From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754904Ab0F0KCq (ORCPT ); Sun, 27 Jun 2010 06:02:46 -0400 Received: from sh.osrg.net ([192.16.179.4]:47942 "EHLO sh.osrg.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754838Ab0F0KCm (ORCPT ); Sun, 27 Jun 2010 06:02:42 -0400 Date: Sun, 27 Jun 2010 19:01:33 +0900 To: hch@lst.de Cc: fujita.tomonori@lab.ntt.co.jp, snitzer@redhat.com, axboe@kernel.dk, dm-devel@redhat.com, James.Bottomley@suse.de, linux-kernel@vger.kernel.org, martin.petersen@oracle.com, akpm@linux-foundation.org Subject: Re: [PATCH 1/2] block: fix leaks associated with discard request payload From: FUJITA Tomonori In-Reply-To: <20100627092652.GA11625@lst.de> References: <1277582211-10725-1-git-send-email-snitzer@redhat.com> <20100627174721D.fujita.tomonori@lab.ntt.co.jp> <20100627092652.GA11625@lst.de> Mime-Version: 1.0 Content-Type: Text/Plain; charset=us-ascii Content-Transfer-Encoding: 7bit Message-Id: <20100627185927K.fujita.tomonori@lab.ntt.co.jp> X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-3.0 (sh.osrg.net [192.16.179.4]); Sun, 27 Jun 2010 19:01:35 +0900 (JST) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Sun, 27 Jun 2010 11:26:52 +0200 Christoph Hellwig wrote: > On Sun, Jun 27, 2010 at 05:49:29PM +0900, FUJITA Tomonori wrote: > > On Sat, 26 Jun 2010 15:56:50 -0400 > > Mike Snitzer wrote: > > > > > Fix leaks introduced via "block: don't allocate a payload for discard > > > request" commit a1d949f5f44. > > > > > > sd_done() is not called for REQ_TYPE_BLOCK_PC commands so cleanup > > > discard request's payload directly in scsi_finish_command(). > > > > Instead of adding another discard hack to scsi_finish_command(), how > > about converting discard to REQ_TYPE_FS request? discard is FS request > > from the perspective of the block layer. It also fixes a problem that > > discard isn't retried in the case of UNIT ATTENTION. > > > > I think that we can get more cleaner code if we handle discard as > > normal (fs) request in the block layer (and scsi-ml). We need more > > changes but this patch is the first step. > > Making discard a REQ_TYPE_FS inside scsi (it already is before entering > sd_prep_fn) means we'll need to special case it all over the I/O > submission and completion path. Having the payload length not matching Hmm, my patch doesn't add any special case in scsi submission and completion. sd_prep_fn already has a hack for discard to set bi->bi_size to rq->__data_size so scsi can tell the block layer to finish discard requests. Adding another special case for discard to scsi_io_completion() doesn't look good. About the block layer, we already have special case for discard everywhere (rq->cmd_flags & REQ_DISCARD). > the transfer length is something we don't expect for FS requests. Yeah, that's tricky. I'm not sure yet which is better; change how the block layer handles the transfer length or let the lower layer to add pages (as we do now). > > index e16185b..9e15c46 100644 > > --- a/block/blk-lib.c > > +++ b/block/blk-lib.c > > @@ -20,6 +20,10 @@ static void blkdev_discard_end_io(struct bio *bio, int err) > > if (bio->bi_private) > > complete(bio->bi_private); > > > > + /* free the page that the lower layer allocated */ > > + if (bio_page(bio)) > > + __free_page(bio_page(bio)); > > + > > This is exactly what this patchkit gets rid off. Having a payload > page that the caller tracks (previously fully, with this patch only for > freeing) makes DM's life a lot harder. Remember we don't actually store > any payload in there before entering sd_prep_fn - it's just that the > scsi commands implementing discards need some payload - either a sector > sizes zero filled buffer for WRITE SAME, or an LBA/len encoding inside > the payload for UNMAP. It's so bad if the block layer frees pages that the lower layer allocates? I thought it's ok if the block layer doesn't allocate. It's better if sd_done() frees a page? As my patch does, if we handle discard as FS in scsi-ml, sd_done() is called. > > - rq->cmd_type = REQ_TYPE_BLOCK_PC; > > + rq->cmd_type = REQ_TYPE_FS; > > No need to set REQ_TYPE_FS here, it's already set up that way. Oops, sure.