diff mbox series

[01/13] iomap: Use a IOMAP_COW/srcmap for a read-modify-write I/O

Message ID 20190802220048.16142-2-rgoldwyn@suse.de (mailing list archive)
State New, archived
Headers show
Series Btrfs iomap | expand

Commit Message

Goldwyn Rodrigues Aug. 2, 2019, 10 p.m. UTC 
From: Goldwyn Rodrigues <rgoldwyn@suse.com>

Introduces a new type IOMAP_COW, which means the data at offset
must be read from a srcmap and copied before performing the
write on the offset.

The srcmap is used to identify where the read is to be performed
from. This is passed to iomap->begin() of the respective
filesystem, which is supposed to put in the details for
reading before performing the copy for CoW.

Signed-off-by: Goldwyn Rodrigues <rgoldwyn@suse.com>
---
 fs/dax.c               |  8 +++++---
 fs/ext2/inode.c        |  2 +-
 fs/ext4/inode.c        |  2 +-
 fs/gfs2/bmap.c         |  3 ++-
 fs/iomap/apply.c       |  5 +++--
 fs/iomap/buffered-io.c | 14 +++++++-------
 fs/iomap/direct-io.c   |  2 +-
 fs/iomap/fiemap.c      |  4 ++--
 fs/iomap/seek.c        |  4 ++--
 fs/iomap/swapfile.c    |  3 ++-
 fs/xfs/xfs_iomap.c     |  9 ++++++---
 include/linux/iomap.h  |  6 ++++--
 12 files changed, 36 insertions(+), 26 deletions(-)

Comments

Darrick J. Wong Aug. 3, 2019, 12:39 a.m. UTC | #1 
On Fri, Aug 02, 2019 at 05:00:36PM -0500, Goldwyn Rodrigues wrote:
> From: Goldwyn Rodrigues <rgoldwyn@suse.com>
> 
> Introduces a new type IOMAP_COW, which means the data at offset
> must be read from a srcmap and copied before performing the
> write on the offset.
> 
> The srcmap is used to identify where the read is to be performed
> from. This is passed to iomap->begin() of the respective
> filesystem, which is supposed to put in the details for
> reading before performing the copy for CoW.
> 
> Signed-off-by: Goldwyn Rodrigues <rgoldwyn@suse.com>
> ---
>  fs/dax.c               |  8 +++++---
>  fs/ext2/inode.c        |  2 +-
>  fs/ext4/inode.c        |  2 +-
>  fs/gfs2/bmap.c         |  3 ++-
>  fs/iomap/apply.c       |  5 +++--
>  fs/iomap/buffered-io.c | 14 +++++++-------
>  fs/iomap/direct-io.c   |  2 +-
>  fs/iomap/fiemap.c      |  4 ++--
>  fs/iomap/seek.c        |  4 ++--
>  fs/iomap/swapfile.c    |  3 ++-
>  fs/xfs/xfs_iomap.c     |  9 ++++++---
>  include/linux/iomap.h  |  6 ++++--
>  12 files changed, 36 insertions(+), 26 deletions(-)
> 
> diff --git a/fs/dax.c b/fs/dax.c
> index a237141d8787..b21d9a9cde2b 100644
> --- a/fs/dax.c
> +++ b/fs/dax.c
> @@ -1090,7 +1090,7 @@ EXPORT_SYMBOL_GPL(__dax_zero_page_range);
>  
>  static loff_t
>  dax_iomap_actor(struct inode *inode, loff_t pos, loff_t length, void *data,
> -		struct iomap *iomap)
> +		struct iomap *iomap, struct iomap *srcmap)
>  {
>  	struct block_device *bdev = iomap->bdev;
>  	struct dax_device *dax_dev = iomap->dax_dev;
> @@ -1248,6 +1248,7 @@ static vm_fault_t dax_iomap_pte_fault(struct vm_fault *vmf, pfn_t *pfnp,
>  	unsigned long vaddr = vmf->address;
>  	loff_t pos = (loff_t)vmf->pgoff << PAGE_SHIFT;
>  	struct iomap iomap = { 0 };
> +	struct iomap srcmap = { 0 };
>  	unsigned flags = IOMAP_FAULT;
>  	int error, major = 0;
>  	bool write = vmf->flags & FAULT_FLAG_WRITE;
> @@ -1292,7 +1293,7 @@ static vm_fault_t dax_iomap_pte_fault(struct vm_fault *vmf, pfn_t *pfnp,
>  	 * the file system block size to be equal the page size, which means
>  	 * that we never have to deal with more than a single extent here.
>  	 */
> -	error = ops->iomap_begin(inode, pos, PAGE_SIZE, flags, &iomap);
> +	error = ops->iomap_begin(inode, pos, PAGE_SIZE, flags, &iomap, &srcmap);
>  	if (iomap_errp)
>  		*iomap_errp = error;
>  	if (error) {
> @@ -1472,6 +1473,7 @@ static vm_fault_t dax_iomap_pmd_fault(struct vm_fault *vmf, pfn_t *pfnp,
>  	struct inode *inode = mapping->host;
>  	vm_fault_t result = VM_FAULT_FALLBACK;
>  	struct iomap iomap = { 0 };
> +	struct iomap srcmap = { 0 };
>  	pgoff_t max_pgoff;
>  	void *entry;
>  	loff_t pos;
> @@ -1546,7 +1548,7 @@ static vm_fault_t dax_iomap_pmd_fault(struct vm_fault *vmf, pfn_t *pfnp,
>  	 * to look up our filesystem block.
>  	 */
>  	pos = (loff_t)xas.xa_index << PAGE_SHIFT;
> -	error = ops->iomap_begin(inode, pos, PMD_SIZE, iomap_flags, &iomap);
> +	error = ops->iomap_begin(inode, pos, PMD_SIZE, iomap_flags, &iomap, &srcmap);

/me wonders aloud if he ought to add a helper function to standardize at
least some of validation of the iomap that gets returned from
->iomap_begin invocations...

>  	if (error)
>  		goto unlock_entry;
>  

<snip>

> diff --git a/fs/iomap/apply.c b/fs/iomap/apply.c
> index 54c02aecf3cd..6cdb362fff36 100644
> --- a/fs/iomap/apply.c
> +++ b/fs/iomap/apply.c
> @@ -24,6 +24,7 @@ iomap_apply(struct inode *inode, loff_t pos, loff_t length, unsigned flags,
>  		const struct iomap_ops *ops, void *data, iomap_actor_t actor)
>  {
>  	struct iomap iomap = { 0 };
> +	struct iomap srcmap = { 0 };
>  	loff_t written = 0, ret;
>  
>  	/*
> @@ -38,7 +39,7 @@ iomap_apply(struct inode *inode, loff_t pos, loff_t length, unsigned flags,
>  	 * expose transient stale data. If the reserve fails, we can safely
>  	 * back out at this point as there is nothing to undo.
>  	 */
> -	ret = ops->iomap_begin(inode, pos, length, flags, &iomap);
> +	ret = ops->iomap_begin(inode, pos, length, flags, &iomap, &srcmap);
>  	if (ret)
>  		return ret;
>  	if (WARN_ON(iomap.offset > pos))

...because I wonder if we ought to have a debugging assert here just in
case an ->iomap_begin returns IOMAP_COW in response to an IOMAP_WRITE
request?  Basic sanity checks to catch accidental API misuse, etc.

Eh we probably ought to have a CONFIG_IOMAP_DEBUG so that non-developers
don't necessarily have to pay the assert costs or something like that.

> @@ -58,7 +59,7 @@ iomap_apply(struct inode *inode, loff_t pos, loff_t length, unsigned flags,
>  	 * we can do the copy-in page by page without having to worry about
>  	 * failures exposing transient data.
>  	 */
> -	written = actor(inode, pos, length, data, &iomap);
> +	written = actor(inode, pos, length, data, &iomap, &srcmap);
>  
>  	/*
>  	 * Now the data has been copied, commit the range we've copied.  This
> diff --git a/fs/iomap/buffered-io.c b/fs/iomap/buffered-io.c
> index e25901ae3ff4..f27756c0b31c 100644
> --- a/fs/iomap/buffered-io.c
> +++ b/fs/iomap/buffered-io.c
> @@ -205,7 +205,7 @@ iomap_read_inline_data(struct inode *inode, struct page *page,
>  
>  static loff_t
>  iomap_readpage_actor(struct inode *inode, loff_t pos, loff_t length, void *data,
> -		struct iomap *iomap)
> +		struct iomap *iomap, struct iomap *srcmap)
>  {
>  	struct iomap_readpage_ctx *ctx = data;
>  	struct page *page = ctx->cur_page;
> @@ -351,7 +351,7 @@ iomap_next_page(struct inode *inode, struct list_head *pages, loff_t pos,
>  
>  static loff_t
>  iomap_readpages_actor(struct inode *inode, loff_t pos, loff_t length,
> -		void *data, struct iomap *iomap)
> +		void *data, struct iomap *iomap, struct iomap *srcmap)
>  {
>  	struct iomap_readpage_ctx *ctx = data;
>  	loff_t done, ret;
> @@ -371,7 +371,7 @@ iomap_readpages_actor(struct inode *inode, loff_t pos, loff_t length,
>  			ctx->cur_page_in_bio = false;
>  		}
>  		ret = iomap_readpage_actor(inode, pos + done, length - done,
> -				ctx, iomap);
> +				ctx, iomap, srcmap);
>  	}
>  
>  	return done;
> @@ -736,7 +736,7 @@ iomap_write_end(struct inode *inode, loff_t pos, unsigned len,
>  
>  static loff_t
>  iomap_write_actor(struct inode *inode, loff_t pos, loff_t length, void *data,
> -		struct iomap *iomap)
> +		struct iomap *iomap, struct iomap *srcmap)
>  {
>  	struct iov_iter *i = data;
>  	long status = 0;
> @@ -853,7 +853,7 @@ __iomap_read_page(struct inode *inode, loff_t offset)
>  
>  static loff_t
>  iomap_dirty_actor(struct inode *inode, loff_t pos, loff_t length, void *data,
> -		struct iomap *iomap)
> +		struct iomap *iomap, struct iomap *srcmap)
>  {
>  	long status = 0;
>  	ssize_t written = 0;
> @@ -942,7 +942,7 @@ static int iomap_dax_zero(loff_t pos, unsigned offset, unsigned bytes,
>  
>  static loff_t
>  iomap_zero_range_actor(struct inode *inode, loff_t pos, loff_t count,
> -		void *data, struct iomap *iomap)
> +		void *data, struct iomap *iomap, struct iomap *srcmap)
>  {
>  	bool *did_zero = data;
>  	loff_t written = 0;
> @@ -1011,7 +1011,7 @@ EXPORT_SYMBOL_GPL(iomap_truncate_page);
>  
>  static loff_t
>  iomap_page_mkwrite_actor(struct inode *inode, loff_t pos, loff_t length,
> -		void *data, struct iomap *iomap)
> +		void *data, struct iomap *iomap, struct iomap *srcmap)
>  {
>  	struct page *page = data;
>  	int ret;
> diff --git a/fs/iomap/direct-io.c b/fs/iomap/direct-io.c
> index 10517cea9682..5279029c7a3c 100644
> --- a/fs/iomap/direct-io.c
> +++ b/fs/iomap/direct-io.c
> @@ -362,7 +362,7 @@ iomap_dio_inline_actor(struct inode *inode, loff_t pos, loff_t length,
>  
>  static loff_t
>  iomap_dio_actor(struct inode *inode, loff_t pos, loff_t length,
> -		void *data, struct iomap *iomap)
> +		void *data, struct iomap *iomap, struct iomap *srcmap)
>  {
>  	struct iomap_dio *dio = data;
>  
> diff --git a/fs/iomap/fiemap.c b/fs/iomap/fiemap.c
> index f26fdd36e383..690ef2d7c6c8 100644
> --- a/fs/iomap/fiemap.c
> +++ b/fs/iomap/fiemap.c
> @@ -44,7 +44,7 @@ static int iomap_to_fiemap(struct fiemap_extent_info *fi,
>  
>  static loff_t
>  iomap_fiemap_actor(struct inode *inode, loff_t pos, loff_t length, void *data,
> -		struct iomap *iomap)
> +		struct iomap *iomap, struct iomap *srcmap)
>  {
>  	struct fiemap_ctx *ctx = data;
>  	loff_t ret = length;
> @@ -111,7 +111,7 @@ EXPORT_SYMBOL_GPL(iomap_fiemap);
>  
>  static loff_t
>  iomap_bmap_actor(struct inode *inode, loff_t pos, loff_t length,
> -		void *data, struct iomap *iomap)
> +		void *data, struct iomap *iomap, struct iomap *srcmap)
>  {
>  	sector_t *bno = data, addr;
>  
> diff --git a/fs/iomap/seek.c b/fs/iomap/seek.c
> index c04bad4b2b43..89f61d93c0bc 100644
> --- a/fs/iomap/seek.c
> +++ b/fs/iomap/seek.c
> @@ -119,7 +119,7 @@ page_cache_seek_hole_data(struct inode *inode, loff_t offset, loff_t length,
>  
>  static loff_t
>  iomap_seek_hole_actor(struct inode *inode, loff_t offset, loff_t length,
> -		      void *data, struct iomap *iomap)
> +		      void *data, struct iomap *iomap, struct iomap *srcmap)
>  {
>  	switch (iomap->type) {
>  	case IOMAP_UNWRITTEN:
> @@ -165,7 +165,7 @@ EXPORT_SYMBOL_GPL(iomap_seek_hole);
>  
>  static loff_t
>  iomap_seek_data_actor(struct inode *inode, loff_t offset, loff_t length,
> -		      void *data, struct iomap *iomap)
>  	switch (iomap->type) {
>  	case IOMAP_HOLE:
> diff --git a/fs/iomap/swapfile.c b/fs/iomap/swapfile.c
> index 152a230f668d..a648dbf6991e 100644
> --- a/fs/iomap/swapfile.c
> +++ b/fs/iomap/swapfile.c
> @@ -76,7 +76,8 @@ static int iomap_swapfile_add_extent(struct iomap_swapfile_info *isi)
>   * distinction between written and unwritten extents.
>   */
>  static loff_t iomap_swapfile_activate_actor(struct inode *inode, loff_t pos,
> -		loff_t count, void *data, struct iomap *iomap)
> +		loff_t count, void *data, struct iomap *iomap,
> +		struct iomap *srcmap)

The switch(iomap->type) probably ought to have a separate printk for the
IOMAP_COW case so that we don't go complaining about "unwritten" extents
in the swap file.

>  {
>  	struct iomap_swapfile_info *isi = data;
>  	int error;
> diff --git a/fs/xfs/xfs_iomap.c b/fs/xfs/xfs_iomap.c
> index 3a4310d7cb59..8321733c16c3 100644
> --- a/fs/xfs/xfs_iomap.c
> +++ b/fs/xfs/xfs_iomap.c
> @@ -922,7 +922,8 @@ xfs_file_iomap_begin(
>  	loff_t			offset,
>  	loff_t			length,
>  	unsigned		flags,
> -	struct iomap		*iomap)
> +	struct iomap		*iomap,
> +	struct iomap		*srcmap)
>  {
>  	struct xfs_inode	*ip = XFS_I(inode);
>  	struct xfs_mount	*mp = ip->i_mount;
> @@ -1145,7 +1146,8 @@ xfs_seek_iomap_begin(
>  	loff_t			offset,
>  	loff_t			length,
>  	unsigned		flags,
> -	struct iomap		*iomap)
> +	struct iomap		*iomap,
> +	struct iomap		*srcmap)
>  {
>  	struct xfs_inode	*ip = XFS_I(inode);
>  	struct xfs_mount	*mp = ip->i_mount;
> @@ -1231,7 +1233,8 @@ xfs_xattr_iomap_begin(
>  	loff_t			offset,
>  	loff_t			length,
>  	unsigned		flags,
> -	struct iomap		*iomap)
> +	struct iomap		*iomap,
> +	struct iomap		*srcmap)
>  {
>  	struct xfs_inode	*ip = XFS_I(inode);
>  	struct xfs_mount	*mp = ip->i_mount;

XFS part looks ok... I guess I'll get to Shiyuan's series next.

> diff --git a/include/linux/iomap.h b/include/linux/iomap.h
> index bc499ceae392..5b2055e8ca8a 100644
> --- a/include/linux/iomap.h
> +++ b/include/linux/iomap.h
> @@ -26,6 +26,7 @@ struct vm_fault;
>  #define IOMAP_MAPPED	0x03	/* blocks allocated at @addr */
>  #define IOMAP_UNWRITTEN	0x04	/* blocks allocated at @addr in unwritten state */
>  #define IOMAP_INLINE	0x05	/* data inline in the inode */
> +#define IOMAP_COW	0x06	/* copy data from srcmap before writing */

Hm, ok, at least the comment references that this is only for writes.
Looks good!

--D

>  
>  /*
>   * Flags for all iomap mappings:
> @@ -110,7 +111,8 @@ struct iomap_ops {
>  	 * The actual length is returned in iomap->length.
>  	 */
>  	int (*iomap_begin)(struct inode *inode, loff_t pos, loff_t length,
> -			unsigned flags, struct iomap *iomap);
> +			unsigned flags, struct iomap *iomap,
> +			struct iomap *srcmap);
>  
>  	/*
>  	 * Commit and/or unreserve space previous allocated using iomap_begin.
> @@ -126,7 +128,7 @@ struct iomap_ops {
>   * Main iomap iterator function.
>   */
>  typedef loff_t (*iomap_actor_t)(struct inode *inode, loff_t pos, loff_t len,
> -		void *data, struct iomap *iomap);
> +		void *data, struct iomap *iomap, struct iomap *srcmap);
>  
>  loff_t iomap_apply(struct inode *inode, loff_t pos, loff_t length,
>  		unsigned flags, const struct iomap_ops *ops, void *data,
> -- 
> 2.16.4
>
Dave Chinner Aug. 5, 2019, 12:06 a.m. UTC | #2 
On Fri, Aug 02, 2019 at 05:00:36PM -0500, Goldwyn Rodrigues wrote:
> From: Goldwyn Rodrigues <rgoldwyn@suse.com>
> 
> Introduces a new type IOMAP_COW, which means the data at offset
> must be read from a srcmap and copied before performing the
> write on the offset.
> 
> The srcmap is used to identify where the read is to be performed
> from. This is passed to iomap->begin() of the respective
> filesystem, which is supposed to put in the details for
> reading before performing the copy for CoW.
> 
> Signed-off-by: Goldwyn Rodrigues <rgoldwyn@suse.com>
> ---
>  fs/dax.c               |  8 +++++---
>  fs/ext2/inode.c        |  2 +-
>  fs/ext4/inode.c        |  2 +-
>  fs/gfs2/bmap.c         |  3 ++-
>  fs/iomap/apply.c       |  5 +++--
>  fs/iomap/buffered-io.c | 14 +++++++-------
>  fs/iomap/direct-io.c   |  2 +-
>  fs/iomap/fiemap.c      |  4 ++--
>  fs/iomap/seek.c        |  4 ++--
>  fs/iomap/swapfile.c    |  3 ++-
>  fs/xfs/xfs_iomap.c     |  9 ++++++---
>  include/linux/iomap.h  |  6 ++++--
>  12 files changed, 36 insertions(+), 26 deletions(-)
> 
> diff --git a/fs/dax.c b/fs/dax.c
> index a237141d8787..b21d9a9cde2b 100644
> --- a/fs/dax.c
> +++ b/fs/dax.c
> @@ -1090,7 +1090,7 @@ EXPORT_SYMBOL_GPL(__dax_zero_page_range);
>  
>  static loff_t
>  dax_iomap_actor(struct inode *inode, loff_t pos, loff_t length, void *data,
> -		struct iomap *iomap)
> +		struct iomap *iomap, struct iomap *srcmap)
>  {
>  	struct block_device *bdev = iomap->bdev;
>  	struct dax_device *dax_dev = iomap->dax_dev;
> @@ -1248,6 +1248,7 @@ static vm_fault_t dax_iomap_pte_fault(struct vm_fault *vmf, pfn_t *pfnp,
>  	unsigned long vaddr = vmf->address;
>  	loff_t pos = (loff_t)vmf->pgoff << PAGE_SHIFT;
>  	struct iomap iomap = { 0 };
> +	struct iomap srcmap = { 0 };

I'm not found of defining multiple iomaps everywhere and passing
them explicitly to every function that might need them.  Perhaps
something like:

	DEFINE_IOMAP(iomap);

#define IOMAP_BASE_MAP		0
#define IOMAP_SOURCE_MAP	1
#define IOMAP_MAX_MAPS		2

#define DEFINE_IOMAP(name)	\
	(struct iomap #name[IOMAP_MAX_MAPS] = {{0}})

#define IOMAP_B(name)		((name)[IOMAP_BASE_MAP])
#define IOMAP_S(name)		((name)[IOMAP_SOURCE_MAP])

And now we only have to pass a single iomap parameter to each
function as "struct iomap **iomap". This makes the code somewhat
simpler, and we only ever need to use IOMAP_S(iomap) when
IOMAP_B(iomap)->type == IOMAP_COW.

The other advantage of this is that if we even need new
functionality that requires 2 (or more) iomaps, we don't have to
change APIs again....

Cheers,

Dave.
diff mbox series

Patch

diff --git a/fs/dax.c b/fs/dax.c
index a237141d8787..b21d9a9cde2b 100644
--- a/fs/dax.c
+++ b/fs/dax.c
@@ -1090,7 +1090,7 @@  EXPORT_SYMBOL_GPL(__dax_zero_page_range);
 
 static loff_t
 dax_iomap_actor(struct inode *inode, loff_t pos, loff_t length, void *data,
-		struct iomap *iomap)
+		struct iomap *iomap, struct iomap *srcmap)
 {
 	struct block_device *bdev = iomap->bdev;
 	struct dax_device *dax_dev = iomap->dax_dev;
@@ -1248,6 +1248,7 @@  static vm_fault_t dax_iomap_pte_fault(struct vm_fault *vmf, pfn_t *pfnp,
 	unsigned long vaddr = vmf->address;
 	loff_t pos = (loff_t)vmf->pgoff << PAGE_SHIFT;
 	struct iomap iomap = { 0 };
+	struct iomap srcmap = { 0 };
 	unsigned flags = IOMAP_FAULT;
 	int error, major = 0;
 	bool write = vmf->flags & FAULT_FLAG_WRITE;
@@ -1292,7 +1293,7 @@  static vm_fault_t dax_iomap_pte_fault(struct vm_fault *vmf, pfn_t *pfnp,
 	 * the file system block size to be equal the page size, which means
 	 * that we never have to deal with more than a single extent here.
 	 */
-	error = ops->iomap_begin(inode, pos, PAGE_SIZE, flags, &iomap);
+	error = ops->iomap_begin(inode, pos, PAGE_SIZE, flags, &iomap, &srcmap);
 	if (iomap_errp)
 		*iomap_errp = error;
 	if (error) {
@@ -1472,6 +1473,7 @@  static vm_fault_t dax_iomap_pmd_fault(struct vm_fault *vmf, pfn_t *pfnp,
 	struct inode *inode = mapping->host;
 	vm_fault_t result = VM_FAULT_FALLBACK;
 	struct iomap iomap = { 0 };
+	struct iomap srcmap = { 0 };
 	pgoff_t max_pgoff;
 	void *entry;
 	loff_t pos;
@@ -1546,7 +1548,7 @@  static vm_fault_t dax_iomap_pmd_fault(struct vm_fault *vmf, pfn_t *pfnp,
 	 * to look up our filesystem block.
 	 */
 	pos = (loff_t)xas.xa_index << PAGE_SHIFT;
-	error = ops->iomap_begin(inode, pos, PMD_SIZE, iomap_flags, &iomap);
+	error = ops->iomap_begin(inode, pos, PMD_SIZE, iomap_flags, &iomap, &srcmap);
 	if (error)
 		goto unlock_entry;
 
diff --git a/fs/ext2/inode.c b/fs/ext2/inode.c
index 7004ce581a32..467c13ff6b40 100644
--- a/fs/ext2/inode.c
+++ b/fs/ext2/inode.c
@@ -801,7 +801,7 @@  int ext2_get_block(struct inode *inode, sector_t iblock,
 
 #ifdef CONFIG_FS_DAX
 static int ext2_iomap_begin(struct inode *inode, loff_t offset, loff_t length,
-		unsigned flags, struct iomap *iomap)
+		unsigned flags, struct iomap *iomap, struct iomap *srcmap)
 {
 	unsigned int blkbits = inode->i_blkbits;
 	unsigned long first_block = offset >> blkbits;
diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
index 420fe3deed39..918f94eff799 100644
--- a/fs/ext4/inode.c
+++ b/fs/ext4/inode.c
@@ -3453,7 +3453,7 @@  static bool ext4_inode_datasync_dirty(struct inode *inode)
 }
 
 static int ext4_iomap_begin(struct inode *inode, loff_t offset, loff_t length,
-			    unsigned flags, struct iomap *iomap)
+			    unsigned flags, struct iomap *iomap, struct iomap *srcmap)
 {
 	struct ext4_sb_info *sbi = EXT4_SB(inode->i_sb);
 	unsigned int blkbits = inode->i_blkbits;
diff --git a/fs/gfs2/bmap.c b/fs/gfs2/bmap.c
index 79581b9bdebb..0bf8e8fa82bd 100644
--- a/fs/gfs2/bmap.c
+++ b/fs/gfs2/bmap.c
@@ -1123,7 +1123,8 @@  static int gfs2_iomap_begin_write(struct inode *inode, loff_t pos,
 }
 
 static int gfs2_iomap_begin(struct inode *inode, loff_t pos, loff_t length,
-			    unsigned flags, struct iomap *iomap)
+			    unsigned flags, struct iomap *iomap,
+			    struct iomap *srcmap)
 {
 	struct gfs2_inode *ip = GFS2_I(inode);
 	struct metapath mp = { .mp_aheight = 1, };
diff --git a/fs/iomap/apply.c b/fs/iomap/apply.c
index 54c02aecf3cd..6cdb362fff36 100644
--- a/fs/iomap/apply.c
+++ b/fs/iomap/apply.c
@@ -24,6 +24,7 @@  iomap_apply(struct inode *inode, loff_t pos, loff_t length, unsigned flags,
 		const struct iomap_ops *ops, void *data, iomap_actor_t actor)
 {
 	struct iomap iomap = { 0 };
+	struct iomap srcmap = { 0 };
 	loff_t written = 0, ret;
 
 	/*
@@ -38,7 +39,7 @@  iomap_apply(struct inode *inode, loff_t pos, loff_t length, unsigned flags,
 	 * expose transient stale data. If the reserve fails, we can safely
 	 * back out at this point as there is nothing to undo.
 	 */
-	ret = ops->iomap_begin(inode, pos, length, flags, &iomap);
+	ret = ops->iomap_begin(inode, pos, length, flags, &iomap, &srcmap);
 	if (ret)
 		return ret;
 	if (WARN_ON(iomap.offset > pos))
@@ -58,7 +59,7 @@  iomap_apply(struct inode *inode, loff_t pos, loff_t length, unsigned flags,
 	 * we can do the copy-in page by page without having to worry about
 	 * failures exposing transient data.
 	 */
-	written = actor(inode, pos, length, data, &iomap);
+	written = actor(inode, pos, length, data, &iomap, &srcmap);
 
 	/*
 	 * Now the data has been copied, commit the range we've copied.  This
diff --git a/fs/iomap/buffered-io.c b/fs/iomap/buffered-io.c
index e25901ae3ff4..f27756c0b31c 100644
--- a/fs/iomap/buffered-io.c
+++ b/fs/iomap/buffered-io.c
@@ -205,7 +205,7 @@  iomap_read_inline_data(struct inode *inode, struct page *page,
 
 static loff_t
 iomap_readpage_actor(struct inode *inode, loff_t pos, loff_t length, void *data,
-		struct iomap *iomap)
+		struct iomap *iomap, struct iomap *srcmap)
 {
 	struct iomap_readpage_ctx *ctx = data;
 	struct page *page = ctx->cur_page;
@@ -351,7 +351,7 @@  iomap_next_page(struct inode *inode, struct list_head *pages, loff_t pos,
 
 static loff_t
 iomap_readpages_actor(struct inode *inode, loff_t pos, loff_t length,
-		void *data, struct iomap *iomap)
+		void *data, struct iomap *iomap, struct iomap *srcmap)
 {
 	struct iomap_readpage_ctx *ctx = data;
 	loff_t done, ret;
@@ -371,7 +371,7 @@  iomap_readpages_actor(struct inode *inode, loff_t pos, loff_t length,
 			ctx->cur_page_in_bio = false;
 		}
 		ret = iomap_readpage_actor(inode, pos + done, length - done,
-				ctx, iomap);
+				ctx, iomap, srcmap);
 	}
 
 	return done;
@@ -736,7 +736,7 @@  iomap_write_end(struct inode *inode, loff_t pos, unsigned len,
 
 static loff_t
 iomap_write_actor(struct inode *inode, loff_t pos, loff_t length, void *data,
-		struct iomap *iomap)
+		struct iomap *iomap, struct iomap *srcmap)
 {
 	struct iov_iter *i = data;
 	long status = 0;
@@ -853,7 +853,7 @@  __iomap_read_page(struct inode *inode, loff_t offset)
 
 static loff_t
 iomap_dirty_actor(struct inode *inode, loff_t pos, loff_t length, void *data,
-		struct iomap *iomap)
+		struct iomap *iomap, struct iomap *srcmap)
 {
 	long status = 0;
 	ssize_t written = 0;
@@ -942,7 +942,7 @@  static int iomap_dax_zero(loff_t pos, unsigned offset, unsigned bytes,
 
 static loff_t
 iomap_zero_range_actor(struct inode *inode, loff_t pos, loff_t count,
-		void *data, struct iomap *iomap)
+		void *data, struct iomap *iomap, struct iomap *srcmap)
 {
 	bool *did_zero = data;
 	loff_t written = 0;
@@ -1011,7 +1011,7 @@  EXPORT_SYMBOL_GPL(iomap_truncate_page);
 
 static loff_t
 iomap_page_mkwrite_actor(struct inode *inode, loff_t pos, loff_t length,
-		void *data, struct iomap *iomap)
+		void *data, struct iomap *iomap, struct iomap *srcmap)
 {
 	struct page *page = data;
 	int ret;
diff --git a/fs/iomap/direct-io.c b/fs/iomap/direct-io.c
index 10517cea9682..5279029c7a3c 100644
--- a/fs/iomap/direct-io.c
+++ b/fs/iomap/direct-io.c
@@ -362,7 +362,7 @@  iomap_dio_inline_actor(struct inode *inode, loff_t pos, loff_t length,
 
 static loff_t
 iomap_dio_actor(struct inode *inode, loff_t pos, loff_t length,
-		void *data, struct iomap *iomap)
+		void *data, struct iomap *iomap, struct iomap *srcmap)
 {
 	struct iomap_dio *dio = data;
 
diff --git a/fs/iomap/fiemap.c b/fs/iomap/fiemap.c
index f26fdd36e383..690ef2d7c6c8 100644
--- a/fs/iomap/fiemap.c
+++ b/fs/iomap/fiemap.c
@@ -44,7 +44,7 @@  static int iomap_to_fiemap(struct fiemap_extent_info *fi,
 
 static loff_t
 iomap_fiemap_actor(struct inode *inode, loff_t pos, loff_t length, void *data,
-		struct iomap *iomap)
+		struct iomap *iomap, struct iomap *srcmap)
 {
 	struct fiemap_ctx *ctx = data;
 	loff_t ret = length;
@@ -111,7 +111,7 @@  EXPORT_SYMBOL_GPL(iomap_fiemap);
 
 static loff_t
 iomap_bmap_actor(struct inode *inode, loff_t pos, loff_t length,
-		void *data, struct iomap *iomap)
+		void *data, struct iomap *iomap, struct iomap *srcmap)
 {
 	sector_t *bno = data, addr;
 
diff --git a/fs/iomap/seek.c b/fs/iomap/seek.c
index c04bad4b2b43..89f61d93c0bc 100644
--- a/fs/iomap/seek.c
+++ b/fs/iomap/seek.c
@@ -119,7 +119,7 @@  page_cache_seek_hole_data(struct inode *inode, loff_t offset, loff_t length,
 
 static loff_t
 iomap_seek_hole_actor(struct inode *inode, loff_t offset, loff_t length,
-		      void *data, struct iomap *iomap)
+		      void *data, struct iomap *iomap, struct iomap *srcmap)
 {
 	switch (iomap->type) {
 	case IOMAP_UNWRITTEN:
@@ -165,7 +165,7 @@  EXPORT_SYMBOL_GPL(iomap_seek_hole);
 
 static loff_t
 iomap_seek_data_actor(struct inode *inode, loff_t offset, loff_t length,
-		      void *data, struct iomap *iomap)
+		      void *data, struct iomap *iomap, struct iomap *srcmap)
 {
 	switch (iomap->type) {
 	case IOMAP_HOLE:
diff --git a/fs/iomap/swapfile.c b/fs/iomap/swapfile.c
index 152a230f668d..a648dbf6991e 100644
--- a/fs/iomap/swapfile.c
+++ b/fs/iomap/swapfile.c
@@ -76,7 +76,8 @@  static int iomap_swapfile_add_extent(struct iomap_swapfile_info *isi)
  * distinction between written and unwritten extents.
  */
 static loff_t iomap_swapfile_activate_actor(struct inode *inode, loff_t pos,
-		loff_t count, void *data, struct iomap *iomap)
+		loff_t count, void *data, struct iomap *iomap,
+		struct iomap *srcmap)
 {
 	struct iomap_swapfile_info *isi = data;
 	int error;
diff --git a/fs/xfs/xfs_iomap.c b/fs/xfs/xfs_iomap.c
index 3a4310d7cb59..8321733c16c3 100644
--- a/fs/xfs/xfs_iomap.c
+++ b/fs/xfs/xfs_iomap.c
@@ -922,7 +922,8 @@  xfs_file_iomap_begin(
 	loff_t			offset,
 	loff_t			length,
 	unsigned		flags,
-	struct iomap		*iomap)
+	struct iomap		*iomap,
+	struct iomap		*srcmap)
 {
 	struct xfs_inode	*ip = XFS_I(inode);
 	struct xfs_mount	*mp = ip->i_mount;
@@ -1145,7 +1146,8 @@  xfs_seek_iomap_begin(
 	loff_t			offset,
 	loff_t			length,
 	unsigned		flags,
-	struct iomap		*iomap)
+	struct iomap		*iomap,
+	struct iomap		*srcmap)
 {
 	struct xfs_inode	*ip = XFS_I(inode);
 	struct xfs_mount	*mp = ip->i_mount;
@@ -1231,7 +1233,8 @@  xfs_xattr_iomap_begin(
 	loff_t			offset,
 	loff_t			length,
 	unsigned		flags,
-	struct iomap		*iomap)
+	struct iomap		*iomap,
+	struct iomap		*srcmap)
 {
 	struct xfs_inode	*ip = XFS_I(inode);
 	struct xfs_mount	*mp = ip->i_mount;
diff --git a/include/linux/iomap.h b/include/linux/iomap.h
index bc499ceae392..5b2055e8ca8a 100644
--- a/include/linux/iomap.h
+++ b/include/linux/iomap.h
@@ -26,6 +26,7 @@  struct vm_fault;
 #define IOMAP_MAPPED	0x03	/* blocks allocated at @addr */
 #define IOMAP_UNWRITTEN	0x04	/* blocks allocated at @addr in unwritten state */
 #define IOMAP_INLINE	0x05	/* data inline in the inode */
+#define IOMAP_COW	0x06	/* copy data from srcmap before writing */
 
 /*
  * Flags for all iomap mappings:
@@ -110,7 +111,8 @@  struct iomap_ops {
 	 * The actual length is returned in iomap->length.
 	 */
 	int (*iomap_begin)(struct inode *inode, loff_t pos, loff_t length,
-			unsigned flags, struct iomap *iomap);
+			unsigned flags, struct iomap *iomap,
+			struct iomap *srcmap);
 
 	/*
 	 * Commit and/or unreserve space previous allocated using iomap_begin.
@@ -126,7 +128,7 @@  struct iomap_ops {
  * Main iomap iterator function.
  */
 typedef loff_t (*iomap_actor_t)(struct inode *inode, loff_t pos, loff_t len,
-		void *data, struct iomap *iomap);
+		void *data, struct iomap *iomap, struct iomap *srcmap);
 
 loff_t iomap_apply(struct inode *inode, loff_t pos, loff_t length,
 		unsigned flags, const struct iomap_ops *ops, void *data,