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

[Ming Lei <ming.lei@redhat.com>]

On Thu, Nov 15, 2018 at 12:22:01PM +1100, Dave Chinner wrote:
> 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...

Indeed, I can reproduce this issue via the following commands:

modprobe scsi_debug virtual_gb=2049 sector_size=512 lbpws10=1 dev_size_mb=512
blkdiscard /dev/sde

> 
> 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.

write_same/write_zeros are usually used by driver directly, so we
may need make the test case on some specific device.

Thanks,
Ming