Re: [PATCH] mmc: core: don't set limits.discard_granularity as 0

On 1/10/20 9:29 am, Coly Li wrote:

On 2020/10/1 14:14, Adrian Hunter wrote:

quoted

On 1/10/20 7:36 am, Coly Li wrote:

quoted

On 2020/10/1 01:23, Adrian Hunter wrote:

quoted

On 30/09/20 7:08 pm, Coly Li wrote:

quoted

In mmc_queue_setup_discard() the mmc driver queue's discard_granularity
might be set as 0 (when card->pref_erase > max_discard) while the mmc
device still declares to support discard operation. This is buggy and
triggered the following kernel warning message,

WARNING: CPU: 0 PID: 135 at __blkdev_issue_discard+0x200/0x294
CPU: 0 PID: 135 Comm: f2fs_discard-17 Not tainted 5.9.0-rc6 #1
Hardware name: Google Kevin (DT)
pstate: 00000005 (nzcv daif -PAN -UAO BTYPE=--)
pc : __blkdev_issue_discard+0x200/0x294
lr : __blkdev_issue_discard+0x54/0x294
sp : ffff800011dd3b10
x29: ffff800011dd3b10 x28: 0000000000000000 x27: ffff800011dd3cc4 x26: ffff800011dd3e18 x25: 000000000004e69b x24: 0000000000000c40 x23: ffff0000f1deaaf0 x22: ffff0000f2849200 x21: 00000000002734d8 x20: 0000000000000008 x19: 0000000000000000 x18: 0000000000000000 x17: 0000000000000000 x16: 0000000000000000 x15: 0000000000000000 x14: 0000000000000394 x13: 0000000000000000 x12: 0000000000000000 x11: 0000000000000000 x10: 00000000000008b0 x9 : ffff800011dd3cb0 x8 : 000000000004e69b x7 : 0000000000000000 x6 : ffff0000f1926400 x5 : ffff0000f1940800 x4 : 0000000000000000 x3 : 0000000000000c40 x2 : 0000000000000008 x1 : 00000000002734d8 x0 : 0000000000000000 Call trace:
__blkdev_issue_discard+0x200/0x294
__submit_discard_cmd+0x128/0x374
__issue_discard_cmd_orderly+0x188/0x244
__issue_discard_cmd+0x2e8/0x33c
issue_discard_thread+0xe8/0x2f0
kthread+0x11c/0x120
ret_from_fork+0x10/0x1c
---[ end trace e4c8023d33dfe77a ]---

This patch fixes the issue by setting discard_granularity as SECTOR_SIZE
instead of 0 when (card->pref_erase > max_discard) is true. Now no more
complain from __blkdev_issue_discard() for the improper value of discard
granularity.

Fixes: commit e056a1b5b67b ("mmc: queue: let host controllers specify maximum discard timeout")

That "Fixes" tag is a bit misleading.  For some time, the block layer had
no problem with discard_granularity of zero, and blk_bio_discard_split()
still doesn't (see below).

static struct bio *blk_bio_discard_split(struct request_queue *q,
					 struct bio *bio,
					 struct bio_set *bs,
					 unsigned *nsegs)
{
	unsigned int max_discard_sectors, granularity;
	int alignment;
	sector_t tmp;
	unsigned split_sectors;

	*nsegs = 1;

	/* Zero-sector (unknown) and one-sector granularities are the same.  */
	granularity = max(q->limits.discard_granularity >> 9, 1U);

quoted

From Documentation/block/queue-sysfs.rst, the discard_granularity is

described as,

discard_granularity (RO)
------------------------
This shows the size of internal allocation of the device in bytes, if
reported by the device. A value of '0' means device does not support
the discard functionality.


And from Documentation/ABI/testing/sysfs-block, the discard_granularity
is described as,

What:           /sys/block/<disk>/queue/discard_granularity
Date:           May 2011
Contact:        Martin K. Petersen [off-list ref]
Description:
                Devices that support discard functionality may
                internally allocate space using units that are bigger
                than the logical block size. The discard_granularity
                parameter indicates the size of the internal allocation
                unit in bytes if reported by the device. Otherwise the
                discard_granularity will be set to match the device's
                physical block size. A discard_granularity of 0 means
                that the device does not support discard functionality.


Therefore I took it as a bug when a driver sets its queue
discard_granularity as 0 but still announces to support discard operation.

But if you don't like the Fixes: tag, it is OK for me to remove it in
next version.

Not at all.  I just wrote "a bit misleading" because people might also want
to know from what patch things stopped working.

Oh maybe I understand you. Yes, although this fixed patch was bug, but
the warning was triggered since the new discard alignment changes got
merged.

Hmm, maybe I should add the Fixes tag to commit b35fd7422c2f ("block:
check queue's limits.discard_granularity in __blkdev_issue_discard()").

How do you think of this commit id ?

Yes that could be mentioned in the commit message or Fixes or both.
With that:

Acked-by: Adrian Hunter <adrian.hunter@intel.com>