linux-kernel - Re: is this a bug?

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Message-ID: <20110325151810.GC9313@home.goodmis.org>
Date:	Fri, 25 Mar 2011 11:18:11 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Jay <jinshan.xiong@...mcloud.com>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: is this a bug?

Strange mail client you have.

On Wed, Mar 23, 2011 at 05:46:56PM -0700, Jay wrote:
> Hi,
> 
> Assuming vmscan is doing its job to steal the last page from an
> inode's namespace, so:
> 
> shrink_page_list -> remove_mapping -> __remove_from_page_cache:
> 
> void __remove_from_page_cache(struct page *page)
> 
> {
> 
>         ...
> 
>         radix_tree_delete(&mapping->page_tree, page->index);
> 
>         page->mapping = NULL;
> 
> >>> after removing the page from radix tree, it stuck at here.
> 
>         mapping->nrpages--;
> 
>         ...
> 
> }
> 
> then another process just calls iput_final to release the inode, so
> iput_final -> evict:
> 
> static void evict(struct inode *inode)
> 
> {
> 
>        ...
> 
>         } else {
> 
>                 if (inode->i_data.nrpages)
> 
> >>> here it finds that nrpage is 1, so go into truncate_inode_pages() but it won't find any page in the page tree.
> 
>                         truncate_inode_pages(&inode->i_data, 0);
> 
> >>> here nrpages remains 1.
> 
>                 end_writeback(inode);
> 
> >>> hit  BUG_ON(inode->i_data.nrpages) in end_writeback().


Did you actually hit this bug?

> 
>         }
> 
>         ...
> 
> }
> 
> The root cause of this problem is that nrpages is accessed w/o holding
> mapping->page_tree. The fix is also easy, just grab ->tree_lock inside
> truncate_inode_pages_range:
> 
> +       spin_lock_irq(&mapping->tree_lock);
> 
> -         if (mapping->nrpages == 0)
> 
> +        if (mapping->nrpages == 0) {
> 
> +               spin_unlock_irq(&mapping->tree_lock);
> 
>                 return;
> 
> +        }
> 
> +        spin_unlock_irq(&mapping->tree_lock);
> 
> Am I missed anything?

Perhaps the comment before the __remove_from_page_cache()

/*
 * Remove a page from the page cache and free it. Caller has to make
 * sure the page is locked and that nobody else uses it - or that usage
 * is safe.  The caller must hold the mapping's tree_lock.
 */

-- Steve

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