On Mon, 15 Aug 2016, Mikulas Patocka wrote: > On Sat, 13 Aug 2016, Mike Snitzer wrote: > > > [top-posting just because others went wild with it] > > > > I don't have a strong opinion but I just assumed the local dm-crypt > > workaround wasn't the way forward. I didn't stage it because Christoph > > disagreed with it: > > https://lkml.org/lkml/2016/6/1/456 > > https://lkml.org/lkml/2016/6/1/477 > > > > Also, this would appear to be a more generic fix: > > "block: make sure big bio is splitted into at most 256 bvecs > > https://lkml.org/lkml/2016/8/12/154 > > (but Christoph disagrees there too, so the way forward isn't clear) > > > > Mike > > > On Wed, Jun 01 2016 at 9:44am -0400, Christoph Hellwig <hch@xxxxxxxxxxxxx> wrote: > > > > > be dm-crypt.c. Maybe you've identified some indirect use of > > > > BIO_MAX_SIZE? > > > > > > I mean the recently introduced BIO_MAX_SIZE in -next tree: > > > > > https://git.kernel.org/cgit/linux/kernel/git/next/linux-next.git/commit/drivers/md/dm-crypt.c?id=4ed89c97b0706477b822ea2182827640c0cec486 > > > > The crazy bcache bios striking back once again. I really think it's > > harmful having a _MAX value and then having a minor driver > > reinterpreting it and sending larger ones. Until we can lift the > > maximum limit in general nad have common code exercise it we really need > > to stop bcache from sending these instead of littering the tree with > > workarounds. > > The bio_kmalloc function allocates bios with up to 1024 vector entries (as > opposed to bio_alloc and bio_alloc_bioset that has a limit of 256 vector > entries). > > Device mapper is using bio_alloc_bioset with a bio set, so it is limited > to 256 vector entries, but other kernel users may use bio_kmalloc and > create larger bios. > > So, if you don't want bios with more than 256 vector entries to exist, you > should impose this limit in bio_kmalloc (and fix all the callers that use > it). FYI, Kent Overstreet notes this about bcache from the other thread here: https://lkml.org/lkml/2016/8/15/620 [paste] >> bcache originally had workaround code to split too-large bios when it >> first went upstream - that was dropped only after the patches to make >> generic_make_request() handle arbitrary size bios went in. So to do what >> you're suggesting would mean reverting that bcache patch and bringing that >> code back, which from my perspective would be a step in the wrong >> direction. I just want to get this over and done with. >> >> re: interactions with other drivers - bio_clone() has already been changed >> to only clone biovecs that are live for current bi_iter, so there >> shouldn't be any safety issues. A driver would have to be intentionally >> doing its own open coded bio cloning that clones all of bi_io_vec, not >> just the active ones - but if they're doing that, they're already broken >> because a driver isn't allowed to look at bi_vcnt if it isn't a bio that >> it owns - bi_vcnt is 0 on bios that don't own their biovec (i.e. that were >> created by bio_clone_fast). >> >> And the cloning and bi_vcnt usage stuff I audited very thoroughly back >> when I was working on immutable biovecs and such back in the day, and I >> had to do a fair amount of cleanup/refactoring before that stuff could go >> in. [/paste] They are making progress in the patch-v3 thread, so perhaps this can be fixed for now in generic_make_request(). -- Eric Wheeler -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
