linux-kernel - Re: [PATCH] fuse: Add support for fuse stacked I/O

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Message-ID: <CA+55aFyFLEkAthR=aUV2noQspgpZ8ascNCCftNBdEGaLvi199A@mail.gmail.com>
Date:	Fri, 15 Jan 2016 13:53:34 -0800
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Nikhilesh Reddy <reddyn@...eaurora.org>,
	Nikolaus Rath <nikolaus@...h.org>,
	fuse-devel <fuse-devel@...ts.sourceforge.net>,
	Al Viro <viro@...iv.linux.org.uk>,
	Greg KH <gregkh@...uxfoundation.org>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	Linux API <linux-api@...r.kernel.org>, Jan Kara <jack@...e.cz>,
	"Theodore Ts'o" <tytso@....edu>, sven.utcke@....de,
	Miklos Szeredi <miklos@...redi.hu>,
	Richard Weinberger <richard@....at>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Antonio SJ Musumeci <trapexit@...wn.link>
Subject: Re: [PATCH] fuse: Add support for fuse stacked I/O

On Fri, Jan 15, 2016 at 1:46 PM, Andy Lutomirski <luto@...capital.net> wrote:

> If mmap sets vm_file to the underlying thing, wouldn't CRIU and
> anything else that uses map_files get confused?  Or did you have
> something else in mind?

Why would they care?

Also, I don't think you actually need to change vm_file - we have this
whole notion of "inode->i_data" vs "inode->i_mapping".

So I think you could set the "i_mapping" of the fuse inode to be the
i_mapping of the passed-through-to inode, and it should just work.

And no, I didn't look into this very deeply, and maybe there is some
annoying detail that would make that not work well. But that's part of
the whole point of the i_mapping indirection: so that you can share
the page cache when you have two separate anchor points. I think coda
uses it for the local caching, and block devices use it to not have
mapping aliases between different inodex that all are the same block
device.

                 Linus