linux-ext4 - Re: [PATCH 1/3] ext4: nonda

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Date:	Wed, 29 Aug 2012 15:28:16 +0200
From:	Jan Kara <jack@...e.cz>
To:	Dmitry Monakhov <dmonakhov@...nvz.org>
Cc:	linux-ext4@...r.kernel.org, tytso@....edu, a-fujita@...jp.nec.com
Subject: Re: [PATCH 1/3] ext4: nonda_switch prevent deadlock

On Tue 28-08-12 20:21:41, Dmitry Monakhov wrote:
> Currently ext4_da_write_begin may deadlock if called with opened journal
> transaction. Real life example:
> ->move_extent_per_page()
>   ->ext4_journal_start()-> hold journal transaction
>   ->write_begin()
>     ->ext4_da_write_begin()
>       ->ext4_nonda_switch()
>         ->writeback_inodes_sb_if_idle()  --> will wait for journal_stop()
> 
> This bug may be easily fixed by code reordering,
> But in some cases it should be possible to call write_begin()
> while holding journal's transaction, in this case caller may avoid
> recoursion by passing AOP_FLAG_NOFS flag.
  Well, I find calling ext4_write_begin() with a transaction started a bug.
Possibly ext4_write_begin() can be tweaked to handle that but things would
be simpler if we didn't have to.

Looking into move_extent_per_page(), calling ->write_begin() doesn't seem
to be quite right there anyway. For example it results in filling holes
under that page which is not desirable. I'm not even sure why do we call
->write_begin() there at all. The data in the page is unchanged. So it
should be enough to just remap buffers and mark the page dirty (but I might
be missing some subtlety here). Fujita-san, can you possibly explain?

								Honza

> ---
>  fs/ext4/inode.c |   28 +++++++++++++++++-----------
>  1 files changed, 17 insertions(+), 11 deletions(-)
> 
> diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
> index 6324f74..d12d30e 100644
> --- a/fs/ext4/inode.c
> +++ b/fs/ext4/inode.c
> @@ -889,6 +889,11 @@ static int ext4_write_begin(struct file *file, struct address_space *mapping,
>  	struct page *page;
>  	pgoff_t index;
>  	unsigned from, to;
> +	int nofs = flags & AOP_FLAG_NOFS;
> +
> +	/* We cannot recurse into the filesystem if the transaction is already
> +	 * started */
> +	BUG_ON(!nofs && journal_current_handle());
>  
>  	trace_ext4_write_begin(inode, pos, len, flags);
>  	/*
> @@ -906,9 +911,6 @@ retry:
>  		ret = PTR_ERR(handle);
>  		goto out;
>  	}
> -
> -	/* We cannot recurse into the filesystem as the transaction is already
> -	 * started */
>  	flags |= AOP_FLAG_NOFS;
>  
>  	page = grab_cache_page_write_begin(mapping, index, flags);
> @@ -957,7 +959,8 @@ retry:
>  		}
>  	}
>  
> -	if (ret == -ENOSPC && ext4_should_retry_alloc(inode->i_sb, &retries))
> +	if (!nofs && ret == -ENOSPC &&
> +	    ext4_should_retry_alloc(inode->i_sb, &retries))
>  		goto retry;
>  out:
>  	return ret;
> @@ -2447,7 +2450,7 @@ out_writepages:
>  }
>  
>  #define FALL_BACK_TO_NONDELALLOC 1
> -static int ext4_nonda_switch(struct super_block *sb)
> +static int ext4_nonda_switch(struct super_block *sb, int writeback_allowed)
>  {
>  	s64 free_blocks, dirty_blocks;
>  	struct ext4_sb_info *sbi = EXT4_SB(sb);
> @@ -2475,7 +2478,7 @@ static int ext4_nonda_switch(struct super_block *sb)
>  	 * Even if we don't switch but are nearing capacity,
>  	 * start pushing delalloc when 1/2 of free blocks are dirty.
>  	 */
> -	if (free_blocks < 2 * dirty_blocks)
> +	if (writeback_allowed && free_blocks < 2 * dirty_blocks)
>  		writeback_inodes_sb_if_idle(sb, WB_REASON_FS_FREE_SPACE);
>  
>  	return 0;
> @@ -2490,10 +2493,14 @@ static int ext4_da_write_begin(struct file *file, struct address_space *mapping,
>  	pgoff_t index;
>  	struct inode *inode = mapping->host;
>  	handle_t *handle;
> +	int nofs = flags & AOP_FLAG_NOFS;
>  
>  	index = pos >> PAGE_CACHE_SHIFT;
> +	/* We cannot recurse into the filesystem if the transaction is already
> +	 * started */
> +	BUG_ON(!nofs && journal_current_handle());
>  
> -	if (ext4_nonda_switch(inode->i_sb)) {
> +	if (ext4_nonda_switch(inode->i_sb, !nofs)) {
>  		*fsdata = (void *)FALL_BACK_TO_NONDELALLOC;
>  		return ext4_write_begin(file, mapping, pos,
>  					len, flags, pagep, fsdata);
> @@ -2512,8 +2519,6 @@ retry:
>  		ret = PTR_ERR(handle);
>  		goto out;
>  	}
> -	/* We cannot recurse into the filesystem as the transaction is already
> -	 * started */
>  	flags |= AOP_FLAG_NOFS;
>  
>  	page = grab_cache_page_write_begin(mapping, index, flags);
> @@ -2538,7 +2543,8 @@ retry:
>  			ext4_truncate_failed_write(inode);
>  	}
>  
> -	if (ret == -ENOSPC && ext4_should_retry_alloc(inode->i_sb, &retries))
> +	if (!nofs && ret == -ENOSPC &&
> +	    ext4_should_retry_alloc(inode->i_sb, &retries))
>  		goto retry;
>  out:
>  	return ret;
> @@ -4791,7 +4797,7 @@ int ext4_page_mkwrite(struct vm_area_struct *vma, struct vm_fault *vmf)
>  	/* Delalloc case is easy... */
>  	if (test_opt(inode->i_sb, DELALLOC) &&
>  	    !ext4_should_journal_data(inode) &&
> -	    !ext4_nonda_switch(inode->i_sb)) {
> +	    !ext4_nonda_switch(inode->i_sb, 1)) {
>  		do {
>  			ret = __block_page_mkwrite(vma, vmf,
>  						   ext4_da_get_block_prep);
> -- 
> 1.7.7.6
> 
> --
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-- 
Jan Kara <jack@...e.cz>
SUSE Labs, CR
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