linux-ext4 - Re: [PATCH 2/2] ext4: Do not create EA inode under buffer lock

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Date: Mon, 25 Mar 2024 20:03:36 +0100
From: Jan Kara <jack@...e.cz>
To: Andreas Dilger <adilger@...ger.ca>
Cc: Jan Kara <jack@...e.cz>, Ted Tso <tytso@....edu>,
	linux-ext4@...r.kernel.org,
	syzbot+a43d4f48b8397d0e41a9@...kaller.appspotmail.com
Subject: Re: [PATCH 2/2] ext4: Do not create EA inode under buffer lock

On Fri 22-03-24 12:06:16, Andreas Dilger wrote:
> On Mar 21, 2024, at 10:26 AM, Jan Kara <jack@...e.cz> wrote:
> > 
> > ext4_xattr_set_entry() creates new EA inodes while holding buffer lock
> > on the external xattr block. This is problematic as it nests all the
> > allocation locking (which acquires locks on other buffers) under the
> > buffer lock. This can even deadlock when the filesystem is corrupted and
> > e.g. quota file is setup to contain xattr block as data block. Move the
> > allocation of EA inode out of ext4_xattr_set_entry() into the callers.
> 
> This looks like it will allocate a new inode for every setxattr called,
> even if the xattr is small and will likely fit inside the inode itself?
> This would seem to add a lot of extra overhead for the 99% of cases when
> an external inode is not needed.

This is not the case AFAICT. We call ext4_xattr_inode_lookup_create() only
in:

       if (i->value && i->in_inode) {

so that means we've already decided we need to put the xattr value in the
EA inode. Note that ext4_xattr_set_handle() for smaller xattr value first
calls ext4_xattr_block_set() with i.in_inode == 0 and if that fails due to
ENOSPC, it sets i.in_inode = 1 and tries again.

So I think everything is fine.

								Honza
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR