linux-kernel - Re: [RFC V2 14/18] famfs_fuse: GET_DAXDEV message and daxdev

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Message-ID: <oolcpxrjdzrkqnmj4xvcymnyb6ovdt7np7trxlgndniqe35l3s@ru5adqnjxexh>
Date: Fri, 15 Aug 2025 11:38:02 -0500
From: John Groves <John@...ves.net>
To: Miklos Szeredi <miklos@...redi.hu>
Cc: 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>, 
	"Darrick J . Wong" <djwong@...nel.org>, Randy Dunlap <rdunlap@...radead.org>, 
	Jeff Layton <jlayton@...nel.org>, Kent Overstreet <kent.overstreet@...ux.dev>, 
	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>, john@...ves.net
Subject: Re: [RFC V2 14/18] famfs_fuse: GET_DAXDEV message and daxdev_table

On 25/08/14 03:58PM, Miklos Szeredi wrote:
> On Thu, 3 Jul 2025 at 20:54, John Groves <John@...ves.net> wrote:
> >
> > * The new GET_DAXDEV message/response is enabled
> > * The command it triggered by the update_daxdev_table() call, if there
> >   are any daxdevs in the subject fmap that are not represented in the
> >   daxdev_dable yet.
> 
> This is rather convoluted, the server *should know* which dax devices
> it has registered, hence it shouldn't need to be explicitly asked.

That's not impossible, but it's also a bit harder than the current
approach for the famfs user space - which I think would need to become
stateful as to which daxdevs had been pushed into the kernel. The
famfs user space is as unstateful as possible ;)

> 
> And there's already an API for registering file descriptors:
> FUSE_DEV_IOC_BACKING_OPEN.  Is there a reason that interface couldn't
> be used by famfs?

FUSE_DEV_IOC_BACKING_OPEN looks pretty specific to passthrough. The
procedure for opening a daxdev is stolen from the way xfs does it in 
fs-dax mode. It calls fs_dax_get() rather then open(), and passes in 
'famfs_fuse_dax_holder_ops' to 1) effect exclusivity, and 2) receive
callbacks in the event of memory errors. See famfs_fuse_get_daxdev()...

A *similar* ioctl could be added to push in a daxdev, but that would
still add statefulness that isn't required in the current implementation.
I didn't go there because there are so few IOCTLs currently (the overall 
model is more 'pull' than 'push').

Keep in mind that the baseline case with famfs will be files that are 
interleaved across strips from many daxdevs. We commonly create files 
that are striped across 16 daxdevs, selected at random from a
significantly larger pool. Because interleaving is essential to memory 
performance...

There is no "device mapper" analog for memory, so famfs really does 
have to span daxdevs. As Darrick commented somewhere, famfs fmaps do 
repeating patterns well (i.e. striping)...

I think there is a certain elegance to the current approach, but
if you feel strongly I will change it.

Thanks!
John