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

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Message-ID: <3cc59ace-d4b5-d10b-05a0-8677c95515ff@intel.com>
Date:   Thu, 1 Oct 2020 09:14:13 +0300
From:   Adrian Hunter <adrian.hunter@...el.com>
To:     Coly Li <colyli@...e.de>
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 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.

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