Re: dm-crypt: Fix error with too large bios

[Mike Snitzer]

On Wed, Aug 31 2016 at  2:26am -0400,
Milan Broz  wrote:

> On 08/31/2016 12:27 AM, Mikulas Patocka wrote:
> 
> ...
> > 
> > Drop that "#ifdef CONFIG_BCACHE". Anyone should be allowed to create a big 
> > bio, not just bcache.
> 
> Yes. Please, do not hide it behind #ifdef.
> If it is in code, it should be enabled always.
> 
> There can third party modules or some new code appears and creating strange
> config dependence only adds more problems later.

I did last night, see:
https://git.kernel.org/cgit/linux/kernel/git/device-mapper/linux-dm.git/commit/?h=dm-4.8=4e870e948fbabf62b78e8410f04c67703e7c816b

Will go to Linus for v4.8-rc5 by the end of the week.
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Re: dm-crypt: Fix error with too large bios

[Milan Broz]

On 08/31/2016 12:27 AM, Mikulas Patocka wrote:

...
> 
> Drop that "#ifdef CONFIG_BCACHE". Anyone should be allowed to create a big 
> bio, not just bcache.

Yes. Please, do not hide it behind #ifdef.
If it is in code, it should be enabled always.

There can third party modules or some new code appears and creating strange
config dependence only adds more problems later.

Milan

> 
> That one test has no performance impact, there is no need to hide it 
> behind #ifdef.
> 
> Mikulas
> 
>> From: Mikulas Patocka <mpato...@redhat.com>
>> Date: Tue, 30 Aug 2016 16:38:42 -0400
>> Subject: [PATCH] dm crypt: fix error with too large bcache bios
>>
>> When dm-crypt processes writes, it allocates a new bio in
>> crypt_alloc_buffer().  The bio is allocated from a bio set and it can
>> have at most BIO_MAX_PAGES vector entries, however the incoming bio can be
>> larger if it was allocated by bcache.  If the incoming bio is larger,
>> bio_alloc_bioset() fails and an error is returned.
>>
>> To avoid the error, we test for a too large bio in the function
>> crypt_map() and use dm_accept_partial_bio() to split the bio.
>> dm_accept_partial_bio() trims the current bio to the desired size and
>> asks DM core to send another bio with the rest of the data.
>>
>> This fix is wrapped with a check for CONFIG_BCACHE because there
>> currently isn't any other code that generates too large bios.  So unless
>> bcache is configured there is no point wasting time making this check.
>>
>> Signed-off-by: Mikulas Patocka 
>> Cc: sta...@vger.kernel.org # v3.16+
>> ---
>>  drivers/md/dm-crypt.c | 6 ++
>>  1 file changed, 6 insertions(+)
>>
>> diff --git a/drivers/md/dm-crypt.c b/drivers/md/dm-crypt.c
>> index eedba67..743f548 100644
>> --- a/drivers/md/dm-crypt.c
>> +++ b/drivers/md/dm-crypt.c
>> @@ -1924,6 +1924,12 @@ static int crypt_map(struct dm_target *ti, struct bio 
>> *bio)
>>  return DM_MAPIO_REMAPPED;
>>  }
>>  
>> +#ifdef CONFIG_BCACHE
>> +if (unlikely(bio->bi_iter.bi_size > (BIO_MAX_PAGES << PAGE_SHIFT)) &&
>> +bio_data_dir(bio) == WRITE)
>> +dm_accept_partial_bio(bio, ((BIO_MAX_PAGES << PAGE_SHIFT) >> 
>> SECTOR_SHIFT));
>> +#endif
>> +
>>  io = dm_per_bio_data(bio, cc->per_bio_data_size);
>>  crypt_io_init(io, cc, bio, dm_target_offset(ti, 
>> bio->bi_iter.bi_sector));
>>  io->ctx.req = (struct skcipher_request *)(io + 1);
>> -- 
>> 2.7.4 (Apple Git-66)
>>
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Re: dm-crypt: Fix error with too large bios

[Mikulas Patocka]

On Tue, 30 Aug 2016, Mike Snitzer wrote:

> On Tue, Aug 30 2016 at  8:19P -0400,
> Mikulas Patocka <mpato...@redhat.com> wrote:
> 
> > 
> > 
> > On Tue, 30 Aug 2016, Ming Lei wrote:
> > 
> > > On Tue, Aug 30, 2016 at 5:57 AM, Mikulas Patocka <mpato...@redhat.com> 
> > > wrote:
> > > >
> > > >
> > > > On Mon, 29 Aug 2016, Ming Lei wrote:
> > > >
> > > >> On Sat, Aug 27, 2016 at 11:09 PM, Mikulas Patocka 
> > > >> <mpato...@redhat.com> wrote:
> > > >> >
> > > >> > But this patch won't work for device mapper, blk_bio_segment_split is
> > > >> > called from blk_queue_split and device mapper doesn't use 
> > > >> > blk_queue_split
> > > >> > (it can't because it frees queue->bio_split).
> > > >> >
> > > >> > We still need these two patches:
> > > >> > https://www.redhat.com/archives/dm-devel/2016-May/msg00211.html
> > > >> > https://www.redhat.com/archives/dm-devel/2016-May/msg00210.html
> > > >>
> > > >> About the 2nd patch, it might not be good enough because in theory
> > > >> a small size bio still may include big bvecs, such as, each bvec points
> > > >> to 512byte buffer, so strictly speaking the bvec number should
> > > >> be checked instead of bio size.
> > > >>
> > > >> Ming Lei
> > > >
> > > > This is not a problem.
> > > 
> > > I meant the following code in your 2nd patch:
> > > 
> > > + if (unlikely(bio->bi_iter.bi_size > BIO_MAX_SIZE) &&
> > > +(bio->bi_rw & (REQ_FLUSH | REQ_DISCARD | REQ_WRITE)) == REQ_WRITE)
> > > + dm_accept_partial_bio(bio, BIO_MAX_SIZE >> SECTOR_SHIFT);
> > > 
> > > And the check on .bi_size may not work.
> > 
> > kcryptd_crypt_write_convert calls:
> > crypt_alloc_buffer(io, io->base_bio->bi_iter.bi_size)
> > 
> > crypt_alloc_buffer does:
> > unsigned int nr_iovecs = (size + PAGE_SIZE - 1) >> PAGE_SHIFT;
> > clone = bio_alloc_bioset(GFP_NOIO, nr_iovecs, cc->bs);
> > 
> > So, if io->base_bio->bi_iter.bi_size <= BIO_MAX_SIZE, then nr_iovecs will 
> > be less or equal than BIO_MAX_PAGES and the function bio_alloc_bioset will 
> > succeed.
> > 
> > (BTW. BIO_MAX_SIZE was removed in the current kernel, we should use 
> > (BIO_MAX_PAGES << PAGE_SHIFT) instead).
> 
> Is this revised patch OK with you?

Drop that "#ifdef CONFIG_BCACHE". Anyone should be allowed to create a big 
bio, not just bcache.

That one test has no performance impact, there is no need to hide it 
behind #ifdef.

Mikulas

> From: Mikulas Patocka <mpato...@redhat.com>
> Date: Tue, 30 Aug 2016 16:38:42 -0400
> Subject: [PATCH] dm crypt: fix error with too large bcache bios
> 
> When dm-crypt processes writes, it allocates a new bio in
> crypt_alloc_buffer().  The bio is allocated from a bio set and it can
> have at most BIO_MAX_PAGES vector entries, however the incoming bio can be
> larger if it was allocated by bcache.  If the incoming bio is larger,
> bio_alloc_bioset() fails and an error is returned.
> 
> To avoid the error, we test for a too large bio in the function
> crypt_map() and use dm_accept_partial_bio() to split the bio.
> dm_accept_partial_bio() trims the current bio to the desired size and
> asks DM core to send another bio with the rest of the data.
> 
> This fix is wrapped with a check for CONFIG_BCACHE because there
> currently isn't any other code that generates too large bios.  So unless
> bcache is configured there is no point wasting time making this check.
> 
> Signed-off-by: Mikulas Patocka 
> Cc: sta...@vger.kernel.org # v3.16+
> ---
>  drivers/md/dm-crypt.c | 6 ++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/drivers/md/dm-crypt.c b/drivers/md/dm-crypt.c
> index eedba67..743f548 100644
> --- a/drivers/md/dm-crypt.c
> +++ b/drivers/md/dm-crypt.c
> @@ -1924,6 +1924,12 @@ static int crypt_map(struct dm_target *ti, struct bio 
> *bio)
>   return DM_MAPIO_REMAPPED;
>   }
>  
> +#ifdef CONFIG_BCACHE
> + if (unlikely(bio->bi_iter.bi_size > (BIO_MAX_PAGES << PAGE_SHIFT)) &&
> + bio_data_dir(bio) == WRITE)
> + dm_accept_partial_bio(bio, ((BIO_MAX_PAGES << PAGE_SHIFT) >> 
> SECTOR_SHIFT));
> +#endif
> +
>   io = dm_per_bio_data(bio, cc->per_bio_data_size);
>   crypt_io_init(io, cc, bio, dm_target_offset(ti, 
> bio->bi_iter.bi_sector));
>   io->ctx.req = (struct skcipher_request *)(io + 1);
> -- 
> 2.7.4 (Apple Git-66)
> 
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Re: dm-crypt: Fix error with too large bios

[Mikulas Patocka]

On Mon, 29 Aug 2016, Mike Snitzer wrote:

> On Sat, Aug 27 2016 at 11:09am -0400,
> Mikulas Patocka  wrote:
> 
> > 
> > 
> > On Fri, 26 Aug 2016, Mike Snitzer wrote:
> > 
> > > On Thu, Aug 25 2016 at  4:13pm -0400,
> > > Jens Axboe  wrote:
> > > 
> > > > On 08/25/2016 12:34 PM, Mikulas Patocka wrote:
> > > > >
> > > > >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.
> > > 
> > > See linux-block commit:
> > > 4d70dca4eadf2f block: make sure a big bio is split into at most 256 bvecs
> > > http://git.kernel.dk/cgit/linux-block/commit/?h=for-linus=4d70dca4eadf2f95abe389116ac02b8439c2d16c
> > 
> > But this patch won't work for device mapper, blk_bio_segment_split is 
> > called from blk_queue_split and device mapper doesn't use blk_queue_split 
> > (it can't because it frees queue->bio_split).
> > 
> > We still need these two patches:
> > https://www.redhat.com/archives/dm-devel/2016-May/msg00211.html
> > https://www.redhat.com/archives/dm-devel/2016-May/msg00210.html
> 
> I'm left wondering whether it'd be better to revert commit dbba42d8a9e
> ("dm: eliminate unused "bioset" process for each bio-based DM device")

That would create one more process per dm device.

There is no need to create another process if the bio can be split in the 
device mapper.

> And also look for other areas of DM that could leverage the block core's
> relatively new splitting capabilities.
> 
> Otherwise, we'll just be sprinking more BIO_MAX_PAGES-based splitting
> code in DM targets because DM is so "special" that it doesn't need the
> block core's splitting (even though it clearly would now benefit).

It would be possible to split bios on BIO_MAX_PAGES boundary in the dm 
core. But I checked all the dm targets and I found problem only in 
dm-crypt and dm-log-writes. So there is no need to split bio bios for the 
other targets.

> Mike

Mikulas
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Re: dm-crypt: Fix error with too large bios

[Mike Snitzer]

On Sat, Aug 27 2016 at 11:09am -0400,
Mikulas Patocka  wrote:

> 
> 
> On Fri, 26 Aug 2016, Mike Snitzer wrote:
> 
> > On Thu, Aug 25 2016 at  4:13pm -0400,
> > Jens Axboe  wrote:
> > 
> > > On 08/25/2016 12:34 PM, Mikulas Patocka wrote:
> > > >
> > > >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.
> > 
> > See linux-block commit:
> > 4d70dca4eadf2f block: make sure a big bio is split into at most 256 bvecs
> > http://git.kernel.dk/cgit/linux-block/commit/?h=for-linus=4d70dca4eadf2f95abe389116ac02b8439c2d16c
> 
> But this patch won't work for device mapper, blk_bio_segment_split is 
> called from blk_queue_split and device mapper doesn't use blk_queue_split 
> (it can't because it frees queue->bio_split).
> 
> We still need these two patches:
> https://www.redhat.com/archives/dm-devel/2016-May/msg00211.html
> https://www.redhat.com/archives/dm-devel/2016-May/msg00210.html

I'm left wondering whether it'd be better to revert commit dbba42d8a9e
("dm: eliminate unused "bioset" process for each bio-based DM device")

And also look for other areas of DM that could leverage the block core's
relatively new splitting capabilities.

Otherwise, we'll just be sprinking more BIO_MAX_PAGES-based splitting
code in DM targets because DM is so "special" that it doesn't need the
block core's splitting (even though it clearly would now benefit).

Mike
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Re: dm-crypt: Fix error with too large bios

[Mike Snitzer]

On Thu, Aug 25 2016 at  4:13pm -0400,
Jens Axboe  wrote:

> On 08/25/2016 12:34 PM, Mikulas Patocka wrote:
> >
> >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.

See linux-block commit:
4d70dca4eadf2f block: make sure a big bio is split into at most 256 bvecs
http://git.kernel.dk/cgit/linux-block/commit/?h=for-linus=4d70dca4eadf2f95abe389116ac02b8439c2d16c
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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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