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

[Mikulas Patocka]

On Thu, 18 Aug 2016, Eric Wheeler wrote:

> > On Wed, Jun 01 2016 at  9:44am -0400, Christoph Hellwig 
> >  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.

Mikulas
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