Re: [PATCH v2] mm: Fix assertion mapping->nrpages == 0 in end_writeback()

[Andrew Morton <akpm@linux-foundation.org>]

On Wed, 15 Jun 2011 17:37:13 +0200
Jan Kara <jack@suse.cz> wrote:

> Under heavy memory and filesystem load, users observe the assertion
> mapping->nrpages == 0 in end_writeback() trigger. This can be caused
> by page reclaim reclaiming the last page from a mapping in the following
> race:
> 	CPU0				CPU1
>   ...
>   shrink_page_list()
>     __remove_mapping()
>       __delete_from_page_cache()
>         radix_tree_delete()
> 					evict_inode()
> 					  truncate_inode_pages()
> 					    truncate_inode_pages_range()
> 					      pagevec_lookup() - finds nothing
> 					  end_writeback()
> 					    mapping->nrpages != 0 -> BUG
>         page->mapping = NULL
>         mapping->nrpages--
> 
> Fix the problem by doing a reliable check of mapping->nrpages under
> mapping->tree_lock in end_writeback().
> 
> Analyzed by Jay <jinshan.xiong@whamcloud.com>, lost in LKML, and dug
> out by Miklos Szeredi <mszeredi@suse.de>.
> 
> CC: Jay <jinshan.xiong@whamcloud.com>
> CC: Miklos Szeredi <mszeredi@suse.de>
> Signed-off-by: Jan Kara <jack@suse.cz>
> ---
>  fs/inode.c         |    7 +++++++
>  include/linux/fs.h |    1 +
>  mm/truncate.c      |    5 +++++
>  3 files changed, 13 insertions(+), 0 deletions(-)
> 
>   Andrew, does this look better?

spose so.

> diff --git a/fs/inode.c b/fs/inode.c
> index 33c963d..1133cb0 100644
> --- a/fs/inode.c
> +++ b/fs/inode.c
> @@ -467,7 +467,14 @@ EXPORT_SYMBOL(remove_inode_hash);
>  void end_writeback(struct inode *inode)
>  {
>  	might_sleep();
> +	/*
> +	 * We have to cycle tree_lock here because reclaim can be still in the
> +	 * process of removing the last page (in __delete_from_page_cache())
> +	 * and we must not free mapping under it.
> +	 */
> +	spin_lock(&inode->i_data.tree_lock);
>  	BUG_ON(inode->i_data.nrpages);
> +	spin_unlock(&inode->i_data.tree_lock);

That's an expensive assertion.  We might want to wrap all this in
CONFIG_DEBUG_VM.

Or we could do

	if (unlikely(inode->i_data.nrpages)) {
		/* comment goes here */
		spin_lock(&inode->i_data.tree_lock);
		BUG_ON(inode->i_data.nrpages);
		spin_unlock(&inode->i_data.tree_lock);
	}

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