linux-kernel - Re: [RFC PATCH 13/19] famfs_fuse: Create files with famfs fmaps

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Message-ID: <CAJfpegsKf8Zog3Q6Vd1kBmD6anLSdyYyxy4BjD-dvcyWOyr4QQ@mail.gmail.com>
Date: Fri, 16 May 2025 12:06:44 +0200
From: Miklos Szeredi <miklos@...redi.hu>
To: "Darrick J. Wong" <djwong@...nel.org>
Cc: John Groves <John@...ves.net>, Dan Williams <dan.j.williams@...el.com>, 
	Bernd Schubert <bschubert@....com>, John Groves <jgroves@...ron.com>, Jonathan Corbet <corbet@....net>, 
	Vishal Verma <vishal.l.verma@...el.com>, Dave Jiang <dave.jiang@...el.com>, 
	Matthew Wilcox <willy@...radead.org>, Jan Kara <jack@...e.cz>, 
	Alexander Viro <viro@...iv.linux.org.uk>, Christian Brauner <brauner@...nel.org>, 
	Luis Henriques <luis@...lia.com>, Randy Dunlap <rdunlap@...radead.org>, 
	Jeff Layton <jlayton@...nel.org>, Kent Overstreet <kent.overstreet@...ux.dev>, 
	Petr Vorel <pvorel@...e.cz>, Brian Foster <bfoster@...hat.com>, linux-doc@...r.kernel.org, 
	linux-kernel@...r.kernel.org, nvdimm@...ts.linux.dev, 
	linux-cxl@...r.kernel.org, linux-fsdevel@...r.kernel.org, 
	Amir Goldstein <amir73il@...il.com>, Jonathan Cameron <Jonathan.Cameron@...wei.com>, 
	Stefan Hajnoczi <shajnocz@...hat.com>, Joanne Koong <joannelkoong@...il.com>, 
	Josef Bacik <josef@...icpanda.com>, Aravind Ramesh <arramesh@...ron.com>, 
	Ajay Joshi <ajayjoshi@...ron.com>
Subject: Re: [RFC PATCH 13/19] famfs_fuse: Create files with famfs fmaps

On Thu, 15 May 2025 at 04:06, Darrick J. Wong <djwong@...nel.org> wrote:

> Yeah, it's confusing.  The design doc tries to clarify this, but this is
> roughly what we need for fuse:
>
> FUSE_IOMAP_OP_WRITE being set means we're writing to the file.
> FUSE_IOMAP_OP_ZERO being set means we're zeroing the file.
> Neither of those being set means we're reading the file.
>
> (3 different operations)

Okay, I get why these need to be distinct cases.

Am I right that the only read is sanely cacheable?

> FUSE_IOMAP_OP_DIRECT being set means directio, and it not being set
> means pagecache.
>
> (and one flag, for 6 different types of IO)

Why does this make a difference?

Okay, maybe I can imagine difference allocation strategies.  Which
means that it only matters for the write case?

> FUSE_IOMAP_OP_REPORT is set all by itself for things like FIEMAP and
> SEEK_DATA/HOLE.

Which should again always be the same as the read case, no?

Thanks,
Miklos