linux-kernel - Re: [PATCH] mmc: core: don't set limits.discard

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Date:   Thu, 1 Oct 2020 14:29:50 +0800
From:   Coly Li <colyli@...e.de>
To:     Adrian Hunter <adrian.hunter@...el.com>
Cc:     linux-mmc@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-block@...r.kernel.org, Vicente Bergas <vicencb@...il.com>,
        Ulf Hansson <ulf.hansson@...aro.org>,
        "Martin K. Petersen" <martin.petersen@...cle.com>
Subject: Re: [PATCH] mmc: core: don't set limits.discard_granularity as 0

On 2020/10/1 14:14, Adrian Hunter wrote:
> 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@...cle.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.

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 ?

Thanks.

Coly Li