From mboxrd@z Thu Jan 1 00:00:00 1970 From: Kent Overstreet Subject: Re: [dm-devel] [PATCH 12/13] Make generic_make_request handle arbitrarily large bios Date: Mon, 21 May 2012 10:55:42 -0700 Message-ID: <20120521175542.GA16598@google.com> References: <1114e7019b0055fc09a54b59b36398d5c54f5e32.1337308722.git.koverstreet@google.com> <20120518180550.0a6cdc34@notabene.brown> <20120518081444.GA27205@dhcp-172-18-216-138.mtv.corp.google.com> <20120521171706.GH23993@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: NeilBrown , axboe@kernel.dk, dm-devel@redhat.com, linux-kernel@vger.kernel.org, tj@kernel.org, linux-bcache@vger.kernel.org, linux-fsdevel@vger.kernel.org, agk@redhat.com To: Vivek Goyal Return-path: Content-Disposition: inline In-Reply-To: <20120521171706.GH23993@redhat.com> Sender: linux-kernel-owner@vger.kernel.org List-Id: linux-fsdevel.vger.kernel.org On Mon, May 21, 2012 at 01:17:06PM -0400, Vivek Goyal wrote: > May be I am missing something, hence I will ask. Is punting to workqueue > will really solve the issue raised by Neil. > > Due to spliting required, we will be holding some bios in the stack and > these bios can't be submitted till further allocation from pool happens. So > will it matter whether we are waiting for allocation in submitting process > context or in worker thread context. Punting to workqueue allows all the bio splits that have already been allocated to be submitted. > IOW, say you have a pool of 2 bios. We allocate 1 bio (say bio A), and submit > it for IO (now 1 bio left in pool). Now, bio A needs to be split up, so we > allocate bio B and submit it (pool is empty now). Now we try to submit bio > B and this also needs to be split. There are no more free bios so we will > wait for some to get free but none of the bios (A and B) have actually > been submitted for IO so nothing will get freed and we have a deadlock > (This is assuming that memory is tight enough that we are not able to do > any allocations from the slab backing the mempool). You're talking about a different issue. You can't safely split a split from the same bio pool - but this code doesn't do that since it adds a separate bio pool for each request_queue that's only for splits.