Re: dm-bufio: adjust the reserved buffer for dm-verify-target.

[Xiao, Jin]

On 8/15/2018 4:32 AM, Mike Snitzer wrote:
> On Wed, Aug 08 2018 at  2:40am -0400,
> xiao jin <jin.xiao@xxxxxxxxx> wrote:
>
> > We hit the BUG() report at include/linux/scatterlist.h:144!
> > The callback is as bellow:
> >    => verity_work
> >    => verity_hash_for_block
> >    => verity_verify_level
> >    => verity_hash
> >    => verity_hash_update
> >    => sg_init_one
> >    => sg_set_buf
> >
> > More debug shows the root cause. When creating dufio client it
> > uses the __vmalloc() to allocate the buffer data for the reserved
> > dm_buffer. The buffer that allocated by the __vmalloc() is invalid
> > according to the __virt_addr_valid().
> >
> > Mostly the reserved dm_buffer is not touched. But occasionally
> > it might fail to allocate the dm_buffer data when we try to
> > allocate in the __alloc_buffer_wait_no_callback(). Then it has
> > to take the reserved dm_buffer for usage. Finally it reports the
> > BUG() as virt_addr_valid() detects the buffer data address is invalid.
> >
> > The patch is to adjust the reserved buffer for dm-verity-target. We
> > allocated two dm_buffers into the reserved buffers list when creating
> > the buffer interface. The first dm_buffer in the reserved buffer list
> > is allocated by the __vmalloc(), it's not used after that. The second
> > dm_buffer in the reserved buffer list is allocated by the
> > __get_free_pages() which can be consumed after that.
> >
> > Signed-off-by: xiao jin <jin.xiao@xxxxxxxxx>
> > ---
> >   drivers/md/dm-bufio.c         | 4 ++--
> >   drivers/md/dm-verity-target.c | 2 +-
> >   2 files changed, 3 insertions(+), 3 deletions(-)
> >
> > diff --git a/drivers/md/dm-bufio.c b/drivers/md/dm-bufio.c
> > index dc385b7..3b7ca5e 100644
> > --- a/drivers/md/dm-bufio.c
> > +++ b/drivers/md/dm-bufio.c
> > @@ -841,7 +841,7 @@ static struct dm_buffer *__alloc_buffer_wait_no_callback(struct dm_bufio_client
> >   			tried_noio_alloc = true;
> >   		}
> >   
> > -		if (!list_empty(&c->reserved_buffers)) {
> > +		if (!c->need_reserved_buffers) {
> >   			b = list_entry(c->reserved_buffers.next,
> >   				       struct dm_buffer, lru_list);
> >   			list_del(&b->lru_list);
> > @@ -1701,7 +1701,7 @@ struct dm_bufio_client *dm_bufio_client_create(struct block_device *bdev, unsign
> >   		goto bad;
> >   	}
> >   
> > -	while (c->need_reserved_buffers) {
> > +	if (list_empty(&c->reserved_buffers)) {
> >   		struct dm_buffer *b = alloc_buffer(c, GFP_KERNEL);
> >   
> >   		if (!b) {
> Point was to allocate N buffers (as accounted in
> c->need_reserved_buffers).  This change just allocates a single one.
> Why?
>
> Your header isn't clear on this at all.

Hi Mike,

Currently alloc_buffer() when creating the client will use the 
__vmalloc() to

get the buffer data for c->reserved_buffers. If the c->reserved_buffers 
is read to

use in the failures case of buffer allocation in the 
__alloc_buffer_wait_no_callback(),

and the CONFIG_DEBUG_SG is enabled, we will hit the BUG() report.

That's the problem I find in reality.


I have some thinking to solve such issue. I think to keep the initial 
buffer with the

data from __vmalloc() in the c->reserved_buffers. But the reserved 
buffer with the data

from __vmalloc() can't be read to use. We can allocate more buffers with the

data mode of DATA_MODE_SLAB or DATA_MODE_GET_FREE_PAGES for 
c->reserved_buffers.

Such reserved buffers can be used in the failures case of buffer allocation

in the __alloc_buffer_wait_no_callback().


I test the code on my device. I never see the BUG() report again. Feel 
free to correct me.


Thanks.


Jin

> > diff --git a/drivers/md/dm-verity-target.c b/drivers/md/dm-verity-target.c
> > index 12decdbd7..40c66fc 100644
> > --- a/drivers/md/dm-verity-target.c
> > +++ b/drivers/md/dm-verity-target.c
> > @@ -1107,7 +1107,7 @@ static int verity_ctr(struct dm_target *ti, unsigned argc, char **argv)
> >   	v->hash_blocks = hash_position;
> >   
> >   	v->bufio = dm_bufio_client_create(v->hash_dev->bdev,
> > -		1 << v->hash_dev_block_bits, 1, sizeof(struct buffer_aux),
> > +		1 << v->hash_dev_block_bits, 2, sizeof(struct buffer_aux),
> >   		dm_bufio_alloc_callback, NULL);
> >   	if (IS_ERR(v->bufio)) {
> >   		ti->error = "Cannot initialize dm-bufio";
> > --
> > 2.7.4
> >
> > --
> > dm-devel mailing list
> > dm-devel@xxxxxxxxxx
> > https://www.redhat.com/mailman/listinfo/dm-devel
> It isn't at all clear from my initial review that what you're doing
> makes any sense.
>
> Seems like you're just papering over bufio's use of !__virt_addr_valid()
> memory in unintuitive ways.
>
> Mikulas, can you see a better way forward?
>
> Mike