Re: dm-log-writes: fix bug with too large bios

[Mike Snitzer <snitzer@redhat.com>]

On Wed, Jun 01 2016 at 12:23am -0400,
James Johnston <johnstonj.public@codenest.com> wrote:

> Hi Mikulas,
> 
> > bio_alloc can allocate a bio with at most BIO_MAX_PAGES (256) vector
> > entries. However, the incoming bio may have more vector entries if it was
> > allocated by other means. For example, bcache submits bios with more than
> > BIO_MAX_PAGES entries. This results in bio_alloc failure.
> > 
> > To avoid the failure, change the code so that it allocates bio with at
> > most BIO_MAX_PAGES entries. If the incoming bio has more entries,
> > bio_add_page will fail and a new bio will be allocated - the code that
> > handles bio_add_page failure already exists in the dm-log-writes target.
> > 
> > Also, move atomic_inc(&lc->io_blocks) before bio_alloc to fix a bug that
> > the target hangs if bio_alloc fails. The error path does put_io_block(lc),
> > so we must do atomic_inc(&lc->io_blocks) before invoking the error path to
> > avoid underflow of lc->io_blocks.
> > 
> > Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
> > Cc: stable@vger.kernel.org	# v4.1+
> 
> How does this relate to the previous patch you made to dm-crypt?  How best
> should I test this?  It looks like the dm-crypt patch fixed the problem.
> 
> Should I test by applying this patch ONLY and reverting the dm-crypt patch?
> (i.e. does this patch also fix the problem.)  Or should I just test with
> both patches applied simultaneously?

The dm-log-writes patch has nothing to do with the dm-crypt patch.  It
is just that both targets have comparable issues with bcache issuing
really large bios.