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

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

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.

Thanks,
Ming