From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Jun'ichi Nomura" Subject: Re: fragmented i/o with 2.6.31? Date: Sat, 19 Sep 2009 00:38:32 +0900 Message-ID: <4AB3A978.60006@ce.jp.nec.com> References: <448b15030909160834j2b127c83jab163e1860fc9aa1@mail.gmail.com> <448b15030909160922o84c2d6gc8ead8226dd8777a@mail.gmail.com> <4AB1ED1F.1010203@ct.jp.nec.com> <4AB1FDEE.5020500@ce.jp.nec.com> <20090917131113.GA8163@redhat.com> <20090918150650.GA28070@redhat.com> Reply-To: device-mapper development Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <20090918150650.GA28070@redhat.com> List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: dm-devel-bounces@redhat.com Errors-To: dm-devel-bounces@redhat.com To: Mike Snitzer , "Martin K. Petersen" Cc: device-mapper development , Alasdair Kergon , Jens Axboe List-Id: dm-devel.ids Mike Snitzer wrote: > On Fri, Sep 18 2009 at 2:00am -0400, > Martin K. Petersen wrote: > >>>>>>> "Mike" == Mike Snitzer writes: >>>>> blk_set_default_limits(limits); >>>>> + limits->max_sectors = 0; >>>>> + limits->max_hw_sectors = 0; >> Mike> Seems like we may want some common variant in block even though >> Mike> I'm not aware of other block drivers that would benefit... >> >> Mike> But I'll defer to Martin and/or Jens on whether these helpers are >> Mike> fine to stay in dm-table.c or should be worked into blk-settings.c >> >> In the pre-topology days we set max_sectors to SAFE_MAX_SECTORS upon >> creation of a queue. This is an old ATA-ism that's been around for a >> ages. >> >> Ideally we'd simply nuke it and drivers that really needed to lower the >> bar would explicitly call blk_queue_max_sectors(). However, I'm afraid >> to change the default because I'm sure there are legacy drivers lurking >> somewhere that depend on it. >> >> Seeing as blk_set_default_limits() is mostly aimed at stacking drivers I >> think I'd prefer moving SAFE_MAX_SECTORS back to blk_queue_make_request >> and then set max_sectors and max_hw_sectors to 0 in default_limits. >> >> Would that work for you guys? > > So you're referring to fact that this commit removed > blk_queue_max_sectors(q, SAFE_MAX_SECTORS) from blk_queue_make_request: > http://git.kernel.org/linus/e475bba2 > > I think I like your proposal. But, to clarify things further, are you > saying: > > By moving SAFE_MAX_SECTORS back to blk_queue_make_request (after its > existing call to blk_set_default_limits right?) and having > blk_set_default_limits set max_sectors and max_hw_sectors to 0: > > DM will be free to establish the proper limit stacking because the DM > limits are not derived from the queue's default limits? Because the DM > device limits are just stacked and copied to the queue, some background > for those following along: > > DM's actual stacking of limits takes place when the DM table is > translated to the DM device's final queue (at table resume time), see: > http://git.kernel.org/linus/754c5fc7e > > drivers/md/dm.c:dm_swap_table() calls dm_calculate_queue_limits() to > stack the limits. > > drivers/md/dm.c:__bind() sets the DM device's queue_limits via > dm_table_set_restrictions() > > drivers/md/dm-table.c:dm_table_set_restrictions() simply copies the > queue_limits established by DM's stacking with: > /* > * Copy table's limits to the DM device's request_queue > */ > q->limits = *limits; > > Now coming full circle: > AFAIK the only piece I'm missing is how/where your proposed changes will > account for the need to establish SAFE_MAX_SECTORS _after_ the stacking > of queue_limits: IFF max_sectors and max_hw_sectors are still 0 (like > Jun'ichi did in DM with the 2nd patch posted). > > But I don't pretend to have this all sorted out in my head. I could > easily be missing some other piece(s) implicit in your proposal. > > Maybe an RFC patch that illustrates your thinking would help further this > discussion? I just sent out revised patchset: [PATCH 1/2] dm: Set safe default max_sectors for targets with no underlying device https://www.redhat.com/archives/dm-devel/2009-September/msg00203.html [PATCH 2/2] block: blk_set_default_limits sets 0 to max_sectors https://www.redhat.com/archives/dm-devel/2009-September/msg00205.html But I wonder better fix might be to provide blk_queue_copy_limits() to replace this in dm-table.c: > q->limits = *limits; where blk_queue_copy_limits() looks like this: void blk_queue_copy_limits(struct request_queue *q, struct queue_limits *lim) { q->limits = *limits; /* fix-up bad values */ if (q->limits.max_sectors == 0 || q->limits.max_hw_sectors == 0) blk_queue_max_sectors(q, SAFE_MAX_SECTORS); } so that block/blk-settings.c has full-control on default value and dm don't need to care about the magic 'SAFE_MAX_SECTORS'. Thanks, -- Jun'ichi Nomura, NEC Corporation