linux-ext4 - Re: How do I clear the append-only flag on symlinks?

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Message-ID: <CAObL_7F+Dhv8hFBkVN6q2oP_CBKnN2EbqmzfmqVh-KjOsyMkTw@mail.gmail.com>
Date:	Sat, 22 Oct 2011 01:57:45 -0700
From:	Andrew Lutomirski <andy@...o.us>
To:	"Ted Ts'o" <tytso@....edu>
Cc:	linux-ext4@...r.kernel.org
Subject: Re: How do I clear the append-only flag on symlinks?

On Fri, Oct 21, 2011 at 10:16 PM, Ted Ts'o <tytso@....edu> wrote:
> On Fri, Oct 21, 2011 at 11:07:51AM -0700, Andrew Lutomirski wrote:
>> By means that are best not admitted to, I managed to mark a large
>> chunk of my filesystem append-only, including symlinks.  I cleared the
>> flag from all the normal files and directories, but chattr and lsattr
>> don't want to operate on symlinks.
>
> Yes.  So how did you set a whole chunk of your file system
> append-only?

I want to be able to write to shared, locked, writable mappings
without sleeping.  Updating file timestamps and stable pages get in
the way.  I was trying out an approach to fixing the file_update_call
in do_wp_page using a new inode flag.  Sadly, I figured out the hard
way that 8196 is not actually a power of two.  (My patch is also
garbage for other reasons, but I'll email out a different approach as
an RFC sometime soon.)

If you have any ideas for how to handle stable pages other than
carrying a patch to disable them, I'd love to hear it.  The obvious
approach of making a new writable copy of the page will involve
pagecache internals that I'm not at all familiar with.

>
>> Any ideas on how to clear the append-only flags?  debug2fs is a little
>> scary, and hacking e2fsprogs to allow operation on symlinks seems to
>> affect symlink targets and not the symlinks themselves.
>
> I'd probably hack it into e2fsck, and let it offer to clear the
> append-only flag on things that aren't regular files, on the theory
> that they should have never been allowed to be set to begin with.
>

I'll take a look on Monday.

--Andy
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