +static void dax_new_buf(void *addr, unsigned size, unsigned first, loff_t pos,
> +			loff_t end)
> +{
> +	loff_t final = end - pos + first; /* The final byte of the buffer */
> +
> +	if (first > 0)
> +		memset(addr, 0, first);
> +	if (final < size)
> +		memset(addr + final, 0, size - final);
> +}
> +
> +static bool buffer_written(struct buffer_head *bh)
> +{
> +	return buffer_mapped(bh) && !buffer_unwritten(bh);
> +}
> +
> +/*
> + * When ext4 encounters a hole, it returns without modifying the buffer_head
> + * which means that we can't trust b_size.  To cope with this, we set b_state
> + * to 0 before calling get_block and, if any bit is set, we know we can trust
> + * b_size.  Unfortunate, really, since ext4 knows precisely how long a hole is
> + * and would save us time calling get_block repeatedly.
> + */
> +static bool buffer_size_valid(struct buffer_head *bh)
> +{
> +	return bh->b_state != 0;
> +}
> +
> +static ssize_t dax_io(int rw, struct inode *inode, struct iov_iter *iter,
> +			loff_t start, loff_t end, get_block_t get_block,
> +			struct buffer_head *bh)
> +{
> +	ssize_t retval = 0;
> +	loff_t pos = start;
> +	loff_t max = start;
> +	loff_t bh_max = start;
> +	void *addr;
> +	bool hole = false;
> +
> +	if (rw != WRITE)
> +		end = min(end, i_size_read(inode));
> +
> +	while (pos < end) {
> +		unsigned len;
> +		if (pos == max) {
> +			unsigned blkbits = inode->i_blkbits;
> +			sector_t block = pos >> blkbits;
> +			unsigned first = pos - (block << blkbits);
> +			long size;
> +
> +			if (pos == bh_max) {
> +				bh->b_size = PAGE_ALIGN(end - pos);
> +				bh->b_state = 0;
> +				retval = get_block(inode, block, bh,
> +								rw == WRITE);
> +				if (retval)
> +					break;
> +				if (!buffer_size_valid(bh))
> +					bh->b_size = 1 << blkbits;
> +				bh_max = pos - first + bh->b_size;
> +			} else {
> +				unsigned done = bh->b_size -
> +						(bh_max - (pos - first));
> +				bh->b_blocknr += done >> blkbits;
> +				bh->b_size -= done;
> +			}
> +			if (rw == WRITE) {
> +				if (!buffer_mapped(bh)) {
> +					retval = -EIO;
> +					/* FIXME: fall back to buffered I/O */

Fallback on buffered I/O would void guarantee about having data stored
into persistent memory after write returns. Not sure we actually want
that.

Thanks,

Mathieu

> +					break;
> +				}
> +				hole = false;
> +			} else {
> +				hole = !buffer_written(bh);
> +			}
> +
> +			if (hole) {
> +				addr = NULL;
> +				size = bh->b_size - first;
> +			} else {
> +				retval = dax_get_addr(bh, &addr, blkbits);
> +				if (retval < 0)
> +					break;
> +				if (buffer_unwritten(bh) || buffer_new(bh))
> +					dax_new_buf(addr, retval, first, pos,
> +									end);
> +				addr += first;
> +				size = retval - first;
> +			}
> +			max = min(pos + size, end);
> +		}
> +
> +		if (rw == WRITE)
> +			len = copy_from_iter(addr, max - pos, iter);
> +		else if (!hole)
> +			len = copy_to_iter(addr, max - pos, iter);
> +		else
> +			len = iov_iter_zero(max - pos, iter);
> +
> +		if (!len)
> +			break;
> +
> +		pos += len;
> +		addr += len;
> +	}
> +
> +	return (pos == start) ? retval : pos - start;
> +}
> +
> +/**
> + * dax_do_io - Perform I/O to a DAX file
> + * @rw: READ to read or WRITE to write
> + * @iocb: The control block for this I/O
> + * @inode: The file which the I/O is directed at
> + * @iter: The addresses to do I/O from or to
> + * @pos: The file offset where the I/O starts
> + * @get_block: The filesystem method used to translate file offsets to blocks
> + * @end_io: A filesystem callback for I/O completion
> + * @flags: See below
> + *
> + * This function uses the same locking scheme as do_blockdev_direct_IO:
> + * If @flags has DIO_LOCKING set, we assume that the i_mutex is held by the
> + * caller for writes.  For reads, we take and release the i_mutex ourselves.
> + * If DIO_LOCKING is not set, the filesystem takes care of its own locking.
> + * As with do_blockdev_direct_IO(), we increment i_dio_count while the I/O
> + * is in progress.
> + */
> +ssize_t dax_do_io(int rw, struct kiocb *iocb, struct inode *inode,
> +			struct iov_iter *iter, loff_t pos,
> +			get_block_t get_block, dio_iodone_t end_io, int flags)
> +{
> +	struct buffer_head bh;
> +	ssize_t retval = -EINVAL;
> +	loff_t end = pos + iov_iter_count(iter);
> +
> +	memset(&bh, 0, sizeof(bh));
> +
> +	if ((flags & DIO_LOCKING) && (rw == READ)) {
> +		struct address_space *mapping = inode->i_mapping;
> +		mutex_lock(&inode->i_mutex);
> +		retval = filemap_write_and_wait_range(mapping, pos, end - 1);
> +		if (retval) {
> +			mutex_unlock(&inode->i_mutex);
> +			goto out;
> +		}
> +	}
> +
> +	/* Protects against truncate */
> +	atomic_inc(&inode->i_dio_count);
> +
> +	retval = dax_io(rw, inode, iter, pos, end, get_block, &bh);
> +
> +	if ((flags & DIO_LOCKING) && (rw == READ))
> +		mutex_unlock(&inode->i_mutex);
> +
> +	if ((retval > 0) && end_io)
> +		end_io(iocb, pos, retval, bh.b_private);
> +
> +	inode_dio_done(inode);
> + out:
> +	return retval;
> +}
> +EXPORT_SYMBOL_GPL(dax_do_io);
> diff --git a/fs/ext2/file.c b/fs/ext2/file.c
> index 7c87b22..a247123 100644
> --- a/fs/ext2/file.c
> +++ b/fs/ext2/file.c
> @@ -81,8 +81,10 @@ const struct file_operations ext2_file_operations = {
>  #ifdef CONFIG_EXT2_FS_XIP
>  const struct file_operations ext2_xip_file_operations = {
>  	.llseek		= generic_file_llseek,
> -	.read		= xip_file_read,
> -	.write		= xip_file_write,
> +	.read		= new_sync_read,
> +	.write		= new_sync_write,
> +	.read_iter	= generic_file_read_iter,
> +	.write_iter	= generic_file_write_iter,
>  	.unlocked_ioctl = ext2_ioctl,
>  #ifdef CONFIG_COMPAT
>  	.compat_ioctl	= ext2_compat_ioctl,
> diff --git a/fs/ext2/inode.c b/fs/ext2/inode.c
> index 0cb0448..3ccd5fd 100644
> --- a/fs/ext2/inode.c
> +++ b/fs/ext2/inode.c
> @@ -859,7 +859,12 @@ ext2_direct_IO(int rw, struct kiocb *iocb, struct iov_iter *iter,
>  	size_t count = iov_iter_count(iter);
>  	ssize_t ret;
>  
> -	ret = blockdev_direct_IO(rw, iocb, inode, iter, offset, ext2_get_block);
> +	if (IS_DAX(inode))
> +		ret = dax_do_io(rw, iocb, inode, iter, offset, ext2_get_block,
> +				NULL, DIO_LOCKING);
> +	else
> +		ret = blockdev_direct_IO(rw, iocb, inode, iter, offset,
> +					 ext2_get_block);
>  	if (ret < 0 && (rw & WRITE))
>  		ext2_write_failed(mapping, offset + count);
>  	return ret;
> @@ -888,6 +893,7 @@ 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,
>  };
>  
>  const struct address_space_operations ext2_nobh_aops = {
> diff --git a/include/linux/fs.h b/include/linux/fs.h
> index e99e5c4..45839e8 100644
> --- a/include/linux/fs.h
> +++ b/include/linux/fs.h
> @@ -2490,17 +2490,22 @@ extern int generic_file_open(struct inode * inode, struct file * filp);
>  extern int nonseekable_open(struct inode * inode, struct file * filp);
>  
>  #ifdef CONFIG_FS_XIP
> -extern ssize_t xip_file_read(struct file *filp, char __user *buf, size_t len,
> -			     loff_t *ppos);
>  extern int xip_file_mmap(struct file * file, struct vm_area_struct * vma);
> -extern ssize_t xip_file_write(struct file *filp, const char __user *buf,
> -			      size_t len, loff_t *ppos);
>  extern int xip_truncate_page(struct address_space *mapping, loff_t from);
> +ssize_t dax_do_io(int rw, struct kiocb *, struct inode *, struct iov_iter *,
> +		loff_t, get_block_t, dio_iodone_t, int flags);
>  #else
>  static inline int xip_truncate_page(struct address_space *mapping, loff_t from)
>  {
>  	return 0;
>  }
> +
> +static inline ssize_t dax_do_io(int rw, struct kiocb *iocb,
> +		struct inode *inode, struct iov_iter *iter, loff_t pos,
> +		get_block_t get_block, dio_iodone_t end_io, int flags)
> +{
> +	return -ENOTTY;
> +}
>  #endif
>  
>  #ifdef CONFIG_BLOCK
> @@ -2657,6 +2662,11 @@ extern int generic_show_options(struct seq_file *m, struct dentry *root);
>  extern void save_mount_options(struct super_block *sb, char *options);
>  extern void replace_mount_options(struct super_block *sb, char *options);
>  
> +static inline bool io_is_direct(struct file *filp)
> +{
> +	return (filp->f_flags & O_DIRECT) || IS_DAX(file_inode(filp));
> +}
> +
>  static inline ino_t parent_ino(struct dentry *dentry)
>  {
>  	ino_t res;
> diff --git a/mm/filemap.c b/mm/filemap.c
> index fec4db9..e69b586 100644
> --- a/mm/filemap.c
> +++ b/mm/filemap.c
> @@ -1690,8 +1690,7 @@ generic_file_read_iter(struct kiocb *iocb, struct iov_iter *iter)
>  	loff_t *ppos = &iocb->ki_pos;
>  	loff_t pos = *ppos;
>  
> -	/* coalesce the iovecs and go direct-to-BIO for O_DIRECT */
> -	if (file->f_flags & O_DIRECT) {
> +	if (io_is_direct(file)) {
>  		struct address_space *mapping = file->f_mapping;
>  		struct inode *inode = mapping->host;
>  		size_t count = iov_iter_count(iter);
> @@ -2581,8 +2580,7 @@ ssize_t __generic_file_write_iter(struct kiocb *iocb, struct iov_iter *from)
>  	if (err)
>  		goto out;
>  
> -	/* coalesce the iovecs and go direct-to-BIO for O_DIRECT */
> -	if (unlikely(file->f_flags & O_DIRECT)) {
> +	if (io_is_direct(file)) {
>  		loff_t endbyte;
>  
>  		written = generic_file_direct_write(iocb, from, pos);
> diff --git a/mm/filemap_xip.c b/mm/filemap_xip.c
> index c8d23e9..f7c37a1 100644
> --- a/mm/filemap_xip.c
> +++ b/mm/filemap_xip.c
> @@ -42,119 +42,6 @@ static struct page *xip_sparse_page(void)
>  }
>  
>  /*
> - * This is a file read routine for execute in place files, and uses
> - * the mapping->a_ops->get_xip_mem() function for the actual low-level
> - * stuff.
> - *
> - * Note the struct file* is not used at all.  It may be NULL.
> - */
> -static ssize_t
> -do_xip_mapping_read(struct address_space *mapping,
> -		    struct file_ra_state *_ra,
> -		    struct file *filp,
> -		    char __user *buf,
> -		    size_t len,
> -		    loff_t *ppos)
> -{
> -	struct inode *inode = mapping->host;
> -	pgoff_t index, end_index;
> -	unsigned long offset;
> -	loff_t isize, pos;
> -	size_t copied = 0, error = 0;
> -
> -	BUG_ON(!mapping->a_ops->get_xip_mem);
> -
> -	pos = *ppos;
> -	index = pos >> PAGE_CACHE_SHIFT;
> -	offset = pos & ~PAGE_CACHE_MASK;
> -
> -	isize = i_size_read(inode);
> -	if (!isize)
> -		goto out;
> -
> -	end_index = (isize - 1) >> PAGE_CACHE_SHIFT;
> -	do {
> -		unsigned long nr, left;
> -		void *xip_mem;
> -		unsigned long xip_pfn;
> -		int zero = 0;
> -
> -		/* nr is the maximum number of bytes to copy from this page */
> -		nr = PAGE_CACHE_SIZE;
> -		if (index >= end_index) {
> -			if (index > end_index)
> -				goto out;
> -			nr = ((isize - 1) & ~PAGE_CACHE_MASK) + 1;
> -			if (nr <= offset) {
> -				goto out;
> -			}
> -		}
> -		nr = nr - offset;
> -		if (nr > len - copied)
> -			nr = len - copied;
> -
> -		error = mapping->a_ops->get_xip_mem(mapping, index, 0,
> -							&xip_mem, &xip_pfn);
> -		if (unlikely(error)) {
> -			if (error == -ENODATA) {
> -				/* sparse */
> -				zero = 1;
> -			} else
> -				goto out;
> -		}
> -
> -		/* If users can be writing to this page using arbitrary
> -		 * virtual addresses, take care about potential aliasing
> -		 * before reading the page on the kernel side.
> -		 */
> -		if (mapping_writably_mapped(mapping))
> -			/* address based flush */ ;
> -
> -		/*
> -		 * Ok, we have the mem, so now we can copy it to user space...
> -		 *
> -		 * The actor routine returns how many bytes were actually used..
> -		 * NOTE! This may not be the same as how much of a user buffer
> -		 * we filled up (we may be padding etc), so we can only update
> -		 * "pos" here (the actor routine has to update the user buffer
> -		 * pointers and the remaining count).
> -		 */
> -		if (!zero)
> -			left = __copy_to_user(buf+copied, xip_mem+offset, nr);
> -		else
> -			left = __clear_user(buf + copied, nr);
> -
> -		if (left) {
> -			error = -EFAULT;
> -			goto out;
> -		}
> -
> -		copied += (nr - left);
> -		offset += (nr - left);
> -		index += offset >> PAGE_CACHE_SHIFT;
> -		offset &= ~PAGE_CACHE_MASK;
> -	} while (copied < len);
> -
> -out:
> -	*ppos = pos + copied;
> -	if (filp)
> -		file_accessed(filp);
> -
> -	return (copied ? copied : error);
> -}
> -
> -ssize_t
> -xip_file_read(struct file *filp, char __user *buf, size_t len, loff_t *ppos)
> -{
> -	if (!access_ok(VERIFY_WRITE, buf, len))
> -		return -EFAULT;
> -
> -	return do_xip_mapping_read(filp->f_mapping, &filp->f_ra, filp,
> -			    buf, len, ppos);
> -}
> -EXPORT_SYMBOL_GPL(xip_file_read);
> -
> -/*
>   * __xip_unmap is invoked from xip_unmap and
>   * xip_write
>   *
> @@ -340,127 +227,6 @@ int xip_file_mmap(struct file * file, struct vm_area_struct * vma)
>  }
>  EXPORT_SYMBOL_GPL(xip_file_mmap);
>  
> -static ssize_t
> -__xip_file_write(struct file *filp, const char __user *buf,
> -		  size_t count, loff_t pos, loff_t *ppos)
> -{
> -	struct address_space * mapping = filp->f_mapping;
> -	const struct address_space_operations *a_ops = mapping->a_ops;
> -	struct inode 	*inode = mapping->host;
> -	long		status = 0;
> -	size_t		bytes;
> -	ssize_t		written = 0;
> -
> -	BUG_ON(!mapping->a_ops->get_xip_mem);
> -
> -	do {
> -		unsigned long index;
> -		unsigned long offset;
> -		size_t copied;
> -		void *xip_mem;
> -		unsigned long xip_pfn;
> -
> -		offset = (pos & (PAGE_CACHE_SIZE -1)); /* Within page */
> -		index = pos >> PAGE_CACHE_SHIFT;
> -		bytes = PAGE_CACHE_SIZE - offset;
> -		if (bytes > count)
> -			bytes = count;
> -
> -		status = a_ops->get_xip_mem(mapping, index, 0,
> -						&xip_mem, &xip_pfn);
> -		if (status == -ENODATA) {
> -			/* we allocate a new page unmap it */
> -			mutex_lock(&xip_sparse_mutex);
> -			status = a_ops->get_xip_mem(mapping, index, 1,
> -							&xip_mem, &xip_pfn);
> -			mutex_unlock(&xip_sparse_mutex);
> -			if (!status)
> -				/* unmap page at pgoff from all other vmas */
> -				__xip_unmap(mapping, index);
> -		}
> -
> -		if (status)
> -			break;
> -
> -		copied = bytes -
> -			__copy_from_user_nocache(xip_mem + offset, buf, bytes);
> -
> -		if (likely(copied > 0)) {
> -			status = copied;
> -
> -			if (status >= 0) {
> -				written += status;
> -				count -= status;
> -				pos += status;
> -				buf += status;
> -			}
> -		}
> -		if (unlikely(copied != bytes))
> -			if (status >= 0)
> -				status = -EFAULT;
> -		if (status < 0)
> -			break;
> -	} while (count);
> -	*ppos = pos;
> -	/*
> -	 * No need to use i_size_read() here, the i_size
> -	 * cannot change under us because we hold i_mutex.
> -	 */
> -	if (pos > inode->i_size) {
> -		i_size_write(inode, pos);
> -		mark_inode_dirty(inode);
> -	}
> -
> -	return written ? written : status;
> -}
> -
> -ssize_t
> -xip_file_write(struct file *filp, const char __user *buf, size_t len,
> -	       loff_t *ppos)
> -{
> -	struct address_space *mapping = filp->f_mapping;
> -	struct inode *inode = mapping->host;
> -	size_t count;
> -	loff_t pos;
> -	ssize_t ret;
> -
> -	mutex_lock(&inode->i_mutex);
> -
> -	if (!access_ok(VERIFY_READ, buf, len)) {
> -		ret=-EFAULT;
> -		goto out_up;
> -	}
> -
> -	pos = *ppos;
> -	count = len;
> -
> -	/* We can write back this queue in page reclaim */
> -	current->backing_dev_info = mapping->backing_dev_info;
> -
> -	ret = generic_write_checks(filp, &pos, &count, S_ISBLK(inode->i_mode));
> -	if (ret)
> -		goto out_backing;
> -	if (count == 0)
> -		goto out_backing;
> -
> -	ret = file_remove_suid(filp);
> -	if (ret)
> -		goto out_backing;
> -
> -	ret = file_update_time(filp);
> -	if (ret)
> -		goto out_backing;
> -
> -	ret = __xip_file_write (filp, buf, count, pos, ppos);
> -
> - out_backing:
> -	current->backing_dev_info = NULL;
> - out_up:
> -	mutex_unlock(&inode->i_mutex);
> -	return ret;
> -}
> -EXPORT_SYMBOL_GPL(xip_file_write);
> -
>  /*
>   * truncate a page used for execute in place
>   * functionality is analog to block_truncate_page but does use get_xip_mem
> -- 
> 2.1.0
> 
> --
> To unsubscribe, send a message with 'unsubscribe linux-mm' in
> the body to majordomo@...ck.org.  For more info on Linux MM,
> see: http://www.linux-mm.org/ .
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> 
> 

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com
Key fingerprint: 2A0B 4ED9 15F2 D3FA 45F5  B162 1728 0A97 8118 6ACF
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