linux-kernel - Re: splice vs execve lockdep trace.

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <20130718034203.GO11674@dastard>
Date:	Thu, 18 Jul 2013 13:42:03 +1000
From:	Dave Chinner <david@...morbit.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Ben Myers <bpm@....com>, Peter Zijlstra <peterz@...radead.org>,
	Oleg Nesterov <oleg@...hat.com>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Dave Jones <davej@...hat.com>, xfs@....sgi.com
Subject: Re: splice vs execve lockdep trace.

On Wed, Jul 17, 2013 at 05:17:36PM -0700, Linus Torvalds wrote:
> On Wed, Jul 17, 2013 at 4:40 PM, Ben Myers <bpm@....com> wrote:
> >>
> >> We're still talking at cross purposes then.
> >>
> >> How the hell do you handle mmap() and page faulting?
> >
> > __xfs_get_blocks serializes access to the block map with the i_lock on the
> > xfs_inode.  This appears to be racy with respect to hole punching.
> 
> Would it be possible to just make __xfs_get_blocks get the i_iolock
> (non-exclusively)?

No. __xfs_get_blocks() operates on metadata (e.g. extent lists), and
as such is protected by the i_ilock (note: not the i_iolock).  i.e.
XFS has a multi-level locking strategy:

	i_iolock is provided for *data IO serialisation*,
	i_ilock is for *inode metadata serialisation*.

Truncate and hole punching require IO level serialisation rather
than metadata or page cache level serialisation as they have to be
safe against direct IO as well as page cache based IO.

> Or, alternatively, do it in the readpage() function?
> 
> That was what I thought you did anyway. Exactly because of the whole
> page faulting issue.

We protect the inode itself with the i_ilock in the page fault path,
but we have no IO level serialisation. Racing faults serialise
access to inode metadata via on the i_ilock, but this doesn't
serialise against IO in progress....

Cheers,

Dave.
-- 
Dave Chinner
david@...morbit.com
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/