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

Jens Axboe
2018-11-15T01:51:52Z

On 11/14/18 6:06 PM, 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.

null_blk has discard support, might be an easier target in terms
of blktests.

--
Jens Axboe