From mboxrd@z Thu Jan 1 00:00:00 1970 From: Jens Axboe Subject: Re: [PATCH] remove use_sg_chaining Date: Sun, 20 Jan 2008 20:29:42 +0100 Message-ID: <20080120192942.GW6258@kernel.dk> References: <1200419579.9273.39.camel@localhost.localdomain> <47939E9B.9020906@panasas.com> <1200857062.3105.15.camel@localhost.localdomain> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Received: from brick.kernel.dk ([87.55.233.238]:8086 "EHLO kernel.dk" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754209AbYATT3r (ORCPT ); Sun, 20 Jan 2008 14:29:47 -0500 Content-Disposition: inline In-Reply-To: <1200857062.3105.15.camel@localhost.localdomain> Sender: linux-scsi-owner@vger.kernel.org List-Id: linux-scsi@vger.kernel.org To: James Bottomley Cc: Boaz Harrosh , linux-scsi On Sun, Jan 20 2008, James Bottomley wrote: > > On Sun, 2008-01-20 at 21:18 +0200, Boaz Harrosh wrote: > > On Tue, Jan 15 2008 at 19:52 +0200, James Bottomley wrote: > > > this patch depends on the sg branch of the block tree > > > > > > James > > > > > > --- > > > From: James Bottomley > > > Date: Tue, 15 Jan 2008 11:11:46 -0600 > > > Subject: remove use_sg_chaining > > > > > > With the sg table code, every SCSI driver is now either chain capable > > > or broken, so there's no need to have a check in the host template. > > > > > > Also tidy up the code by moving the scatterlist size defines into the > > > SCSI includes and permit the last entry of the scatterlist pools not > > > to be a power of two. > > > --- > > > > I have a theoretical problem that BUGed me from the beginning. > > > > Could it happen that a memory critical IO, (that is needed to free > > memory), be collected into an sg-chained large IO, and the allocation > > of the multiple sg-pool-allocations fail, thous dead locking on > > out-of-memory? Is there a mechanism in place that will split large IO's > > into smaller chunks in the event of out-of-memory condition in prep_fn? > > > > Is it possible to call blk_rq_map_sg() with less then what is present > > at request to only map the starting portion? > > Obviously, that's why I was worrying about mempool size and default > blocks a while ago. > > However, the deadlock only occurs if the device is swap or backing a > filesystem with memory mapped files. The use cases for this are really > tapes and other entities that need huge buffers. That's why we're > keeping the system sector size at 1024 unless you alter it through sysfs > (here gun, there foot ...) Alternatively (and much safer, imho), we allow blk_rq_map_sg() return smaller than nr_phys_segments and just ensure that the request is continued nicely through the normal 'request if residual' logic. -- Jens Axboe