linux-kernel - Re: [RFC PATCH] 9p: case-insensitive host filesystems

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Date:   Mon, 23 May 2022 19:59:55 +0200
From:   Christian Schoenebeck <qemu_oss@...debyte.com>
To:     qemu-devel@...gnu.org
Cc:     Latchesar Ionkov <lucho@...kov.net>,
        Eric Van Hensbergen <ericvh@...il.com>,
        linux-kernel@...r.kernel.org, qemu-devel@...gnu.org,
        Greg Kurz <groug@...d.org>,
        Keno Fischer <keno@...iacomputing.com>,
        Michael Roitzsch <reactorcontrol@...oud.com>,
        Will Cohen <wwcohen@...il.com>, netdev@...r.kernel.org,
        v9fs-developer@...ts.sourceforge.net,
        Dominique Martinet <asmadeus@...ewreck.org>
Subject: Re: [RFC PATCH] 9p: case-insensitive host filesystems

On Freitag, 22. April 2022 21:57:40 CEST Dominique Martinet wrote:
> Christian Schoenebeck wrote on Fri, Apr 22, 2022 at 08:02:46PM +0200:
> > So maybe it's better to handle case-insensitivity entirely on client side?
> > I've read that some generic "case fold" code has landed in the Linux
> > kernel
> > recently that might do the trick?
> 
> I haven't tried, but settings S_CASEFOLD on every inodes i_flags might do
> what you want client-side.
> That's easy enough to test and could be a mount option

I just made a quick test using:

diff --git a/fs/9p/vfs_inode.c b/fs/9p/vfs_inode.c
index 08f48b70a741..5d8e77daed53 100644
--- a/fs/9p/vfs_inode.c
+++ b/fs/9p/vfs_inode.c
@@ -257,6 +257,7 @@ int v9fs_init_inode(struct v9fs_session_info *v9ses,
        inode->i_atime = inode->i_mtime = inode->i_ctime = 
current_time(inode);
        inode->i_mapping->a_ops = &v9fs_addr_operations;
        inode->i_private = NULL;
+       inode->i_flags |= S_CASEFOLD;
 
        switch (mode & S_IFMT) {
        case S_IFIFO:

Unfortunately that did not help much. I still get EEXIST error e.g. when 
trying 'ln -s foo FOO'.

I am not sure though whether there would be more code places to touch or 
whether that's even the expected behaviour with S_CASEFOLD for some reason.

> Even with that it's possible to do a direct open without readdir first
> if one knows the path and I that would only be case-insensitive if the
> backing server is case insensitive though, so just setting the option
> and expecting it to work all the time might be a little bit
> optimistic... I believe guess that should be an optimization at best.
> 
> Ideally the server should tell the client they are casefolded somehow,
> but 9p doesn't have any capability/mount time negotiation besides msize
> so that's difficult with the current protocol.