On Mon, 2017-01-23 at 17:35 -0500, Jeff Layton wrote:

On Mon, 2017-01-23 at 11:09 +0100, Kevin Wolf wrote:

quoted

However, if we look at the greater problem of hanging requests that
came
up in the more recent emails of this thread, it is only moved
rather
than solved. Chances are that already write() would hang now
instead of
only fsync(), but we still have a hard time dealing with this.

Well, it _is_ better with O_DIRECT as you can usually at least break
out
of the I/O with SIGKILL.

When I last looked at this, the problem with buffered I/O was that
you
often end up waiting on page bits to clear (usually PG_writeback or
PG_dirty), in non-killable sleeps for the most part.

Maybe the fix here is as simple as changing that?

At the risk of kicking off another O_PONIES discussion: Add an
open(O_TIMEOUT) flag that would let the kernel know that the
application is prepared to handle timeouts from operations such as
read(), write() and fsync(), then add an ioctl() or syscall to allow
said application to set the timeout value.


-- 
Trond Myklebust
Linux NFS client maintainer, PrimaryData
trond.myklebust@primarydata.com
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