From mboxrd@z Thu Jan 1 00:00:00 1970 From: Dave Chinner Subject: Re: fallocate mode flag for "unshare blocks"? Date: Thu, 31 Mar 2016 12:18:13 +1100 Message-ID: <20160331011813.GJ30721@dastard> References: <20160302155007.GB7125@infradead.org> <20160330182755.GC2236@birch.djwong.org> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Content-Disposition: inline In-Reply-To: <20160330182755.GC2236@birch.djwong.org> Sender: linux-btrfs-owner@vger.kernel.org To: "Darrick J. Wong" Cc: Christoph Hellwig , xfs@oss.sgi.com, linux-fsdevel , linux-btrfs , linux-api@vger.kernel.org List-Id: linux-api@vger.kernel.org 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