From: Dan Smith <danms@xxxxxxxxxx> Subject: Re: [PATCH 1/2] Add userspace device-mapper target Date: Mon, 19 Feb 2007 07:16:29 -0800 > I've got a hacked-up version running that allows userspace to provide > a pointer to a buffer to be written after the original bio completes. Great. > This seems to work pretty well, but I haven't added support to the > library or my cow application, so I'm not sure how the performance > differs. > > There is something that needs to be resolved, however. Currently, > bio_map_user() assumes it is being run in process context. Since we > defer to a kthread for all ring buffer processing, we can't easily > construct the extra bios. So, there are two paths forward that I can > think of: > > 1. Redesign how we process messages going to the kernel. Userspace > could mark them as "tentatively ready" and then do the write() > which just goes through and constructs extra bios and marks them > "ready". This seems undesirable to me, because we're increasing > the amount of time that userspace is blocked in the kernel. Yeah, it's not the best way for the performance perspective. But your original code used this logic, didn't it? I guess that the code doesn't give notable performance difference even if dmu_ctl_write checks new requests and calls bio_map_user if necessary. > 2. Add a bio_map_user_from() call to the kernel, which behaves just > like bio_map_user(), but takes a struct task struct pointer to the > process to map from, thus allowing the kthread to construct the > bios asynchronously. I have a patch cooked up to do this, but I'm > worried that it might be rather controversial. This path isn't easy, -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
