linux-kernel - Re: [PATCH] scsi: sd: Keep the discard mode stable

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Date: Mon, 17 Jun 2024 17:03:03 +0800
From: Li Feng <fengli@...rtx.com>
To: Christoph Hellwig <hch@...radead.org>
Cc: "James E.J. Bottomley" <James.Bottomley@...senpartnership.com>,
 linux-kernel <linux-kernel@...r.kernel.org>,
 "linux-scsi@...r.kernel.org" <linux-scsi@...r.kernel.org>,
 "Martin K. Petersen" <martin.petersen@...cle.com>
Subject: Re: [PATCH] scsi: sd: Keep the discard mode stable

> 2024年6月17日 14:17，Christoph Hellwig <hch@...radead.org> 写道：
> 
> On Sat, Jun 15, 2024 at 12:03:47AM +0800, Li Feng wrote:
>> + /*
>> + * When the discard mode has been set to UNMAP, it should not be set to
> 
> Overly long line here.
OK.

> 
>> + * WRITE SAME with UNMAP.
>> + */
>> + if (!sdkp->max_unmap_blocks)
>> + sd_config_discard(sdkp, SD_LBP_WS16);
> 
> But more importantly this doesn't really scale to all the variations
> of reported / guessed at probe time vs overriden.  I think you just
> need an explicit override flag that skips the discard settings.
> 
I think we only need to prevent the temporary change of discard mode 
from UNMAP to WS16, and this patch should be enough.

Maybe it is a good idea to remove the call to sd_config_discard 
from read_capacity_16 . Because the unmap_alignment/ unmap_granularity
used by sd_config_discard are assigned in sd_read_block_limits. 

sd_read_block_limits is enough to negotiate the discard parameter. 
It is redundant for read_capacity to modify the discard parameter.  In this way, 
when the SCSI probe sends read_capacity first and then read block limits, 
it avoids the change of discard from DISABLE to WS16 to UNMAP.

Thanks,
Li