linux-kernel - Re: [RFC] block: Make MMC respect REQ

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Message-ID: <20250508055951.GA28277@lst.de>
Date: Thu, 8 May 2025 07:59:51 +0200
From: Christoph Hellwig <hch@....de>
To: Ahmad Fatoum <a.fatoum@...gutronix.de>
Cc: "Darrick J. Wong" <djwong@...nel.org>, Christoph Hellwig <hch@....de>,
	Jens Axboe <axboe@...nel.dk>, Ulf Hansson <ulf.hansson@...aro.org>,
	Jan Lübbe <jlu@...gutronix.de>,
	"kernel@...gutronix.de" <kernel@...gutronix.de>,
	linux-block@...r.kernel.org, linux-mmc@...r.kernel.org,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [RFC] block: Make MMC respect REQ_NOUNMAP?

On Wed, May 07, 2025 at 06:24:34PM +0200, Ahmad Fatoum wrote:
>     blk_ioctl_zeroout
>         blkdev_issue_zeroout(..., BLKDEV_ZERO_NOUNMAP)
>             __blkdev_issue_write_zeroes(..., BLKDEV_ZERO_NOUNMAP)
> 	    submit_bio_wait(bio->bi_opf = REQ_NOUNMAP)
> 
> __REQ_NOUNMAP has comment saying 'do not free blocks when zeroing',
> but as shown above, TRIM allows the card to unmap the indicated region.

__REQ_NOUNMAP is a hint.  All the storage specs are a mess in this
area as they usually get the polarity wrong and/or just have very
vague semantics.  Also actually writing zeroes and not unmapping is
totally pointless on flash storage as it just increases PE cycles
for no good reason.