linux-ext4 - Re: ext4_orphan_del() sleeps in non-journal mode

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Date:	Sat, 15 Sep 2012 15:28:19 -0700
From:	Anatol Pomozov <anatol.pomozov@...il.com>
To:	Dmitry Monakhov <dmonakhov@...nvz.org>
Cc:	linux-ext4@...r.kernel.org, "Theodore Ts'o" <tytso@....edu>
Subject: Re: ext4_orphan_del() sleeps in non-journal mode

D'oh, sorry.

I did not realize that it is our local modification that calls
ext4_orphan_del(NULL) from ext4_end_io_dio. So please ignore the issue
"ext4_orphan_del sleeps in softirq context".

But the other more general issue "ext4_orphan_del sleeps in no-journal
mode" still applies. As Dmitry mentioned in this commit 3d287de3b828
such sleep might degrade performance. In no-journal mode we do not
need to manipulate with i_orphan list and no reason to take the mutex.

I checked all ext4_orphan_del(NULL,...) usages and some of them look
good for me, e.g.

		handle = ext4_journal_start(inode, 2);
		if (IS_ERR(handle)) {
			if (inode->i_nlink)
				ext4_orphan_del(NULL, inode);

In this example we take handle and important thing to note here is
that IS_ERR(handle) can be true only in journal mode when starting
journal failed. In non-journal mode ext4_journal_start() *always*
returns a fake handle that is non-error (see ext4_get_nojournal). So
the example above never sleeps in ext4_orphan_del().

But there are other examples where sleep might happen (at current HEAD
3f0c3c8fe):

inode.c:281
inode.c:956
inode.c:1069
inode.c:1111
inode.c:1177

I think we need to fix these 5 places. In all other cases we check
IS_ERR(handle) and thus do not call ext4_orphan_del() in no-journal
mode.
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