Re: [PATCH 1/3] block: invalidate the page cache when issuing BLKZEROOUT.

On Mon, Jun 20, 2016 at 02:35:29PM +0200, Bart Van Assche wrote:

On 06/17/2016 03:18 AM, Darrick J. Wong wrote:

quoted

Invalidate the page cache (as a regular O_DIRECT write would do) to avoid
returning stale cache contents at a later time.

v5: Refactor the 4.4 refactoring of the ioctl code into separate functions.
Split the page invalidation and the new ioctl into separate patches.

Signed-off-by: Darrick J. Wong <redacted>
Reviewed-by: Christoph Hellwig <redacted>
---
block/ioctl.c |   29 +++++++++++++++++++++++------
1 file changed, 23 insertions(+), 6 deletions(-)

diff --git a/block/ioctl.c b/block/ioctl.c
index ed2397f..d001f52 100644
--- a/block/ioctl.c
+++ b/block/ioctl.c

@@ -225,7 +225,9 @@ static int blk_ioctl_zeroout(struct block_device *bdev, fmode_t mode,

		unsigned long arg)
{
	uint64_t range[2];
-	uint64_t start, len;
+	struct address_space *mapping;
+	uint64_t start, end, len;
+	int ret;

	if (!(mode & FMODE_WRITE))
		return -EBADF;

@@ -235,18 +237,33 @@ static int blk_ioctl_zeroout(struct block_device *bdev, fmode_t mode,

	start = range[0];
	len = range[1];
+	end = start + len - 1;

	if (start & 511)
		return -EINVAL;
	if (len & 511)
		return -EINVAL;
-	start >>= 9;
-	len >>= 9;
-
-	if (start + len > (i_size_read(bdev->bd_inode) >> 9))
+	if (end >= (uint64_t)i_size_read(bdev->bd_inode))
+		return -EINVAL;
+	if (end < start)
		return -EINVAL;

-	return blkdev_issue_zeroout(bdev, start, len, GFP_KERNEL, false);
+	/* Invalidate the page cache, including dirty pages */
+	mapping = bdev->bd_inode->i_mapping;
+	truncate_inode_pages_range(mapping, start, end);
+
+	ret = blkdev_issue_zeroout(bdev, start >> 9, len >> 9, GFP_KERNEL,
+				    false);
+	if (ret)
+		return ret;
+
+	/*
+	 * Invalidate again; if someone wandered in and dirtied a page,
+	 * the caller will be given -EBUSY.
+	 */
+	return invalidate_inode_pages2_range(mapping,
+					     start >> PAGE_SHIFT,
+					     end >> PAGE_SHIFT);
}

Hello Darrick,

Maybe this has already been discussed, but anyway: in the POSIX spec
(http://pubs.opengroup.org/onlinepubs/9699919799/functions/write.html) I
found the following: "This volume of POSIX.1-2008 does not specify behavior
of concurrent writes to a file from multiple processes. Applications should
use some form of concurrency control."

Do we really need the invalidate_inode_pages2_range() call?

It's not strictly necessary.  I like the idea of having the kernel bonking
userspace when they don't coordinate and collide, but we could just jump
out after the blkdev_*() calls and let userspace fend for themselves. :)

--D

Thanks,

Bart.