block: fix 32 bit overflow in __blkdev_issue_discard()

From: Dave Chinner <[email protected]>

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.

Fixes ba5d73851e71 ("block: cleanup __blkdev_issue_discard")

Signed-off-by Dave Chinner <[email protected]>

---
 block/blk-lib.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

block/blk-lib.c +4 -1

index e8b3bb9bf375..144e156ed341 100644
@@ -55,9 +55,12 @@ int __blkdev_issue_discard(struct block_device *bdev, sector_t sector,
 		return -EINVAL;
 
 	while (nr_sects) {
-		unsigned int req_sects = min_t(unsigned int, nr_sects,
+		sector_t req_sects = min_t(sector_t, nr_sects,
 				bio_allowed_max_sectors(q));
 
+		WARN_ON_ONCE(req_sects == 0);
+		WARN_ON_ONCE((req_sects << 9) > UINT_MAX);
+
 		bio = blk_next_bio(bio, 0, gfp_mask);
 		bio->bi_iter.bi_sector = sector;
 		bio_set_dev(bio, bdev);

Signature · 1 line

2.19.1

From: Dave Chinner <[email protected]>

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.

Fixes ba5d73851e71 ("block: cleanup __blkdev_issue_discard")

Signed-off-by Dave Chinner <[email protected]>

---
 block/blk-lib.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

block/blk-lib.c +0 -0

index e8b3bb9bf375..144e156ed341 100644

Quoted 32 lines · Dave Chinner

From: Dave Chinner <[email protected]>

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.

Fixes ba5d73851e71 ("block: cleanup __blkdev_issue_discard")

Signed-off-by Dave Chinner <[email protected]>

---
 block/blk-lib.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

block/blk-lib.c +0 -0

index e8b3bb9bf375..144e156ed341 100644

block/blk-lib.c int __blkdev_issue_discard(struct block_device *bdev, sector_t sector, · line 59

Quoted 6 lines

block/blk-lib.c +1 -1

@@ -55,9 +55,12 @@ int __blkdev_issue_discard(struct block_device *bdev, sector_t sector,
                return -EINVAL;
 
        while (nr_sects) {
-               unsigned int req_sects = min_t(unsigned int, nr_sects,
+               sector_t req_sects = min_t(sector_t, nr_sects,
                                bio_allowed_max_sectors(q));

bio_allowed_max_sectors(q) is always < UINT_MAX, and 'sector_t' is only required during the comparison, so another simpler fix might be the following, could you test if it is workable?

block/blk-lib.c +0 -0

index e8b3bb9bf375..6ef44f99e83f 100644

block/blk-lib.c int __blkdev_issue_discard(struct block_device *bdev, sector_t sector, · line 60

Quoted 2 lines

block/blk-lib.c +1 -0

+               WARN_ON_ONCE(req_sects == 0);

The above line isn't necessary given 'nr_sects' can't be zero.

Quoted 52 lines · Ming Lei

Quoted 32 lines · Dave Chinner

From: Dave Chinner <[email protected]>

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.

Fixes ba5d73851e71 ("block: cleanup __blkdev_issue_discard")

Signed-off-by Dave Chinner <[email protected]>

---
 block/blk-lib.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

block/blk-lib.c +0 -0

index e8b3bb9bf375..144e156ed341 100644

block/blk-lib.c int __blkdev_issue_discard(struct block_device *bdev, sector_t sector, · line 59

Quoted 6 lines

block/blk-lib.c +1 -1

@@ -55,9 +55,12 @@ int __blkdev_issue_discard(struct block_device *bdev, sector_t sector,
                return -EINVAL;
 
        while (nr_sects) {
-               unsigned int req_sects = min_t(unsigned int, nr_sects,
+               sector_t req_sects = min_t(sector_t, nr_sects,
                                bio_allowed_max_sectors(q));

bio_allowed_max_sectors(q) is always < UINT_MAX, and 'sector_t' is only required during the comparison, so another simpler fix might be the following, could you test if it is workable?

block/blk-lib.c +0 -0

index e8b3bb9bf375..6ef44f99e83f 100644
@@ -55,7 +55,7 @@ int __blkdev_issue_discard(struct block_device
 *bdev, sector_t sector,
                return -EINVAL;
 

block/blk-lib.c int __blkdev_issue_discard(struct block_device · line 60

Quoted 4 lines

block/blk-lib.c +1 -1

        while (nr_sects) {
-               unsigned int req_sects = min_t(unsigned int, nr_sects,
+               unsigned int req_sects = min_t(sector_t, nr_sects,
                                bio_allowed_max_sectors(q));

Rearrange the deck chairs all you like, just make sure you fix your regression test suite to exercise obvious boundary conditions like this so the next cleanup doesn't break the code again.

Quoted 5 lines

block/blk-lib.c int __blkdev_issue_discard(struct block_device *bdev, sector_t sector, · line 60

Quoted 3 lines

block/blk-lib.c +1 -0

+               WARN_ON_ONCE(req_sects == 0);

The above line isn't necessary given 'nr_sects' can't be zero.

Except it was 0 and it caused the bug I had to fix. So it should have a warning on it.

From: Dave Chinner <[email protected]>

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.

Quoted 12 lines · Dave Chinner

From: Dave Chinner <[email protected]>

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.

Quoted 7 lines

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.

Quoted 21 lines · Jens Axboe

Quoted 19 lines · Dave Chinner

From: Dave Chinner <[email protected]>

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.

Quoted 2 lines

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.

Quoted 22 lines · Ming Lei

Quoted 21 lines · Jens Axboe

Quoted 19 lines · Dave Chinner

From: Dave Chinner <[email protected]>

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.

Quoted 6 lines

Quoted 2 lines

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.

Quoted 30 lines · Dave Chinner

Quoted 22 lines · Ming Lei

Quoted 21 lines · Jens Axboe

Quoted 19 lines · Dave Chinner

From: Dave Chinner <[email protected]>

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.

Quoted 6 lines

Quoted 2 lines

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.

Quoted 7 lines

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

Quoted 1 line

Quoted 4 lines

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.

Quoted 50 lines · Ming Lei

Quoted 30 lines · Dave Chinner

Quoted 22 lines · Ming Lei

Quoted 21 lines · Jens Axboe

Quoted 19 lines · Dave Chinner

From: Dave Chinner <[email protected]>

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.

Quoted 6 lines

Quoted 2 lines

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.

Quoted 7 lines

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

Quoted 5 lines

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.

Quoted 2 lines

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

My local linux iscsi server and client advertise support for them. It definitely does not ships zeros across the wire(*) when I use things like FALLOC_FL_ZERO_RANGE, but fstests does not have block device fallocate() tests for zeroing or punching...

Cheers,

Dave.

(*) but the back end storage is a sparse file on an XFS filesystem, and the iscsi server fails to translate write_zeroes or WRITE_SAME(0) to FALLOC_FL_ZERO_RANGE on the storage side and hence is really slow because it physically writes zeros to the XFS file. i.e. the client offloads the operation to the server to minimise wire traffic, but then the server doesn't offload the operation to the storage....

Cheers,

Dave.

Signature · 2 lines

Dave Chinner
[email protected]

Quoted 54 lines · Dave Chinner

Quoted 52 lines · Ming Lei

Quoted 30 lines · Dave Chinner

Quoted 22 lines · Ming Lei

Quoted 21 lines · Jens Axboe

Quoted 19 lines · Dave Chinner

From: Dave Chinner <[email protected]>

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.

Quoted 6 lines

Quoted 2 lines

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.

Quoted 7 lines

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

Quoted 5 lines

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.

Quoted 4 lines

My local linux iscsi server and client advertise support for them. It definitely does not ships zeros across the wire(*) when I use things like FALLOC_FL_ZERO_RANGE, but fstests does not have block device fallocate() tests for zeroing or punching...

fstests does (generic/{349,350,351}) but those basic functionality tests don't include creating a 2^32 block device and seeing if overflows happen... :/

They don't run on my test machines because they require a modular kernel and I run a monolithic kernel specified externally by the qemu command line on all my test VMs.

generic/349     [not run] scsi_debug module not found
generic/350     [not run] scsi_debug module not found
generic/351     [not run] scsi_debug module not found

Quoted 7 lines · Dave Chinner

They don't run on my test machines because they require a modular kernel and I run a monolithic kernel specified externally by the qemu command line on all my test VMs.

generic/349     [not run] scsi_debug module not found
generic/350     [not run] scsi_debug module not found
generic/351     [not run] scsi_debug module not found

Same here, btw. Any test that requires modules is a rather bad idea.

Quoted 5 lines

Quoted 3 lines

generic/349     [not run] scsi_debug module not found
generic/350     [not run] scsi_debug module not found
generic/351     [not run] scsi_debug module not found

Same here, btw. Any test that requires modules is a rather bad idea.

I'll plug my vm.py script that supports running a kernel build with modules without installing them into the VM (by mounting the modules over 9p): https://github.com/osandov/osandov-linux#vm-setup

Quoted 55 lines · Darrick J. Wong

Quoted 54 lines · Dave Chinner

Quoted 52 lines · Ming Lei

Quoted 30 lines · Dave Chinner

Quoted 22 lines · Ming Lei

Quoted 21 lines · Jens Axboe

Quoted 19 lines · Dave Chinner

From: Dave Chinner <[email protected]>

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.

Quoted 6 lines

Quoted 2 lines

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.

Quoted 7 lines

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

Quoted 5 lines

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.

Quoted 8 lines

Quoted 4 lines

My local linux iscsi server and client advertise support for them. It definitely does not ships zeros across the wire(*) when I use things like FALLOC_FL_ZERO_RANGE, but fstests does not have block device fallocate() tests for zeroing or punching...

fstests does (generic/{349,350,351}) but those basic functionality tests don't include creating a 2^32 block device and seeing if overflows happen... :/

They don't run on my test machines because they require a modular kernel and I run a monolithic kernel specified externally by the qemu command line on all my test VMs.

generic/349     [not run] scsi_debug module not found
generic/350     [not run] scsi_debug module not found
generic/351     [not run] scsi_debug module not found

Quoted 21 lines · Jens Axboe

Quoted 19 lines · Dave Chinner

From: Dave Chinner <[email protected]>

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.

Quoted 2 lines

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.