linux-kernel - Re: [PATCH v2 2/7] iomap: Add zero unwritten mappings dio support

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Message-ID: <20250109075412.GA19081@lst.de>
Date: Thu, 9 Jan 2025 08:54:12 +0100
From: Christoph Hellwig <hch@....de>
To: "Darrick J. Wong" <djwong@...nel.org>
Cc: John Garry <john.g.garry@...cle.com>, Christoph Hellwig <hch@....de>,
	brauner@...nel.org, cem@...nel.org, dchinner@...hat.com,
	ritesh.list@...il.com, linux-xfs@...r.kernel.org,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	martin.petersen@...cle.com
Subject: Re: [PATCH v2 2/7] iomap: Add zero unwritten mappings dio support

On Tue, Jan 07, 2025 at 05:26:36PM -0800, Darrick J. Wong wrote:
> "I think we should wire it up as a new FALLOC_FL_WRITE_ZEROES mode,
> document very vigorously that it exists to facilitate pure overwrites
> (specifically that it returns EOPNOTSUPP for always-cow files), and not
> add more ioctls."
> 
> If we added this new fallocate mode to set up written mappings, would it
> be enough to write in the programming manuals that applications should
> use it to prepare a file for block-untorn writes?  Perhaps we should
> change the errno code to EMEDIUMTYPE for the mixed mappings case.
> 
> Alternately, maybe we /should/ let programs open a lease-fd on a file
> range, do their untorn writes through the lease fd, and if another
> thread does something to break the lease, then the lease fd returns EIO
> until you close it.

This still violates the "no unexpected errors" paradigm.  The whole
FALLOC_FL_WRITE_ZEROES (I hate that name btw) model would only work
if we had a software fallback that make the operations slower but
still work in case of an unexpected change to the extent mapping.