linux-kernel - Re: [RFC PATCH -next v3 01/10] block: introduce BLK_FEAT_WRITE_ZEROES

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Message-ID: <Z_d_VDvgBkgt4UhS@kbusch-mbp>
Date: Thu, 10 Apr 2025 09:20:36 +0100
From: Keith Busch <kbusch@...nel.org>
To: Christoph Hellwig <hch@....de>
Cc: Zhang Yi <yi.zhang@...weicloud.com>, linux-fsdevel@...r.kernel.org,
	linux-ext4@...r.kernel.org, linux-block@...r.kernel.org,
	dm-devel@...ts.linux.dev, linux-nvme@...ts.infradead.org,
	linux-scsi@...r.kernel.org, linux-xfs@...r.kernel.org,
	linux-kernel@...r.kernel.org, tytso@....edu, djwong@...nel.org,
	john.g.garry@...cle.com, bmarzins@...hat.com, chaitanyak@...dia.com,
	shinichiro.kawasaki@....com, yi.zhang@...wei.com,
	chengzhihao1@...wei.com, yukuai3@...wei.com, yangerkun@...wei.com
Subject: Re: [RFC PATCH -next v3 01/10] block: introduce
 BLK_FEAT_WRITE_ZEROES_UNMAP to queue limits features

On Thu, Apr 10, 2025 at 09:15:59AM +0200, Christoph Hellwig wrote:
> On Thu, Apr 10, 2025 at 11:52:17AM +0800, Zhang Yi wrote:
> > 
> > Thank you for your review and comments. However, I'm not sure I fully
> > understand your points. Could you please provide more details?
> > 
> > AFAIK, the NVMe protocol has the following description in the latest
> > NVM Command Set Specification Figure 82 and Figure 114:
> > 
> > ===
> > Deallocate (DEAC): If this bit is set to `1´, then the host is
> > requesting that the controller deallocate the specified logical blocks.
> > If this bit is cleared to `0´, then the host is not requesting that
> > the controller deallocate the specified logical blocks...
> > 
> > DLFEAT:
> > Write Zeroes Deallocation Support (WZDS): If this bit is set to `1´,
> > then the controller supports the Deallocate bit in the Write Zeroes
> > command for this namespace...
> 
> Yes.  The host is requesting, not the controller shall.  It's not
> guaranteed behavior and the controller might as well actually write
> zeroes to the media.  That is rather stupid, but still.

I guess some controllers _really_ want specific alignments to
successfully do a proper discard. While still not guaranteed in spec, I
think it is safe to assume a proper deallocation will occur if you align
to NPDA and NPDG. Otherwise, the controller may do a read-modify-write
to ensure zeroes are returned for the requested LBA range on anything
that straddles an implementation specific boundary.