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

[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

 



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.

-Eric


--
To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html


[Index of Archives]     [Linux Ext4 Filesystem]     [Union Filesystem]     [Filesystem Testing]     [Ceph Users]     [Ecryptfs]     [AutoFS]     [Kernel Newbies]     [Share Photos]     [Security]     [Netfilter]     [Bugtraq]     [Yosemite News]     [MIPS Linux]     [ARM Linux]     [Linux Security]     [Linux Cachefs]     [Reiser Filesystem]     [Linux RAID]     [Samba]     [Device Mapper]     [CEPH Development]
  Powered by Linux