On Wed, Mar 30, 2016 at 11:27:55AM -0700, Darrick J. Wong wrote:

Or is it ok that fallocate could block, potentially for a long time as
we stream cows through the page cache (or however unshare works
internally)?  Those same programs might not be expecting fallocate to
take a long time.

Yes, it's perfectly fine for fallocate to block for long periods of
time. See what gfs2 does during preallocation of blocks - it ends up
calling sb_issue_zerout() because it doesn't have unwritten
extents, and hence can block for long periods of time....

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com