From: Eric Wheeler git@linux.ewheeler.net
According to drbd.conf documentation, "To not break established and expected behaviour, and suddenly cause fstrim on thin-provisioned LVs to run out-of-space instead of freeing up space, the default value is yes."
This behavior regressed in the REQ_OP_WRITE_ZEROES refactor near 45c21793 drbd: implement REQ_OP_WRITE_ZEROES 0dbed96 drbd: make intelligent use of blkdev_issue_zeroout which caused dm-thin backed DRBD volumes to zero blocks and run out of space instead of passing discard to the backing device as defined by the discard_zeroes_if_aligned option.
A helper function could reduce code duplication.
Signed-off-by: Eric Wheeler drbd@linux.ewheeler.net Cc: stable@vger.kernel.org # 4.14 --- drivers/block/drbd/drbd_receiver.c | 22 ++++++++++++++++++++-- drivers/block/drbd/drbd_req.c | 23 +++++++++++++++++++++-- 2 files changed, 41 insertions(+), 4 deletions(-)
diff --git a/drivers/block/drbd/drbd_receiver.c b/drivers/block/drbd/drbd_receiver.c index 62a902f..58f0e43 100644 --- a/drivers/block/drbd/drbd_receiver.c +++ b/drivers/block/drbd/drbd_receiver.c @@ -1507,9 +1507,27 @@ void drbd_bump_write_ordering(struct drbd_resource *resource, struct drbd_backin static void drbd_issue_peer_discard(struct drbd_device *device, struct drbd_peer_request *peer_req) { struct block_device *bdev = device->ldev->backing_bdev; + struct disk_conf *dc; + bool discard_zeroes_if_aligned, zeroout;
- if (blkdev_issue_zeroout(bdev, peer_req->i.sector, peer_req->i.size >> 9, - GFP_NOIO, 0)) + rcu_read_lock(); + dc = rcu_dereference(device->ldev->disk_conf); + discard_zeroes_if_aligned = dc->discard_zeroes_if_aligned; + rcu_read_unlock(); + + /* Use zeroout unless discard_zeroes_if_aligned is set. + * If blkdev_issue_discard fails, then retry with blkdev_issue_zeroout. + * See also drbd_process_discard_req() in drbd_req.c. + */ + zeroout = true; + if (discard_zeroes_if_aligned && + blkdev_issue_discard(bdev, peer_req->i.sector, + peer_req->i.size >> 9, GFP_NOIO, 0) == 0) + zeroout = false; + + if (zeroout && + blkdev_issue_zeroout(bdev, peer_req->i.sector, + peer_req->i.size >> 9, GFP_NOIO, 0) != 0) peer_req->flags |= EE_WAS_ERROR;
drbd_endio_write_sec_final(peer_req); diff --git a/drivers/block/drbd/drbd_req.c b/drivers/block/drbd/drbd_req.c index de8566e..070b5e7 100644 --- a/drivers/block/drbd/drbd_req.c +++ b/drivers/block/drbd/drbd_req.c @@ -1158,9 +1158,28 @@ static int drbd_process_write_request(struct drbd_request *req) static void drbd_process_discard_req(struct drbd_request *req) { struct block_device *bdev = req->device->ldev->backing_bdev; + struct drbd_device *device = req->device; + struct disk_conf *dc; + bool discard_zeroes_if_aligned, zeroout;
- if (blkdev_issue_zeroout(bdev, req->i.sector, req->i.size >> 9, - GFP_NOIO, 0)) + rcu_read_lock(); + dc = rcu_dereference(device->ldev->disk_conf); + discard_zeroes_if_aligned = dc->discard_zeroes_if_aligned; + rcu_read_unlock(); + + /* Use zeroout unless discard_zeroes_if_aligned is set. + * If blkdev_issue_discard fails, then retry with blkdev_issue_zeroout. + * See also drbd_issue_peer_discard() in drbd_receiver.c. + */ + zeroout = true; + if (discard_zeroes_if_aligned && + blkdev_issue_discard(bdev, req->i.sector, req->i.size >> 9, + GFP_NOIO, 0) == 0) + zeroout = false; + + if (zeroout && + blkdev_issue_zeroout(bdev, req->i.sector, req->i.size >> 9, + GFP_NOIO, 0) != 0) req->private_bio->bi_status = BLK_STS_IOERR; bio_endio(req->private_bio); }
NAK. Calling a discard and expecting zeroing is simply buggy.
And double NAK for patches like this without a linux-block Cc.
On Mon, Jan 15, 2018 at 11:26:15PM -0800, Christoph Hellwig wrote:
NAK. Calling a discard and expecting zeroing is simply buggy.
What he said.
The bug/misunderstanding was that we now use zeroout even for discards, with the assumption that it would try to do discards where it can.
Which is even true.
But our expectations of when zeroout "should" use unmap, and where it actually can do that safely based on the information it has, don't really match: our expectations where wrong, we assumed too much. (in the general case).
Which means in DRBD we have to stop use zeroout for discards, and again pass down normal discard for discards.
In the specific case where the backend to DRBD is lvm-thin, AND it does de-alloc blocks on discard, AND it does NOT have skip_block_zeroing set or it's backend does zero on discard and it does discard passdown, and we tell DRBD about all of that (using the discard_zeroes_if_aligned flag) then we can do this "zero head and tail, discard full blocks", and expect them to be zero.
If skip_block_zeroing is set however, and there is no discard passdown in thin, or the backend of thin does not zero on discard, DRBD can still pass down discards to thin, and expect them do be de-allocated, but can not expect discarded ranges to remain "zero", any later partial write to an unallocated area could pull in different "undefined" garbage on different replicas for the not-written-to part of a new allocated block.
The end result is that you have areas of the block device that return different data depending on which replica you read from.
But that is the case even eithout discard in that setup, don't do that then or live with it.
"undefined data" is undefined, you have that directly on thin in that setup already, with DRBD on top you now have several versions of "undefined".
Anything on top of such a setup must not do "read-modify-write" of "undefined" data and expect a defined result, adding DRBD on top does not change that.
DRBD can deal with that just fine, but our "online verify" will report loads of boring "mismatches" for those areas.
TL;DR: we'll stop doing "discard-is-zeroout" (our assumptions were wrong). We still won't do exactly "discard-is-discard", but re-enable our "discard-is-discard plus zeroout on head and tail", because in some relevant setups, this gives us the best result, and avoids the false positives in our online-verify.
Lars
On Mon, 15 Jan 2018, Christoph Hellwig wrote:
NAK. Calling a discard and expecting zeroing is simply buggy.
But of course, that would be silly.
We don't expect discard to zero---but we do expect discard to discard!
And double NAK for patches like this without a linux-block Cc.
My appologies, I thought this was internal to DRBD.
What is the general rule here?
Should linux-block always be Cc'ed with a patch?
-- Eric Wheeler
On Tue, 16 Jan 2018, Lars Ellenberg wrote:
On Mon, Jan 15, 2018 at 11:26:15PM -0800, Christoph Hellwig wrote:
NAK. Calling a discard and expecting zeroing is simply buggy.
What he said.
The bug/misunderstanding was that we now use zeroout even for discards, with the assumption that it would try to do discards where it can.
Which is even true.
But our expectations of when zeroout "should" use unmap, and where it actually can do that safely based on the information it has, don't really match: our expectations where wrong, we assumed too much. (in the general case).
Which means in DRBD we have to stop use zeroout for discards, and again pass down normal discard for discards.
In the specific case where the backend to DRBD is lvm-thin, AND it does de-alloc blocks on discard, AND it does NOT have skip_block_zeroing set or it's backend does zero on discard and it does discard passdown, and we tell DRBD about all of that (using the discard_zeroes_if_aligned flag) then we can do this "zero head and tail, discard full blocks", and expect them to be zero.
If skip_block_zeroing is set however, and there is no discard passdown in thin, or the backend of thin does not zero on discard, DRBD can still pass down discards to thin, and expect them do be de-allocated, but can not expect discarded ranges to remain "zero", any later partial write to an unallocated area could pull in different "undefined" garbage on different replicas for the not-written-to part of a new allocated block.
The end result is that you have areas of the block device that return different data depending on which replica you read from.
But that is the case even eithout discard in that setup, don't do that then or live with it.
"undefined data" is undefined, you have that directly on thin in that setup already, with DRBD on top you now have several versions of "undefined".
Anything on top of such a setup must not do "read-modify-write" of "undefined" data and expect a defined result, adding DRBD on top does not change that.
DRBD can deal with that just fine, but our "online verify" will report loads of boring "mismatches" for those areas.
TL;DR: we'll stop doing "discard-is-zeroout" (our assumptions were wrong). We still won't do exactly "discard-is-discard", but re-enable our "discard-is-discard plus zeroout on head and tail", because in some relevant setups, this gives us the best result, and avoids the false positives in our online-verify.
Lars
Hi Lars,
We just tried 4.19.x and this bugs still exists. We applied the patch which was originally submitted to this thread and it still applies cleanly and seems to work for our use case. You mentioned that you had some older code which zeroed out unaligned discard requests (or perhaps it was for a different purpose) that you may be able to use to patch this. Could you dig those up and see if we can get this solved?
It would be nice to be able to use drbd with thin backing volumes from the vanilla kernel. If this has already been fixed in something newer than 4.19, then please point me to the commit.
Thank you for your help!
-- Eric Wheeler
linux-stable-mirror@lists.linaro.org
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Christoph Hellwig -
Eric Wheeler -
Lars Ellenberg