linux-kernel - RE: [RFC PATCH] fs: file freeze support

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Date:	Fri, 16 Jan 2015 15:48:04 +0900
From:	Namjae Jeon <namjae.jeon@...sung.com>
To:	'Jan Kara' <jack@...e.cz>
Cc:	'Dave Chinner' <david@...morbit.com>,
	'Theodore Ts'o' <tytso@....edu>,
	'Alexander Viro' <viro@...iv.linux.org.uk>,
	'Brian Foster' <bfoster@...hat.com>,
	'Dmitry Monakhov' <dmonakhov@...nvz.org>,
	'Lukáš Czerner' <lczerner@...hat.com>,
	linux-fsdevel@...r.kernel.org,
	'Ashish Sangwan' <a.sangwan@...sung.com>,
	linux-kernel@...r.kernel.org
Subject: RE: [RFC PATCH] fs: file freeze support


> 
>   Hello,
Hi Jan,
> 

> > +
> > +int file_write_unfreeze(struct inode *inode)
> > +{
> > +	struct super_block *sb = inode->i_sb;
> > +
> > +	if (!S_ISREG(inode->i_mode))
> > +		return -EINVAL;
> > +
> > +	spin_lock(&inode->i_lock);
> > +
> > +	if (!(inode->i_state & I_WRITE_FREEZED)) {
> > +		spin_unlock(&inode->i_lock);
> > +		return -EINVAL;
> > +	}
> > +
> > +	inode->i_state &= ~I_WRITE_FREEZED;
> > +	smp_wmb();
> > +	wake_up(&sb->s_writers.wait_unfrozen);
> > +	spin_unlock(&inode->i_lock);
> > +	return 0;
> > +}
> > +EXPORT_SYMBOL(file_write_unfreeze);
>   So I was looking at the implementation and I have a few comments:
> 1) The trick with freezing superblock looks nice but I'm somewhat worried
> that if we wanted to heavily use per-inode freezing to defrag the whole
> filesystem it may be too slow to freeze the whole fs, mark one inode as
> frozen and then unfreeze the fs. But I guess we'll see that once have some
> reasonably working implementation.
Dmitry has given a good idea to avoid multiple freeze fs and unfreeze fs
calls.

ioctl(sb,FIFREEZE)
while (f = pop(files_list))
  ioctl(f,FS_IOC_FWFREEZE)
ioctl(sb,FITHAW)

In file_write_freeze, we could first check if the fs is already frozen,
if yes than we can directly set inode write freeze state after taking
relevant lock to prevent fs_thaw while the inode state is being set.

> 
> 2) The tests you are currently doing are racy. If
> things happen as:
>   CPU1					CPU2
> inode_start_write()
> 					file_write_freeze()
> sb_start_pagefault()
> Do modifications.
> 
> Then you have a CPU modifying a file while file_write_freeze() has
> succeeded so it should be frozen.
> 
> If you swap inode_start_write() with sb_start_pagefault() the above race
> doesn't happen but userspace program has to be really careful not to hit a
> deadlock. E.g. if you tried to freeze two inodes the following could happen:
>   CPU1					CPU2
> 					file_write_freeze(inode1)
> fault on inode1:
> sb_start_pagefault()
> inode_start_write() -> blocks
> 					file_write_freeze(inode2)
> 					  blocks in freeze_super()
> 
> So I don't think this is a good scheme for inode freezing...
To solve this race, we can fold inode_start_write with sb_start_write and use
similar appraoch of __sb_start_write.
How about the below scheme ?

void inode_start_write(struct inode *inode)
{
	struct super_block *sb = inode->i_sb;

retry:

	if (unlikely(inode->i_state & I_WRITE_FREEZED)) {
		DEFINE_WAIT(wait);

		prepare_to_wait(&sb->s_writers.wait_unfrozen, &wait,
			TASK_UNINTERRUPTIBLE);
		schedule();
		finish_wait(&sb->s_writers.wait_unfrozen, &wait);

		goto retry;
	}

	sb_start_write(sb);

	/* check if file_write_freeze race with us */
	if (unlikely(inode->i_state & I_WRITE_FREEZED) {
		sb_end_write(sb);
		goto retry;
	}
}

Thanks for your review!
> 
> 								Honza
> --
> Jan Kara <jack@...e.cz>
> SUSE Labs, CR

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