linux-kernel - Re: regression introduced by "block: Add support for DAX reads/writes to block devices"

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Message-ID: <x49fv3wgl4f.fsf@segfault.boston.devel.redhat.com>
Date:	Thu, 06 Aug 2015 17:30:56 -0400
From:	Jeff Moyer <jmoyer@...hat.com>
To:	"Wilcox\, Matthew R" <matthew.r.wilcox@...el.com>
Cc:	"linda.knippers\@hp.com" <linda.knippers@...com>,
	"linux-kernel\@vger.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-fsdevel\@vger.kernel.org" <linux-fsdevel@...r.kernel.org>
Subject: Re: regression introduced by "block: Add support for DAX reads/writes to block devices"

"Wilcox, Matthew R" <matthew.r.wilcox@...el.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;

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