Re: [PATCH] block: fix 32 bit overflow in __blkdev_issue_discard()

Dave Chinner
2018-11-15T01:22:06Z

On Thu, Nov 15, 2018 at 09:06:52AM +0800, Ming Lei wrote:
> On Wed, Nov 14, 2018 at 08:18:24AM -0700, Jens Axboe wrote:
> > On 11/13/18 2:43 PM, Dave Chinner wrote:
> > > From: Dave Chinner <dchinner@redhat.com>
> > >
> > > A discard cleanup merged into 4.20-rc2 causes fstests xfs/259 to
> > > fall into an endless loop in the discard code. The test is creating
> > > a device that is exactly 2^32 sectors in size to test mkfs boundary
> > > conditions around the 32 bit sector overflow region.
> > >
> > > mkfs issues a discard for the entire device size by default, and
> > > hence this throws a sector count of 2^32 into
> > > blkdev_issue_discard(). It takes the number of sectors to discard as
> > > a sector_t - a 64 bit value.
> > >
> > > The commit ba5d73851e71 ("block: cleanup __blkdev_issue_discard")
> > > takes this sector count and casts it to a 32 bit value before
> > > comapring it against the maximum allowed discard size the device
> > > has. This truncates away the upper 32 bits, and so if the lower 32
> > > bits of the sector count is zero, it starts issuing discards of
> > > length 0. This causes the code to fall into an endless loop, issuing
> > > a zero length discards over and over again on the same sector.
> >
> > Applied, thanks. Ming, can you please add a blktests test for
> > this case? This is the 2nd time it's been broken.
>
> OK, I will add zram discard test in blktests, which should cover the
> 1st report. For the xfs/259, I need to investigate if it is easy to
> do in blktests.

Just write a test that creates block devices of 2^32 + (-1,0,1)
sectors and runs a discard across the entire device. That's all that
xfs/259 it doing - exercising mkfs on 2TB, 4TB and 16TB boundaries.
i.e. the boundaries where sectors and page cache indexes (on 4k page
size systems) overflow 32 bit int and unsigned int sizes. mkfs
issues a discard for the entire device, so it's testing that as
well...

You need to write tests that exercise write_same, write_zeros and
discard operations around these boundaries, because they all take
a 64 bit sector count and stuff them into 32 bit size fields in
the bio tha tis being submitted.

Cheers,

Dave.
--
Dave Chinner
david@fromorbit.com