xfs
[Top] [All Lists]

Re: [PATCH 1/8] fs: Improve filesystem freezing handling

To: Eric Sandeen <sandeen@xxxxxxxxxxx>
Subject: Re: [PATCH 1/8] fs: Improve filesystem freezing handling
From: Jan Kara <jack@xxxxxxx>
Date: Mon, 6 Feb 2012 16:17:45 +0100
Cc: Jan Kara <jack@xxxxxxx>, linux-fsdevel@xxxxxxxxxxxxxxx, Dave Chinner <dchinner@xxxxxxxxxx>, Surbhi Palande <csurbhi@xxxxxxxxx>, Kamal Mostafa <kamal@xxxxxxxxxxxxx>, Christoph Hellwig <hch@xxxxxxxxxxxxx>, LKML <linux-kernel@xxxxxxxxxxxxxxx>, xfs@xxxxxxxxxxx, linux-ext4@xxxxxxxxxxxxxxx
In-reply-to: <4F2C9FF8.2010207@xxxxxxxxxxx>
References: <1327091686-23177-1-git-send-email-jack@xxxxxxx> <1327091686-23177-2-git-send-email-jack@xxxxxxx> <4F2C9FF8.2010207@xxxxxxxxxxx>
User-agent: Mutt/1.5.20 (2009-06-14)
On Fri 03-02-12 21:03:20, Eric Sandeen wrote:
> On 1/20/12 2:34 PM, Jan Kara wrote:
> > vfs_check_frozen() tests are racy since the filesystem can be frozen just 
> > after
> > the test is performed. Thus in write paths we can end up marking some pages 
> > or
> > inodes dirty even though filesystem is already frozen. This creates problems
> > with flusher thread hanging on frozen filesystem.
> > 
> > Another problem is that exclusion between ->page_mkwrite() and filesystem
> > freezing has been handled by setting page dirty and then verifying s_frozen.
> > This guaranteed that either the freezing code sees the faulted page, writes 
> > it,
> > and writeprotects it again or we see s_frozen set and bail out of page 
> > fault.
> > This works to protect from page being marked writeable while filesystem
> > freezing is running but has an unpleasant artefact of leaving dirty 
> > (although
> > unmodified and writeprotected) pages on frozen filesystem resulting in 
> > similar
> > problems with flusher thread as the first problem.
> > 
> > This patch aims at providing exclusion between write paths and filesystem
> > freezing. We implement a writer-freeze read-write semaphores in the 
> > superblock
> > for each freezing level (currently there are two - SB_FREEZE_WRITE for data 
> > and
> > SB_FREEZE_TRANS for metadata). Write paths which should block freezing on 
> > given
> > level (e.g. ->block_page_mkwrite(), ->aio_write() for SB_FREEZE_WRITE level;
> > transaction lifetime for SB_FREEZE_TRANS level) hold reader side of the
> > semaphore. Code freezing the filesystem to a given level takes the writer 
> > side.
> > 
> > Only that we don't really want to bounce cachelines of the semaphore between
> > CPUs for each write happening. So we implement the reader side of the 
> > semaphore
> > as a per-cpu counter and the writer side is implemented using s_frozen
> > superblock field.
> > 
> > Acked-by: "Theodore Ts'o" <tytso@xxxxxxx>
> > Signed-off-by: Jan Kara <jack@xxxxxxx>
> 
> ...
> 
> > @@ -135,6 +157,11 @@ static struct super_block *alloc_super(struct 
> > file_system_type *type)
> >  #else
> >             INIT_LIST_HEAD(&s->s_files);
> >  #endif
> > +           if (init_sb_writers(s, SB_FREEZE_WRITE, "sb_writers_write"))
> > +                   goto err_out;
> > +           if (init_sb_writers(s, SB_FREEZE_TRANS, "sb_writers_trans"))
> > +                   goto err_out;
> > +
> >             s->s_bdi = &default_backing_dev_info;
> >             INIT_LIST_HEAD(&s->s_instances);
> >             INIT_HLIST_BL_HEAD(&s->s_anon);
> > @@ -186,6 +213,17 @@ static struct super_block *alloc_super(struct 
> > file_system_type *type)
> >     }
> >  out:
> >     return s;
> > +err_out:
> > +   security_sb_free(s);
> > +#ifdef CONFIG_SMP
> > +   if (s->s_files)
> > +           free_percpu(s->s_files);
> > +#endif
> > +   destroy_sb_writers(s, SB_FREEZE_WRITE);
> > +   destroy_sb_writers(s, SB_FREEZE_TRANS);
> 
> You probably ran into this already but the writer percpu vars need
> to be torn down in destroy_super() as well.
  Actually not. Thanks for spotting this.

                                                                Honza
-- 
Jan Kara <jack@xxxxxxx>
SUSE Labs, CR

<Prev in Thread] Current Thread [Next in Thread>