Re: [PATCH 07/19] iomap: use a srcmap for a read-modify-write I/O

["Darrick J. Wong" <darrick.wong@oracle.com>]

On Mon, Sep 16, 2019 at 11:42:37AM -0700, Darrick J. Wong wrote:
> On Mon, Sep 09, 2019 at 08:27:10PM +0200, Christoph Hellwig wrote:

<snip>

> > diff --git a/fs/iomap/apply.c b/fs/iomap/apply.c
> > index 54c02aecf3cd..67efd86675e6 100644
> > --- a/fs/iomap/apply.c
> > +++ b/fs/iomap/apply.c
> > @@ -24,7 +24,9 @@ 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;
> > +	u64 end;
> >  
> >  	/*
> >  	 * Need to map a range from start position for length bytes. This can
> > @@ -38,7 +40,7 @@ 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))
> > @@ -50,15 +52,26 @@ iomap_apply(struct inode *inode, loff_t pos, loff_t length, unsigned flags,
> >  	 * Cut down the length to the one actually provided by the filesystem,
> >  	 * as it might not be able to give us the whole size that we requested.
> >  	 */
> > -	if (iomap.offset + iomap.length < pos + length)
> > -		length = iomap.offset + iomap.length - pos;
> > +	end = iomap.offset + iomap.length;
> > +	if (srcmap.type)
> 
> Do we need a "#define IOMAP_NULL 0" type to make this more explicit?
> 
> Or at least a comment in iomap.h above the IOMAP_HOLE definition
> pointing out that type == 0 is valid for srcmap and not valid for iomap?
> 
> Other than that the rest looks ok.

FWIW, my opinions about this have evolved since two days ago; please see
my reply to patch #12 about initializing @srcmap.type = IOMAP_HOLE and
using "type == IOMAP_HOLE" to decide if we're going to use the srcmap
for anything.

--D

> --D
> 
> (This reminds me that at some point I ought to work on iomap debugging
> checks for the mappings returned by ->iomap_begin ...)
> 
> > +		end = min(end, srcmap.offset + srcmap.length);
> > +	if (pos + length > end)
> > +		length = end - pos;
> >  
> >  	/*
> > -	 * Now that we have guaranteed that the space allocation will succeed.
> > +	 * Now that we have guaranteed that the space allocation will succeed,
> >  	 * we can do the copy-in page by page without having to worry about
> >  	 * failures exposing transient data.
> > +	 *
> > +	 * To support COW operations, we read in data for partially blocks from
> > +	 * the srcmap if the file system filled it in.  In that case we the
> > +	 * length needs to be limited to the earlier of the ends of the iomaps.
> > +	 * If the file system did not provide a srcmap we pass in the normal
> > +	 * iomap into the actors so that they don't need to have special
> > +	 * handling for the two cases.
> >  	 */
> > -	written = actor(inode, pos, length, data, &iomap);
> > +	written = actor(inode, pos, length, data, &iomap,
> > +			srcmap.type ? &srcmap : &iomap);
> >  
> >  	/*
> >  	 * 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 a421977a9496..58fb7a894ad4 100644
> > --- a/fs/iomap/buffered-io.c
> > +++ b/fs/iomap/buffered-io.c
> > @@ -219,7 +219,7 @@ static inline bool iomap_block_needs_zeroing(struct inode *inode,
> >  
> >  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;
> > @@ -367,7 +367,7 @@ 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;
> > @@ -387,7 +387,7 @@ 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;
> > @@ -567,7 +567,7 @@ iomap_read_page_sync(loff_t block_start, struct page *page, unsigned poff,
> >  
> >  static int
> >  __iomap_write_begin(struct inode *inode, loff_t pos, unsigned len, int flags,
> > -		struct page *page, struct iomap *iomap)
> > +		struct page *page, struct iomap *srcmap)
> >  {
> >  	struct iomap_page *iop = iomap_page_create(inode, page);
> >  	loff_t block_size = i_blocksize(inode);
> > @@ -590,7 +590,7 @@ __iomap_write_begin(struct inode *inode, loff_t pos, unsigned len, int flags,
> >  		    (to <= poff || to >= poff + plen))
> >  			continue;
> >  
> > -		if (iomap_block_needs_zeroing(inode, iomap, block_start)) {
> > +		if (iomap_block_needs_zeroing(inode, srcmap, block_start)) {
> >  			if (WARN_ON_ONCE(flags & IOMAP_WRITE_F_UNSHARE))
> >  				return -EIO;
> >  			zero_user_segments(page, poff, from, to, poff + plen);
> > @@ -599,7 +599,7 @@ __iomap_write_begin(struct inode *inode, loff_t pos, unsigned len, int flags,
> >  		}
> >  
> >  		status = iomap_read_page_sync(block_start, page, poff, plen,
> > -				iomap);
> > +				srcmap);
> >  		if (status)
> >  			return status;
> >  	} while ((block_start += plen) < block_end);
> > @@ -609,13 +609,15 @@ __iomap_write_begin(struct inode *inode, loff_t pos, unsigned len, int flags,
> >  
> >  static int
> >  iomap_write_begin(struct inode *inode, loff_t pos, unsigned len, unsigned flags,
> > -		struct page **pagep, struct iomap *iomap)
> > +		struct page **pagep, struct iomap *iomap, struct iomap *srcmap)
> >  {
> >  	const struct iomap_page_ops *page_ops = iomap->page_ops;
> >  	struct page *page;
> >  	int status = 0;
> >  
> >  	BUG_ON(pos + len > iomap->offset + iomap->length);
> > +	if (srcmap != iomap)
> > +		BUG_ON(pos + len > srcmap->offset + srcmap->length);
> >  
> >  	if (fatal_signal_pending(current))
> >  		return -EINTR;
> > @@ -633,13 +635,13 @@ iomap_write_begin(struct inode *inode, loff_t pos, unsigned len, unsigned flags,
> >  		goto out_no_page;
> >  	}
> >  
> > -	if (iomap->type == IOMAP_INLINE)
> > -		iomap_read_inline_data(inode, page, iomap);
> > +	if (srcmap->type == IOMAP_INLINE)
> > +		iomap_read_inline_data(inode, page, srcmap);
> >  	else if (iomap->flags & IOMAP_F_BUFFER_HEAD)
> > -		status = __block_write_begin_int(page, pos, len, NULL, iomap);
> > +		status = __block_write_begin_int(page, pos, len, NULL, srcmap);
> >  	else
> >  		status = __iomap_write_begin(inode, pos, len, flags, page,
> > -				iomap);
> > +				srcmap);
> >  
> >  	if (unlikely(status))
> >  		goto out_unlock;
> > @@ -725,16 +727,16 @@ iomap_write_end_inline(struct inode *inode, struct page *page,
> >  }
> >  
> >  static int
> > -iomap_write_end(struct inode *inode, loff_t pos, unsigned len,
> > -		unsigned copied, struct page *page, struct iomap *iomap)
> > +iomap_write_end(struct inode *inode, loff_t pos, unsigned len, unsigned copied,
> > +		struct page *page, struct iomap *iomap, struct iomap *srcmap)
> >  {
> >  	const struct iomap_page_ops *page_ops = iomap->page_ops;
> >  	loff_t old_size = inode->i_size;
> >  	int ret;
> >  
> > -	if (iomap->type == IOMAP_INLINE) {
> > +	if (srcmap->type == IOMAP_INLINE) {
> >  		ret = iomap_write_end_inline(inode, page, iomap, pos, copied);
> > -	} else if (iomap->flags & IOMAP_F_BUFFER_HEAD) {
> > +	} else if (srcmap->flags & IOMAP_F_BUFFER_HEAD) {
> >  		ret = block_write_end(NULL, inode->i_mapping, pos, len, copied,
> >  				page, NULL);
> >  	} else {
> > @@ -765,7 +767,7 @@ 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;
> > @@ -799,7 +801,8 @@ iomap_write_actor(struct inode *inode, loff_t pos, loff_t length, void *data,
> >  			break;
> >  		}
> >  
> > -		status = iomap_write_begin(inode, pos, bytes, 0, &page, iomap);
> > +		status = iomap_write_begin(inode, pos, bytes, 0, &page, iomap,
> > +				srcmap);
> >  		if (unlikely(status))
> >  			break;
> >  
> > @@ -810,8 +813,8 @@ iomap_write_actor(struct inode *inode, loff_t pos, loff_t length, void *data,
> >  
> >  		flush_dcache_page(page);
> >  
> > -		status = iomap_write_end(inode, pos, bytes, copied, page,
> > -				iomap);
> > +		status = iomap_write_end(inode, pos, bytes, copied, page, iomap,
> > +				srcmap);
> >  		if (unlikely(status < 0))
> >  			break;
> >  		copied = status;
> > @@ -864,7 +867,7 @@ EXPORT_SYMBOL_GPL(iomap_file_buffered_write);
> >  
> >  static loff_t
> >  iomap_unshare_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;
> > @@ -873,7 +876,7 @@ iomap_unshare_actor(struct inode *inode, loff_t pos, loff_t length, void *data,
> >  	if (!(iomap->flags & IOMAP_F_SHARED))
> >  		return length;
> >  	/* don't bother with holes or unwritten extents */
> > -	if (iomap->type == IOMAP_HOLE || iomap->type == IOMAP_UNWRITTEN)
> > +	if (srcmap->type == IOMAP_HOLE || srcmap->type == IOMAP_UNWRITTEN)
> >  		return length;
> >  
> >  	do {
> > @@ -882,11 +885,12 @@ iomap_unshare_actor(struct inode *inode, loff_t pos, loff_t length, void *data,
> >  		struct page *page;
> >  
> >  		status = iomap_write_begin(inode, pos, bytes,
> > -				IOMAP_WRITE_F_UNSHARE, &page, iomap);
> > +				IOMAP_WRITE_F_UNSHARE, &page, iomap, srcmap);
> >  		if (unlikely(status))
> >  			return status;
> >  
> > -		status = iomap_write_end(inode, pos, bytes, bytes, page, iomap);
> > +		status = iomap_write_end(inode, pos, bytes, bytes, page, iomap,
> > +				srcmap);
> >  		if (unlikely(status <= 0)) {
> >  			if (WARN_ON_ONCE(status == 0))
> >  				return -EIO;
> > @@ -925,19 +929,19 @@ iomap_file_unshare(struct inode *inode, loff_t pos, loff_t len,
> >  EXPORT_SYMBOL_GPL(iomap_file_unshare);
> >  
> >  static int iomap_zero(struct inode *inode, loff_t pos, unsigned offset,
> > -		unsigned bytes, struct iomap *iomap)
> > +		unsigned bytes, struct iomap *iomap, struct iomap *srcmap)
> >  {
> >  	struct page *page;
> >  	int status;
> >  
> > -	status = iomap_write_begin(inode, pos, bytes, 0, &page, iomap);
> > +	status = iomap_write_begin(inode, pos, bytes, 0, &page, iomap, srcmap);
> >  	if (status)
> >  		return status;
> >  
> >  	zero_user(page, offset, bytes);
> >  	mark_page_accessed(page);
> >  
> > -	return iomap_write_end(inode, pos, bytes, bytes, page, iomap);
> > +	return iomap_write_end(inode, pos, bytes, bytes, page, iomap, srcmap);
> >  }
> >  
> >  static int iomap_dax_zero(loff_t pos, unsigned offset, unsigned bytes,
> > @@ -949,14 +953,14 @@ 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;
> >  	int status;
> >  
> >  	/* already zeroed?  we're done. */
> > -	if (iomap->type == IOMAP_HOLE || iomap->type == IOMAP_UNWRITTEN)
> > +	if (srcmap->type == IOMAP_HOLE || srcmap->type == IOMAP_UNWRITTEN)
> >  		return count;
> >  
> >  	do {
> > @@ -968,7 +972,8 @@ iomap_zero_range_actor(struct inode *inode, loff_t pos, loff_t count,
> >  		if (IS_DAX(inode))
> >  			status = iomap_dax_zero(pos, offset, bytes, iomap);
> >  		else
> > -			status = iomap_zero(inode, pos, offset, bytes, iomap);
> > +			status = iomap_zero(inode, pos, offset, bytes, iomap,
> > +					srcmap);
> >  		if (status < 0)
> >  			return status;
> >  
> > @@ -1018,7 +1023,7 @@ 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 1fc28c2da279..e3ccbf7daaae 100644
> > --- a/fs/iomap/direct-io.c
> > +++ b/fs/iomap/direct-io.c
> > @@ -358,7 +358,7 @@ 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 d340db666c06..f1c3ff27c666 100644
> > --- a/fs/xfs/xfs_iomap.c
> > +++ b/fs/xfs/xfs_iomap.c
> > @@ -928,7 +928,8 @@ 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;
> > @@ -1154,7 +1155,8 @@ 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;
> > @@ -1240,7 +1242,8 @@ 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 3a0f0975a57e..6d795924baa7 100644
> > --- a/include/linux/iomap.h
> > +++ b/include/linux/iomap.h
> > @@ -127,7 +127,8 @@ 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.
> > @@ -143,7 +144,7 @@ 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,
> > -- 
> > 2.20.1
> >