linux-ext4 - Re: current->journal_info got nested! (was Re: [syzbot] [xfs?] [ext4?] general protection fault in jbd2__journal

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Date: Tue, 30 Jan 2024 22:17:58 -0800
From: Christoph Hellwig <hch@...radead.org>
To: Dave Chinner <david@...morbit.com>
Cc: Matthew Wilcox <willy@...radead.org>, Theodore Ts'o <tytso@....edu>,
	syzbot <syzbot+cdee56dbcdf0096ef605@...kaller.appspotmail.com>,
	adilger.kernel@...ger.ca, chandan.babu@...cle.com, jack@...e.com,
	linux-ext4@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-xfs@...r.kernel.org,
	syzkaller-bugs@...glegroups.com
Subject: Re: current->journal_info got nested! (was Re: [syzbot] [xfs?]
 [ext4?] general protection fault in jbd2__journal_start)

On Wed, Jan 31, 2024 at 05:02:25PM +1100, Dave Chinner wrote:
> This may not be true for other filesystems, but I don't think we
> can really say "page faults in any filesystem transaction are unsafe
> and should be banned"....

I think the point is page faults with current->journal_info set is
unsafe, as the can recurse into another file system using it.  If we
don't set current->journal_info (and your ideas for that sound sensible
to me), the question of page faults in transactions is decoupled from
that.  We just need to ensure we never recurse into a transaction in
the same or a dependent fs, which ot me suggest we'd better avoid it
if we easily can.