[03/15] iomap: Read page from srcmap for IOMAP_COW
diff mbox series

Message ID 20190901200836.14959-4-rgoldwyn@suse.de
State New
Headers show
Series
  • Btrfs iomap
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Commit Message

Goldwyn Rodrigues Sept. 1, 2019, 8:08 p.m. UTC
From: Goldwyn Rodrigues <rgoldwyn@suse.com>

In case of a IOMAP_COW, read a page from the srcmap before
performing a write on the page.

Signed-off-by: Goldwyn Rodrigues <rgoldwyn@suse.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
---
 fs/iomap/buffered-io.c | 23 ++++++++++++++---------
 1 file changed, 14 insertions(+), 9 deletions(-)

Comments

Christoph Hellwig Sept. 2, 2019, 4:31 p.m. UTC | #1
On Sun, Sep 01, 2019 at 03:08:24PM -0500, Goldwyn Rodrigues wrote:

> +		iomap_assert(!(iomap->flags & IOMAP_F_BUFFER_HEAD));
> +		iomap_assert(srcmap->type == IOMAP_HOLE || srcmap->addr > 0);

0 can be a valid address in various file systems, so I don't think we
can just exclude it.  Then again COWing from a hole seems pointless,
doesn't it?

So just check for addr != IOMAP_NULL_ADDR here?

>  
> @@ -961,7 +966,7 @@ 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);

This introduces an > 80 character line.
Darrick J. Wong Sept. 2, 2019, 5:01 p.m. UTC | #2
On Mon, Sep 02, 2019 at 06:31:24PM +0200, Christoph Hellwig wrote:
> On Sun, Sep 01, 2019 at 03:08:24PM -0500, Goldwyn Rodrigues wrote:
> 
> > +		iomap_assert(!(iomap->flags & IOMAP_F_BUFFER_HEAD));
> > +		iomap_assert(srcmap->type == IOMAP_HOLE || srcmap->addr > 0);
> 
> 0 can be a valid address in various file systems, so I don't think we
> can just exclude it.  Then again COWing from a hole seems pointless,
> doesn't it?

XFS does that if you set a cowextsize hint and a speculative cow
preallocation ends up covering a hole.  Granted I don't think there's
much point in reading from a COW fork extent to fill in an unaligned
buffered write since it /should/ just end up zero-filling the pagecache
regardless of fork... but I don't see much harm in doing that.

--D

> So just check for addr != IOMAP_NULL_ADDR here?
> 
> >  
> > @@ -961,7 +966,7 @@ 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);
> 
> This introduces an > 80 character line.
Christoph Hellwig Sept. 2, 2019, 5:13 p.m. UTC | #3
On Mon, Sep 02, 2019 at 10:01:09AM -0700, Darrick J. Wong wrote:
> On Mon, Sep 02, 2019 at 06:31:24PM +0200, Christoph Hellwig wrote:
> > On Sun, Sep 01, 2019 at 03:08:24PM -0500, Goldwyn Rodrigues wrote:
> > 
> > > +		iomap_assert(!(iomap->flags & IOMAP_F_BUFFER_HEAD));
> > > +		iomap_assert(srcmap->type == IOMAP_HOLE || srcmap->addr > 0);
> > 
> > 0 can be a valid address in various file systems, so I don't think we
> > can just exclude it.  Then again COWing from a hole seems pointless,
> > doesn't it?
> 
> XFS does that if you set a cowextsize hint and a speculative cow
> preallocation ends up covering a hole.  Granted I don't think there's
> much point in reading from a COW fork extent to fill in an unaligned
> buffered write since it /should/ just end up zero-filling the pagecache
> regardless of fork... but I don't see much harm in doing that.

That assumes you'd set the iomap-level COW flag for anything that
writes to the COW fork in XFS.  Which doesn't sound right to me - the
iomap-level indicates that we actually need to read some data, which
for a hole is rather pointless as you said.

Patch
diff mbox series

diff --git a/fs/iomap/buffered-io.c b/fs/iomap/buffered-io.c
index f27756c0b31c..99d63ba14d1b 100644
--- a/fs/iomap/buffered-io.c
+++ b/fs/iomap/buffered-io.c
@@ -581,7 +581,7 @@  __iomap_write_begin(struct inode *inode, loff_t pos, unsigned len,
 
 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;
 	pgoff_t index = pos >> PAGE_SHIFT;
@@ -605,12 +605,17 @@  iomap_write_begin(struct inode *inode, loff_t pos, unsigned len, unsigned flags,
 		goto out_no_page;
 	}
 
-	if (iomap->type == IOMAP_INLINE)
+	if (iomap->type == IOMAP_INLINE) {
 		iomap_read_inline_data(inode, page, iomap);
-	else if (iomap->flags & IOMAP_F_BUFFER_HEAD)
+	} else if (iomap->type == IOMAP_COW) {
+		iomap_assert(!(iomap->flags & IOMAP_F_BUFFER_HEAD));
+		iomap_assert(srcmap->type == IOMAP_HOLE || srcmap->addr > 0);
+		status = __iomap_write_begin(inode, pos, len, page, srcmap);
+	} else if (iomap->flags & IOMAP_F_BUFFER_HEAD) {
 		status = __block_write_begin_int(page, pos, len, NULL, iomap);
-	else
+	} else {
 		status = __iomap_write_begin(inode, pos, len, page, iomap);
+	}
 
 	if (unlikely(status))
 		goto out_unlock;
@@ -772,7 +777,7 @@  iomap_write_actor(struct inode *inode, loff_t pos, loff_t length, void *data,
 		}
 
 		status = iomap_write_begin(inode, pos, bytes, flags, &page,
-				iomap);
+				iomap, srcmap);
 		if (unlikely(status))
 			break;
 
@@ -871,7 +876,7 @@  iomap_dirty_actor(struct inode *inode, loff_t pos, loff_t length, void *data,
 			return PTR_ERR(rpage);
 
 		status = iomap_write_begin(inode, pos, bytes,
-					   AOP_FLAG_NOFS, &page, iomap);
+					   AOP_FLAG_NOFS, &page, iomap, srcmap);
 		put_page(rpage);
 		if (unlikely(status))
 			return status;
@@ -917,13 +922,13 @@  iomap_file_dirty(struct inode *inode, loff_t pos, loff_t len,
 EXPORT_SYMBOL_GPL(iomap_file_dirty);
 
 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, AOP_FLAG_NOFS, &page,
-				   iomap);
+				   iomap, srcmap);
 	if (status)
 		return status;
 
@@ -961,7 +966,7 @@  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;