linux-ext4 - [PATCH 1/7] ext2: tell DAX the size of allocation holes

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Date:	Mon, 15 Aug 2016 13:09:12 -0600
From:	Ross Zwisler <ross.zwisler@...ux.intel.com>
To:	linux-kernel@...r.kernel.org
Cc:	Ross Zwisler <ross.zwisler@...ux.intel.com>,
	"Theodore Ts'o" <tytso@....edu>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Andreas Dilger <adilger.kernel@...ger.ca>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Dan Williams <dan.j.williams@...el.com>,
	Dave Chinner <david@...morbit.com>, Jan Kara <jack@...e.com>,
	linux-ext4@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	linux-mm@...ck.org, linux-nvdimm@...ts.01.org
Subject: [PATCH 1/7] ext2: tell DAX the size of allocation holes

When DAX calls ext2_get_block() and the file offset points to a hole we
currently don't set bh_result->b_size.  When we re-enable PMD faults DAX
will need bh_result->b_size to tell it the size of the hole so it can
decide whether to fault in a 4 KiB zero page or a 2 MiB zero page.

For ext2 we always want DAX to use 4 KiB zero pages, so we just tell DAX
that all holes are 4 KiB in size.

Signed-off-by: Ross Zwisler <ross.zwisler@...ux.intel.com>
---
 fs/ext2/inode.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/fs/ext2/inode.c b/fs/ext2/inode.c
index d5c7d09..c6d9763 100644
--- a/fs/ext2/inode.c
+++ b/fs/ext2/inode.c
@@ -773,6 +773,12 @@ int ext2_get_block(struct inode *inode, sector_t iblock, struct buffer_head *bh_
 	if (ret > 0) {
 		bh_result->b_size = (ret << inode->i_blkbits);
 		ret = 0;
+	} else if (ret == 0 && IS_DAX(inode)) {
+		/*
+		 * We have hit a hole.  Tell DAX it is 4k in size so that it
+		 * uses PTE faults.
+		 */
+		bh_result->b_size = PAGE_SIZE;
 	}
 	return ret;
 
-- 
2.9.0

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