On 05/19/2018 08:29 PM, Theodore Y. Ts'o wrote:

On Thu, Feb 08, 2018 at 12:59:48PM -0600, Goldwyn Rodrigues wrote:

quoted

From: Goldwyn Rodrigues <redacted>

In case direct I/O encounters an error midway, it returns the error.
Instead it should be returning the number of bytes transferred so far.

Test case for filesystems (with ENOSPC):
1. Create an almost full filesystem
2. Create a file, say /mnt/lastfile, until the filesystem is full.
3. Direct write() with count > sizeof /mnt/lastfile.

Result: write() returns -ENOSPC. However, file content has data written
in step 3.

Added a sysctl entry: dio_short_writes which is on by default. This is
to support applications which expect either and error or the bytes submitted
as a return value for the write calls.

This fixes fstest generic/472.

Signed-off-by: Goldwyn Rodrigues <redacted>

Hi,

I was wondering if you could give an update regarding what's up with
this patch?

There doesn't seem to be any movement on this patch in a while, and so
I still have xfstests generic/472 suppressed in {kvm,gce}-xfstests.

From earlier discussions, In between errors of a direct I/O cannot be

handled correctly and may need a lot of tracking that it is not worth
performing.

It would be better to drop this test case and add in the documentation
that a direct I/O error could mean that the write() may or may not have
occurred and underlying data may be inconsistent. I am proposing:

diff --git a/man2/write.2 b/man2/write.2
index f8a94f3ff..86f655e26 100644
--- a/man2/write.2
+++ b/man2/write.2

@@ -274,6 +274,14 @@ On Linux,
 returning the number of bytes actually transferred.
 .\" commit e28cc71572da38a5a12c1cfe4d7032017adccf69
 (This is true on both 32-bit and 64-bit systems.)
+.PP
+While performing
+.BR write()
+using direct I/O, an error returned does not mean the
+entire write has failed. Partial data may be written
+and the file offset to length on which the
+.BR write()
+was attempted should be considered inconsistent.
 .SH BUGS
 According to POSIX.1-2008/SUSv4 Section XSI 2.9.7
 ("Thread Interactions with Regular File Operations"):

-- 
Goldwyn