[dm-devel] dm-crypt: Fix error with too large bios

[Eric Wheeler]

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 at infradead.org> 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