Re: [PATCH 4/8] btrfs: Switch to iomap_dio_rw() for dio

[Goldwyn Rodrigues <rgoldwyn@suse.de>]

On  6:42 21/12, Christoph Hellwig wrote:
> So Ilooked into the "unlocked" direct I/O case, and I think the current
> code using dio_sem is really sketchy.  What btrfs really needs to do is
> take i_rwsem shared by default for direct writes, and only upgrade to
> the exclusive lock when needed, similar to xfs and the WIP ext4 code.

Sketchy in what sense? I am not trying to second-guess, but I want to
know where it could fail. I would want it to be simpler as well, but if
we can perform direct writes without locking, why should we introduce
locks.

> 
> While looking for that I also noticed two other things:
> 
>  - check_direct_IO looks pretty bogus
>  - btrfs_direct_IO really should be split and folded into the two
>    callers

Thanks for the cleanups. I will incorporate these.

> 
> Untested patches attached.  The first should probably go into a prep
> patch, and the second could be folded into this one.

> From bc285e440a50140beb456f11e545a049bdf51ec1 Mon Sep 17 00:00:00 2001
> From: Christoph Hellwig <hch@lst.de>
> Date: Sat, 21 Dec 2019 15:17:26 +0100
> Subject: btrfs: remove direct I/O aligment checks
> 
> The direct I/O code itself already checks for the proper sector
> size alignment, so remove the duplicate checks.  The remainder of
> check_direct_IO is not ony needed for reads and can be moved to
> file.c and outside of i_rwsem.
> 
> Signed-off-by: Christoph Hellwig <hch@lst.de>
> ---
>  fs/btrfs/file.c  | 34 +++++++++++++++++++++++++++-------
>  fs/btrfs/inode.c | 37 -------------------------------------
>  2 files changed, 27 insertions(+), 44 deletions(-)
> 
> diff --git a/fs/btrfs/file.c b/fs/btrfs/file.c
> index a6d41d7bf362..0522f6d45a98 100644
> --- a/fs/btrfs/file.c
> +++ b/fs/btrfs/file.c
> @@ -3444,21 +3444,41 @@ static int btrfs_file_open(struct inode *inode, struct file *filp)
>  	return generic_file_open(inode, filp);
>  }
>  
> -static ssize_t btrfs_file_read_iter(struct kiocb *iocb, struct iov_iter *to)
> +/*
> + * If there are duplicate iov_base's in this iovec, fall back to buffered I/O
> + * to avoid checksum errors.
> + */
> +static bool btrfs_direct_read_ok(struct kiocb *iocb, struct iov_iter *iter)
>  {
> -	ssize_t ret = 0;
> +	int seg, i;
>  
> -	if (iocb->ki_flags & IOCB_DIRECT) {
> +	if (!iter_is_iovec(iter))
> +		return true;
> +
> +	for (seg = 0; seg < iter->nr_segs; seg++) {
> +		for (i = seg + 1; i < iter->nr_segs; i++) {
> +			if (iter->iov[seg].iov_base == iter->iov[i].iov_base)
> +				return false;
> +		}
> +	}
> +
> +	return true;
> +}
> +
> +
> +static ssize_t btrfs_file_read_iter(struct kiocb *iocb, struct iov_iter *to)
> +{
> +	if ((iocb->ki_flags & IOCB_DIRECT) && btrfs_direct_read_ok(iocb, to)) {
>  		struct inode *inode = file_inode(iocb->ki_filp);
> +		ssize_t ret;
>  
>  		inode_lock_shared(inode);
>  		ret = btrfs_direct_IO(iocb, to);
>  		inode_unlock_shared(inode);
> -		if (ret < 0)
> -			return ret;
> -	}
>  
> -	return generic_file_buffered_read(iocb, to, ret);
> +		return ret;
> +	}
> +	return generic_file_buffered_read(iocb, to, 0);
>  }
>  
>  const struct file_operations btrfs_file_operations = {
> diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c
> index 824f318cee5e..18d153a62655 100644
> --- a/fs/btrfs/inode.c
> +++ b/fs/btrfs/inode.c
> @@ -8581,39 +8581,6 @@ static blk_qc_t btrfs_submit_direct(struct bio *dio_bio, struct file *file,
>  	return BLK_QC_T_NONE;
>  }
>  
> -static ssize_t check_direct_IO(struct btrfs_fs_info *fs_info,
> -			       const struct iov_iter *iter, loff_t offset)
> -{
> -	int seg;
> -	int i;
> -	unsigned int blocksize_mask = fs_info->sectorsize - 1;
> -	ssize_t retval = -EINVAL;
> -
> -	if (offset & blocksize_mask)
> -		goto out;
> -
> -	if (iov_iter_alignment(iter) & blocksize_mask)
> -		goto out;
> -
> -	/* If this is a write we don't need to check anymore */
> -	if (iov_iter_rw(iter) != READ || !iter_is_iovec(iter))
> -		return 0;
> -	/*
> -	 * Check to make sure we don't have duplicate iov_base's in this
> -	 * iovec, if so return EINVAL, otherwise we'll get csum errors
> -	 * when reading back.
> -	 */
> -	for (seg = 0; seg < iter->nr_segs; seg++) {
> -		for (i = seg + 1; i < iter->nr_segs; i++) {
> -			if (iter->iov[seg].iov_base == iter->iov[i].iov_base)
> -				goto out;
> -		}
> -	}
> -	retval = 0;
> -out:
> -	return retval;
> -}
> -
>  static const struct iomap_ops btrfs_dio_iomap_ops = {
>  	.iomap_begin            = btrfs_dio_iomap_begin,
>  	.iomap_end		= btrfs_dio_iomap_end,
> @@ -8635,7 +8602,6 @@ ssize_t btrfs_direct_IO(struct kiocb *iocb, struct iov_iter *iter)
>  {
>  	struct file *file = iocb->ki_filp;
>  	struct inode *inode = file->f_mapping->host;
> -	struct btrfs_fs_info *fs_info = btrfs_sb(inode->i_sb);
>  	struct extent_changeset *data_reserved = NULL;
>  	loff_t offset = iocb->ki_pos;
>  	size_t count = 0;
> @@ -8644,9 +8610,6 @@ ssize_t btrfs_direct_IO(struct kiocb *iocb, struct iov_iter *iter)
>  
>  	lockdep_assert_held(&inode->i_rwsem);
>  
> -	if (check_direct_IO(fs_info, iter, offset))
> -		return 0;
> -
>  	count = iov_iter_count(iter);
>  	if (iov_iter_rw(iter) == WRITE) {
>  		/*
> -- 
> 2.24.0
> 

> From 7194fa1986a48af46d2b01457865066cdbd14e35 Mon Sep 17 00:00:00 2001
> From: Christoph Hellwig <hch@lst.de>
> Date: Sat, 21 Dec 2019 15:23:41 +0100
> Subject: btrfs: split btrfs_direct_IO
> 
> The read and write versions don't have anything in common except
> for the call to iomap_dio_rw.  So split this function, and merge
> each half into its only caller.
> 
> Signed-off-by: Christoph Hellwig <hch@lst.de>
> ---
>  fs/btrfs/ctree.h |  4 ++-
>  fs/btrfs/file.c  | 44 +++++++++++++++++++++++++----
>  fs/btrfs/inode.c | 72 ++++--------------------------------------------
>  3 files changed, 48 insertions(+), 72 deletions(-)
> 
> diff --git a/fs/btrfs/ctree.h b/fs/btrfs/ctree.h
> index 8faa069b0a73..fccbbfebdf88 100644
> --- a/fs/btrfs/ctree.h
> +++ b/fs/btrfs/ctree.h
> @@ -28,6 +28,7 @@
>  #include <linux/dynamic_debug.h>
>  #include <linux/refcount.h>
>  #include <linux/crc32c.h>
> +#include <linux/iomap.h>
>  #include "extent-io-tree.h"
>  #include "extent_io.h"
>  #include "extent_map.h"
> @@ -2904,7 +2905,8 @@ int btrfs_writepage_cow_fixup(struct page *page, u64 start, u64 end);
>  void btrfs_writepage_endio_finish_ordered(struct page *page, u64 start,
>  					  u64 end, int uptodate);
>  extern const struct dentry_operations btrfs_dentry_operations;
> -ssize_t btrfs_direct_IO(struct kiocb *iocb, struct iov_iter *iter);
> +const struct iomap_ops btrfs_dio_iomap_ops;
> +const struct iomap_dio_ops btrfs_dio_ops;
>  
>  /* ioctl.c */
>  long btrfs_ioctl(struct file *file, unsigned int cmd, unsigned long arg);
> diff --git a/fs/btrfs/file.c b/fs/btrfs/file.c
> index 0522f6d45a98..ed0b2e015d8d 100644
> --- a/fs/btrfs/file.c
> +++ b/fs/btrfs/file.c
> @@ -1822,17 +1822,50 @@ static noinline ssize_t btrfs_buffered_write(struct kiocb *iocb,
>  	return num_written ? num_written : ret;
>  }
>  
> -static ssize_t __btrfs_direct_write(struct kiocb *iocb, struct iov_iter *from)
> +static ssize_t btrfs_direct_write(struct kiocb *iocb, struct iov_iter *from)
>  {
>  	struct file *file = iocb->ki_filp;
>  	struct inode *inode = file_inode(file);
> -	loff_t pos;
> +	size_t count = iov_iter_count(from);
> +	struct extent_changeset *data_reserved = NULL;
> +	loff_t pos = iocb->ki_pos;
>  	ssize_t written;
>  	ssize_t written_buffered;
>  	loff_t endbyte;
> +	bool relock = false;
>  	int err;
>  
> -	written = btrfs_direct_IO(iocb, from);
> +	/*
> +	 * If the write DIO is beyond the EOF, we need update the isize, but
> +	 * it is protected by i_mutex. So we can not unlock the i_mutex in
> +	 * this case.
> +	 */
> +	if (pos + count <= inode->i_size) {
> +		inode_unlock(inode);
> +		relock = true;
> +	} else {
> +		if (iocb->ki_flags & IOCB_NOWAIT)
> +			return -EAGAIN;
> +	}
> +
> +	err = btrfs_delalloc_reserve_space(inode, &data_reserved, pos, count);
> +	if (err) {
> +		if (relock)
> +			inode_lock(inode);
> +		return err;
> +	}
> +
> +	down_read(&BTRFS_I(inode)->dio_sem);
> +	written = iomap_dio_rw(iocb, from, &btrfs_dio_iomap_ops, &btrfs_dio_ops,
> +			is_sync_kiocb(iocb));
> +	up_read(&BTRFS_I(inode)->dio_sem);
> +	if (written >= 0 && (size_t)written < count)
> +		btrfs_delalloc_release_space(inode, data_reserved,
> +				pos, count - (size_t)written, true);
> +	btrfs_delalloc_release_extents(BTRFS_I(inode), count);
> +	if (relock)
> +		inode_lock(inode);
> +	extent_changeset_free(data_reserved);
>  
>  	if (written < 0 || !iov_iter_count(from))
>  		return written;
> @@ -1975,7 +2008,7 @@ static ssize_t btrfs_file_write_iter(struct kiocb *iocb,
>  		atomic_inc(&BTRFS_I(inode)->sync_writers);
>  
>  	if (iocb->ki_flags & IOCB_DIRECT) {
> -		num_written = __btrfs_direct_write(iocb, from);
> +		num_written = btrfs_direct_write(iocb, from);
>  	} else {
>  		num_written = btrfs_buffered_write(iocb, from);
>  		if (num_written > 0)
> @@ -3473,7 +3506,8 @@ static ssize_t btrfs_file_read_iter(struct kiocb *iocb, struct iov_iter *to)
>  		ssize_t ret;
>  
>  		inode_lock_shared(inode);
> -		ret = btrfs_direct_IO(iocb, to);
> +		ret = iomap_dio_rw(iocb, to, &btrfs_dio_iomap_ops,
> +				   &btrfs_dio_ops, is_sync_kiocb(iocb));
>  		inode_unlock_shared(inode);
>  
>  		return ret;
> diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c
> index 18d153a62655..7b747270ec40 100644
> --- a/fs/btrfs/inode.c
> +++ b/fs/btrfs/inode.c
> @@ -29,7 +29,6 @@
>  #include <linux/iversion.h>
>  #include <linux/swap.h>
>  #include <linux/sched/mm.h>
> -#include <linux/iomap.h>
>  #include <asm/unaligned.h>
>  #include "misc.h"
>  #include "ctree.h"
> @@ -7856,6 +7855,11 @@ static int btrfs_dio_iomap_end(struct inode *inode, loff_t pos, loff_t length,
>  	return 0;
>  }
>  
> +const struct iomap_ops btrfs_dio_iomap_ops = {
> +	.iomap_begin            = btrfs_dio_iomap_begin,
> +	.iomap_end		= btrfs_dio_iomap_end,
> +};
> +
>  static inline blk_status_t submit_dio_repair_bio(struct inode *inode,
>  						 struct bio *bio,
>  						 int mirror_num)
> @@ -8581,74 +8585,10 @@ static blk_qc_t btrfs_submit_direct(struct bio *dio_bio, struct file *file,
>  	return BLK_QC_T_NONE;
>  }
>  
> -static const struct iomap_ops btrfs_dio_iomap_ops = {
> -	.iomap_begin            = btrfs_dio_iomap_begin,
> -	.iomap_end		= btrfs_dio_iomap_end,
> -};
> -
> -static const struct iomap_dio_ops btrfs_dops = {
> +const struct iomap_dio_ops btrfs_dio_ops = {
>  	.submit_io		= btrfs_submit_direct,
>  };
>  
> -
> -/*
> - * btrfs_direct_IO - perform direct I/O
> - * inode->i_rwsem must be locked before calling this function, shared or exclusive.
> - * @iocb - kernel iocb
> - * @iter - iter to/from data is copied
> - */
> -
> -ssize_t btrfs_direct_IO(struct kiocb *iocb, struct iov_iter *iter)
> -{
> -	struct file *file = iocb->ki_filp;
> -	struct inode *inode = file->f_mapping->host;
> -	struct extent_changeset *data_reserved = NULL;
> -	loff_t offset = iocb->ki_pos;
> -	size_t count = 0;
> -	bool relock = false;
> -	ssize_t ret;
> -
> -	lockdep_assert_held(&inode->i_rwsem);
> -
> -	count = iov_iter_count(iter);
> -	if (iov_iter_rw(iter) == WRITE) {
> -		/*
> -		 * If the write DIO is beyond the EOF, we need update
> -		 * the isize, but it is protected by i_mutex. So we can
> -		 * not unlock the i_mutex at this case.
> -		 */
> -		if (offset + count <= inode->i_size) {
> -			inode_unlock(inode);
> -			relock = true;
> -		} else if (iocb->ki_flags & IOCB_NOWAIT) {
> -			ret = -EAGAIN;
> -			goto out;
> -		}
> -		ret = btrfs_delalloc_reserve_space(inode, &data_reserved,
> -						   offset, count);
> -		if (ret)
> -			goto out;
> -
> -		down_read(&BTRFS_I(inode)->dio_sem);
> -	}
> -
> -	ret = iomap_dio_rw(iocb, iter, &btrfs_dio_iomap_ops, &btrfs_dops,
> -			is_sync_kiocb(iocb));
> -
> -	if (iov_iter_rw(iter) == WRITE) {
> -		up_read(&BTRFS_I(inode)->dio_sem);
> -		if (ret >= 0 && (size_t)ret < count)
> -			btrfs_delalloc_release_space(inode, data_reserved,
> -					offset, count - (size_t)ret, true);
> -		btrfs_delalloc_release_extents(BTRFS_I(inode), count);
> -	}
> -out:
> -	if (relock)
> -		inode_lock(inode);
> -	extent_changeset_free(data_reserved);
> -	return ret;
> -}
> -
>  #define BTRFS_FIEMAP_FLAGS	(FIEMAP_FLAG_SYNC)
>  
>  static int btrfs_fiemap(struct inode *inode, struct fiemap_extent_info *fieinfo,
> -- 
> 2.24.0
> 


-- 
Goldwyn