linux-kernel - Re: [PATCH 14/39] ovl: stack file ops

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Message-ID: <20180615054717.GC30522@ZenIV.linux.org.uk>
Date:   Fri, 15 Jun 2018 06:47:17 +0100
From:   Al Viro <viro@...IV.linux.org.uk>
To:     Miklos Szeredi <miklos@...redi.hu>
Cc:     Miklos Szeredi <mszeredi@...hat.com>,
        overlayfs <linux-unionfs@...r.kernel.org>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH 14/39] ovl: stack file ops

On Wed, Jun 13, 2018 at 11:21:30AM +0200, Miklos Szeredi wrote:
> On Tue, Jun 12, 2018 at 8:31 PM, Al Viro <viro@...iv.linux.org.uk> wrote:
> > On Tue, Jun 12, 2018 at 07:24:23PM +0100, Al Viro wrote:
> 
> >> I hate it, but... consider path_open() objections withdrawn for now.
> 
> Is that an ACK for the pull if I follow up with fixes for mmap botch, etc?

Yes.

> >> Uses of ->vm_file (and rules for those) are too convoluted to untangle
> >> at the moment.  I still would love to get that straightened out, but
> >> it's not this cycle fodder, more's the pity...
> 
> Looked at some other options...  What coda mmap does looks very
> dubious.  It only sets f_mapping, not vm_file.  That's going to get
> into all sorts of trouble when underlying fs tries to look at
> file_inode() or worse, ->private_data.  Looks like that should be
> converted to what overlayfs does, to have a remote chance of actually
> not crashing on most filesystems.  Does anybody actually use coda
> still?

Keep in mind that coda is using the local fs only as cache; IOW, its needs
are much more limited than those of overlayfs - local r/w filesystem,
disk-backed or tmpfs, used pretty much as a scratch space.

> > PS: conversion of ->f_path.dentry is easy and that can probably go this
> > cycle - it's a fairly trivial change, with no functional changes unless
> > overlayfs is used with <filesystem>, fixing really bad shit if it ever
> > gets used thus.  I'm not asking to put that into overlayfs pull *and*
> > it's independent from the "want to kill that fucking kludge" stuff.
> > The latter is too hard for this cycle, unfortunately.
> 
> So this is about adding a file_dentry_check() (or whatever we want to
> call it) helper to be used by all filesystems when dereferecing
> f_path.dentry, right?

file_dentry(), and some of the users should be converted to file_inode().
There's also a missing helper for debugfs uses - more or less a combination
of file_dentry() and debugfs_file_get() (if not a conversion of
debugfs_file_get() to taking struct file - almost all users are of that
form, if not entirely all of them).  I've some of that done in local
branch...