linux-kernel - Re: dirty balancing deadlock

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: <20070221235532.2361f827.akpm@linux-foundation.org>
Date:	Wed, 21 Feb 2007 23:55:32 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Miklos Szeredi <miklos@...redi.hu>
Cc:	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: dirty balancing deadlock

> On Thu, 22 Feb 2007 08:42:26 +0100 Miklos Szeredi <miklos@...redi.hu> wrote:
> > > 
> > > Index: linux/mm/page-writeback.c
> > > ===================================================================
> > > --- linux.orig/mm/page-writeback.c	2007-02-19 17:32:41.000000000 +0100
> > > +++ linux/mm/page-writeback.c	2007-02-19 18:05:28.000000000 +0100
> > > @@ -198,6 +198,25 @@ static void balance_dirty_pages(struct a
> > >  			dirty_thresh)
> > >  				break;
> > >  
> > > +		/*
> > > +		 * Acquit this producer if there's little or nothing
> > > +		 * to write back to this particular queue
> > > +		 *
> > > +		 * Without this check a deadlock is possible in the
> > > +		 * following case:
> > > +		 *
> > > +		 * - filesystem A writes data through filesystem B
> > > +		 * - filesystem A has dirty pages over dirty_thresh
> > > +		 * - writeback is started, this triggers a write in B
> > > +		 * - balance_dirty_pages() is called synchronously
> > > +		 * - the write to B blocks
> > > +		 * - the writeback completes, but dirty is still over threshold
> > > +		 * - the blocking write prevents futher writes from happening
> > > +		 */
> > > +		if (atomic_long_read(&bdi->nr_dirty) +
> > > +		    atomic_long_read(&bdi->nr_writeback) < 16)
> > > +			break;
> > > +
> > 
> > The problem seems to that little "- the write to B blocks".
> > 
> > How come it blocks?  I mean, if we cannot retire writes to that filesystem
> > then we're screwed anyway.
> 
> Sorry about the sloppy description.  I mean, it's not the lowlevel
> write that will block, but rather the VFS one
> (generic_file_aio_write).  It will block (or rather loop forever with
> 0.1 second sleeps) in balance_dirty_pages().  That means, that for
> this inode, i_mutex is held and no other writer can continue the work.

"this inode" I assume is the inode against filesystem A?

Why does holding that inode's i_mutex prevent further writeback of pages in A?


-
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/