On 2011-04-18 09:25, NeilBrown wrote: > On Mon, 18 Apr 2011 08:38:24 +0200 Jens Axboe <jaxboe@xxxxxxxxxxxx> wrote: > >> On 2011-04-18 00:19, NeilBrown wrote: >>> On Mon, 11 Apr 2011 14:11:58 +0200 Jens Axboe <jaxboe@xxxxxxxxxxxx> wrote: >>> >>>>> Yes. But I need to know when to release the requests that I have stored. >>>>> I need to know when ->write_pages or ->read_pages or whatever has finished >>>>> submitting a pile of pages so that I can start processing the request that I >>>>> have put aside. So I need a callback from blk_finish_plug. >>>> >>>> OK fair enough, I'll add your callback patch. >>>> >>> >>> But you didn't did you? You added a completely different patch which is >>> completely pointless. >>> If you don't like my patch I would really prefer you said so rather than >>> silently replace it with something completely different (and broken). >> >> First of all, you were CC'ed on all that discussion, yet didn't speak up >> until now. This was last week. Secondly, please change your tone. > > Yes, I was CC'ed on a discussion. In that discussion it was never mentioned > that you had completely changed the patch I sent you, and it never contained > the new patch in-line for review. Nothing that was discussed was > particularly relevant to md's needs so there was nothing to speak up about. > > Yes- there were 'git pull' requests and I could have done a pull myself to > review the code but there seemed to be no urgency because you had already > agreed to apply my patch. > When I did finally pull the patches (after all the other issues had settle > down and I had time to finish of the RAID side) I found ... what I found. > > I apologise for my tone, but I was very frustrated. > >> >>> I'll try to explain again. >>> >>> md does not use __make_request. At all. >>> md does not use 'struct request'. At all. >>> >>> The 'list' in 'struct blk_plug' is a list of 'struct request'. >> >> I'm well aware of how these facts, but thanks for bringing it up. >> >>> Therefore md cannot put anything useful on the list in 'struct blk_plug'. >>> >>> So when blk_flush_plug_list calls queue_unplugged() on a queue that belonged >>> to a request found on the blk_plug list, that queue cannot possibly ever be >>> for an 'md' device (because no 'struct request' ever belongs to an md device, >>> because md doesn't not use 'struct request'). >>> >>> So your patch (commit f75664570d8b) doesn't help MD at all. >>> >>> For md, I need to attach something to blk_plug which somehow identifies an md >>> device, so that blk_finish_plug can get to that device and let it unplug. >>> The most sensible thing to have is a completely generic callback. That way >>> different block devices (which choose not to use __make_request) can attach >>> different sorts of things to blk_plug. >>> >>> So can we please have my original patch applied? (Revised version using >>> list_splice_init included below). >>> >>> Or if not, a clear explanation of why not? >> >> So correct me if I'm wrong here, but the _only_ real difference between >> this patch and the current code in the tree, is the checking of the >> callback list indicating a need to flush the callbacks. And that's >> definitely an oversight. It should be functionally equivelant if md >> would just flag this need to get a callback, eg instead of queueing a >> callback on the list, just set plug->need_unplug from md instead of >> queuing a callback and have blk_needs_flush_plug() do: >> >> return plug && (!list_empty(&plug->list) || plug->need_unplug); >> >> instead. Something like the below, completely untested. >> > > No, that is not the only real difference. > > The real difference is that in the current code, md has no way to register > anything with a blk_plug because you can only register a 'struct request' on a > blk_plug, and md doesn't make any use of 'struct request'. > > As I said in the Email you quote above: > >>> Therefore md cannot put anything useful on the list in 'struct blk_plug'. > > That is the heart of the problem. Hmm, I don't really see a way to avoid the list in that case. You really do need some way to queue items, a single callback or flag or pointer will not suffice. I've added the patch and removed the (now) useless ->unplugged_fn callback. I suggest you base your md changes on top of my for-linus branch and tell me when you are confident it looks good, then I'll pull in your MD changes and submit them later today. OK with you? -- Jens Axboe -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
