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

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Date:   Thu, 1 Oct 2020 12:36:25 +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 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.

(CC Martin because he is the origin of the above information)

Thanks.

Coly Li