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SubjectRe: [PATCH] mmc: core: don't set limits.discard_granularity as 0
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On 1/10/20 7:36 am, Coly Li wrote:
> On 2020/10/1 01:23, Adrian Hunter wrote:
>> On 30/09/20 7:08 pm, Coly Li wrote:
>>> 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);
>>
>
>>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 <martin.petersen@oracle.com>
> 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.

>
> (CC Martin because he is the origin of the above information)
>
> Thanks.
>
> Coly Li
>

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