RE: regression introduced by "block: Add support for DAX reads/writes to block devices"

["Wilcox, Matthew R" <matthew.r.wilcox@intel.com>]

So ... what you're doing here is if somebody requests the last 512 bytes, you're asking for the last page.  That's going to work as long as the partition is a multiple of PAGE_SIZE, but not if the partition happens to be misaligned.  It's an improvement, although I'd be tempted to do this as:

		if (pos == max) {
 			unsigned blkbits = inode->i_blkbits;
- 			sector_t block = pos >> blkbits;
+			long page = pos >> PAGE_SHIFT;
+			sector_t block = page << (PAGE_SHIFT - blkbits);
			unsigned first = pos - (block << blkbits);
			long size;

We need to cope with the case where the end of a partition isn't on a page boundary though.

-----Original Message-----
From: Jeff Moyer [mailto:jmoyer@redhat.com] 
Sent: Thursday, August 06, 2015 2:31 PM
To: Wilcox, Matthew R
Cc: linda.knippers@hp.com; linux-kernel@vger.kernel.org; linux-fsdevel@vger.kernel.org
Subject: Re: regression introduced by "block: Add support for DAX reads/writes to block devices"

"Wilcox, Matthew R" <matthew.r.wilcox@intel.com> writes:

> I think I see the problem.  I'm kind of wrapped up in other things
> right now; can you try replacing the line in dax_io():
>
> -				bh->b_size = PAGE_ALIGN(end - pos);
> +				bh->b_size = ALIGN(end - pos, 1 << blkbits);

This works for me.  If it looks okay to others, I'll submit a properly
signed-off patch.

Cheers,
Jeff

diff --git a/fs/dax.c b/fs/dax.c
index a7f77e1..b6c4f93 100644
--- a/fs/dax.c
+++ b/fs/dax.c
@@ -98,6 +98,10 @@ static bool buffer_size_valid(struct buffer_head *bh)
 	return bh->b_state != 0;
 }
 
+/*
+ * This function assumes file system block size (represented by
+ * inode->i_blkbits) is less than or equal to the system page size.
+ */
 static ssize_t dax_io(struct inode *inode, struct iov_iter *iter,
 		      loff_t start, loff_t end, get_block_t get_block,
 		      struct buffer_head *bh)
@@ -117,9 +121,15 @@ static ssize_t dax_io(struct inode *inode, struct iov_iter *iter,
 		if (pos == max) {
 			unsigned blkbits = inode->i_blkbits;
 			sector_t block = pos >> blkbits;
-			unsigned first = pos - (block << blkbits);
+			long page = pos >> PAGE_SHIFT;
+			unsigned first; /* byte offset into block */
 			long size;
 
+			/* we can only map entire pages */
+			if (pos & (PAGE_SIZE-1))
+				block = page << (PAGE_SHIFT - blkbits);
+			first = pos - (block << blkbits);
+
 			if (pos == bh_max) {
 				bh->b_size = PAGE_ALIGN(end - pos);
 				bh->b_state = 0;