linux-ext4 - Re: [PATCH 1/8] fs: Improve filesystem freezing handling

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Date:	Fri, 03 Feb 2012 21:03:20 -0600
From:	Eric Sandeen <sandeen@...deen.net>
To:	Jan Kara <jack@...e.cz>
CC:	linux-fsdevel@...r.kernel.org, Dave Chinner <dchinner@...hat.com>,
	Surbhi Palande <csurbhi@...il.com>,
	Kamal Mostafa <kamal@...onical.com>,
	Christoph Hellwig <hch@...radead.org>,
	LKML <linux-kernel@...r.kernel.org>, xfs@....sgi.com,
	linux-ext4@...r.kernel.org
Subject: Re: [PATCH 1/8] fs: Improve filesystem freezing handling

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@....edu>
> Signed-off-by: Jan Kara <jack@...e.cz>

...

> @@ -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.

-Eric


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