linux-kernel - Re: [PATCH 2/9] vfs: export do_splice

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Date:	Tue, 19 Mar 2013 19:32:42 +0100
From:	Miklos Szeredi <miklos@...redi.hu>
To:	Al Viro <viro@...iv.linux.org.uk>
Cc:	Jan Kara <jack@...e.cz>, David Howells <dhowells@...hat.com>,
	torvalds@...ux-foundation.org, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org, hch@...radead.org,
	akpm@...ux-foundation.org, apw@...onical.com, nbd@...nwrt.org,
	neilb@...e.de, jordipujolp@...il.com, ezk@....cs.sunysb.edu,
	sedat.dilek@...glemail.com, hooanon05@...oo.co.jp, mszeredi@...e.cz
Subject: Re: [PATCH 2/9] vfs: export do_splice_direct() to modules

On Tue, Mar 19, 2013 at 6:03 PM, Al Viro <viro@...iv.linux.org.uk> wrote:
> On Tue, Mar 19, 2013 at 11:29:41AM +0100, Miklos Szeredi wrote:
>
>> Copy up is a once-in-a-lifetime event for an object.  Optimizing it is
>> way down in the list of things to do.  I'd drop splice in a jiffy if
>> it's in the way.
>
> What makes you think that write is any better?  Same deadlock there - check
> generic_file_aio_write(), it calls the same sb_start_write()...  IOW,
> switching from splice to write won't help at all.

Okay, I missed that. Yeah, that needs fixing...

>> Much more interesting question:  what happens if we crash during a
>> rename?  Whiteout implemented in the filesystem won't save us.  And
>> the results are interesting: old versions of files become visible and
>> similar fun.  Far from likely to happen, but ...
>>
>> Add a rename-with-whiteout primitive on filesystems?  That one is not
>> going to be as simple as plain whiteout.  Or?
>
> Umm...  If/when we start caring about that kind of atomicity (and I agree
> that we ought to) overlayfs approach to whiteouts will actually have much
> harder time - it doesn't take much to teach a journalling fs how to do that
> kind of ->rename() in a single transaction; the only question is how to tell
> it that we want to leave a whiteout behind us.  Hell knows; one variant is
> to add a flag, of course.  Another might be more interesting - we want some
> kind of "directory is opaque" flag, so if we start reshuffling the methods,
> we might try to merge unlink/rmdir/whiteout.  Rules:
>         * victim is negative => create a whiteout
>         * victim is a directory, parent opaque => rmdir
>         * victim is a non-directory, parent opaque => unlink
>         * victim is positive, parent _not_ opaque => replace with whiteout
>         * old_dir in case of ->rename() is opaque => normal rename
>         * old_dir in case of ->rename() is not opaque => leave whiteout behind
> Non-unioned => opaque, of course (nothing showing through it).
>

I dunnow.  Overloading common paths with overlay/union specific things
doesn't look very clean to me.

I have a similar problem with union-mounts: it's hooking into lots of
common paths in the VFS for the sake of a very specialized feature.

> Getting good behaviour on rename interrupted by crash is going to be _very_
> tricky with any strategy other than whiteouts-in-fs, AFAICS.
>

One idea is to add a journal to the overlay itself (yeah, namespace issues).

Thanks,
Miklos
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