From: Jan Kara <jack@suse.cz>
To: Kirill Tkhai <ktkhai@virtuozzo.com>
Cc: linux-ext4@vger.kernel.org, tytso@mit.edu, jack@suse.com,
adilger.kernel@dilger.ca
Subject: Re: [Q] e4defrag and append-only files
Date: Tue, 10 Dec 2019 11:10:17 +0100 [thread overview]
Message-ID: <20191210101017.GD1551@quack2.suse.cz> (raw)
In-Reply-To: <a2b2fcf4-4b71-e78c-5a10-627097df44fb@virtuozzo.com>
Hi!
On Mon 09-12-19 13:54:24, Kirill Tkhai wrote:
> on one of production nodes I observe the situation, when many fragmented files
> never become defragmented, becase of they have "a" extended attribute.
> The reason is append-only file can't be open for write without O_APPEND attribute:
>
> $lsattr a.txt
> -----a--------e----- a.txt
>
> $strace e4defrag a.txt
> openat(AT_FDCWD, "a.txt", O_RDWR) = -1 EPERM (Operation not permitted)
>
> Simple O_APPEND passed to open() solves the situation.
>
> The question is: can't we just do this?
>
> Let's observe the file restrictions we may have.
>
> "Append-only" extended attribute restriction is weaker, than RO file permissions (0444).
> But RO files are being processed by e4defrag, since e4defrag runs by root, and it easily
> ignores RO file permissions, while "append-only" files are always ignored by the util.
> Is there a fundamental reason we must skip them?
I don't think there's a technical reason why we cannot defragment inodes
with 'append-only' or even 'immutable' attribute. Just e4defrag would have
to remove these flags so that open can succeed (because neither append-only
nor immutable flags are overridden by CAP_DAC_OVERRIDE capability - unlike
standard unix permissions or xattrs) and then restore the flags after the
fact.
Whether we really want to do this is another question. I don't think
e4defrag should touch immutable files, for append-only files I don't have a
strong opinion...
Honza
--
Jan Kara <jack@suse.com>
SUSE Labs, CR
prev parent reply other threads:[~2019-12-10 10:10 UTC|newest]
Thread overview: 2+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-12-09 10:54 [Q] e4defrag and append-only files Kirill Tkhai
2019-12-10 10:10 ` Jan Kara [this message]
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