Re: [PATCH v4 03/11] fs: Initial atomic write support

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On 19/02/2024 22:44, Dave Chinner wrote:
On Mon, Feb 19, 2024 at 01:01:01PM +0000, John Garry wrote:
@@ -3523,4 +3535,26 @@ extern int vfs_fadvise(struct file *file, loff_t offset, loff_t len,
  extern int generic_fadvise(struct file *file, loff_t offset, loff_t len,
  			   int advice);
+static inline bool atomic_write_valid(loff_t pos, struct iov_iter *iter,
+			   unsigned int unit_min, unsigned int unit_max)
+	size_t len = iov_iter_count(iter);
+	if (!iter_is_ubuf(iter))
+		return false;
+	if (len == unit_min || len == unit_max) {
+		/* ok if exactly min or max */
+	} else if (len < unit_min || len > unit_max) {
+		return false;
+	} else if (!is_power_of_2(len)) {
+		return false;
+	}
This doesn't need if else if else if and it doesn't need to check
for exact unit min/max matches.

This is fastpath code, and I thought it quicker to just check if min/max first. Based on recent discussions, for FS support I expect typically len == unit_max.

But I can change to your simpler checking and later change to the current method if those FS assumptions hold true.

The exact matches require the
length to be a power of 2, so the checks are simply:

	if (len < unit_min || len > unit_max)
		return false;
	if (!is_power_of_2(len))
		return false;

+	if (pos & (len - 1))
+		return false;
This has typing issues - 64 bit value, 32 bit mask. probably should

	if (!IS_ALIGNED(pos, len))
		return false;

ok, good idea.


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