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

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Date:	Fri, 15 Jan 2016 13:46:00 -0800
From:	Andy Lutomirski <luto@...capital.net>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
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 1:43 PM, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
> On Fri, Jan 15, 2016 at 11:29 AM, Nikhilesh Reddy <reddyn@...eaurora.org> wrote:
>>
>> FUSE allows users to implement extensions to filesystems ..such as enforcing policy or permissions without having to modify the kernel or maintain the policy in the kernel.
>>
>> One such example is what was quoted by Antonio above ..
>> Another example is a fuse based filesystem that tries to enforce additional permissions on a FAT based mount point.
>>
>> From what i could google there are many FUSE based filesystems out there that do things during the open call but simply pass through the read/and write I/O calls to the local "lower" filesystem where they actually store the data.
>
> So I think these are valid use-cases, and I just think that they should
>
>  (a) be documented in the commit message as explanations of why people
> would do this/
>
>  (b) not be called "stacked", because that tends to have some other
> connotations to fs people.
>
> I don't know what a better term would be, but you yourself used "pass
> through". Maybe that (perhaps together with a clarification that it's
> a per-file thing) might work fine.
>
> Btw, why is mmap not passed through? That sounds fairly simple and
> straightforward, I'm not seeing why it would be missing.
>

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?

--Andy