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

[Jens Axboe]

On 08/25/2016 12:34 PM, Mikulas Patocka wrote:
>
>
> On Thu, 18 Aug 2016, Eric Wheeler wrote:
>
>>> 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
>
> Device mapper can't split the bio in generic_make_request - it frees the
> md->queue->bio_split bioset, to save one kernel thread per device. Device
> mapper uses its own splitting mechanism.
>
> So what is the final decision? - should device mapper split the big bio or
> should bcache not submit big bios?
>
> I think splitting big bios in the device mapper is better - simply because
> it is much less code than reworking bcache to split bios internally.
>
> BTW. In the device mapper, we have a layer dm-io, that was created to work
> around bio size limitations - it accepts unlimited I/O request and splits
> it to several bios. When bio size limitations are gone, we could simplify
> dm-io too.

The patch from Ming Lei was applied for 4.8 the other day.

-- 
Jens Axboe