Re: tmpfs: kernel BUG at mm/shmem.c:814

[Kel Modderman <kel@otaku42.de>]

Hi Hugh,

On Sunday 27 July 2008 08:28:32 Hugh Dickins wrote:
> On Sat, 26 Jul 2008, Kel Modderman wrote:
> > 
> > I am able to reproduce the triggering of BUG_ON() in shmem_delete_inode
> > function of mm/shmem.c, line 814, while testing the insserv program in a dir
> > on a tmpfs mount point with Linux 2.6.25.x and 2.6.26.
> 
> Outstanding bug report and steps to reproduce: thank you so much.
> You should find this fixes it; though we may have some more work to
> do, maybe other filesystems are surprised by readahead on directories.

Ack, confirmed.

Thanks for such a prompt fix.

> 
> 
> [PATCH] tmpfs: fix kernel BUG in shmem_delete_inode
> 
> SuSE's insserve initscript ordering program hits kernel BUG at mm/shmem.c:814
> on 2.6.26.  It's using posix_fadvise on directories, and the shmem_readpage
> method added in 2.6.23 is letting POSIX_FADV_WILLNEED allocate useless pages
> to a tmpfs directory, incrementing i_blocks count but never decrementing it.
> 
> Fix this by assigning shmem_aops (pointing to readpage and writepage and
> set_page_dirty) only when it's needed, on a regular file or a long symlink.
> 
> Many thanks to Kel for outstanding bugreport and steps to reproduce it.
> 
> Reported-by: Kel Modderman <kel@otaku42.de>
> Signed-off-by: Hugh Dickins <hugh@veritas.com>
> Cc: stable@kernel.org
> ---
> Other filesystems... ramfs looks as if it would allocate useless pages too,
> but should free them, and wouldn't BUG; are there other filesystems to be
> surprised by readahead on directories?  I have not looked through yet.
> 
>  mm/shmem.c |    3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> --- 2.6.26-git/mm/shmem.c	2008-07-26 12:33:28.000000000 +0100
> +++ linux/mm/shmem.c	2008-07-26 22:46:28.000000000 +0100
> @@ -1513,7 +1513,6 @@ shmem_get_inode(struct super_block *sb, 
>  		inode->i_uid = current->fsuid;
>  		inode->i_gid = current->fsgid;
>  		inode->i_blocks = 0;
> -		inode->i_mapping->a_ops = &shmem_aops;
>  		inode->i_mapping->backing_dev_info = &shmem_backing_dev_info;
>  		inode->i_atime = inode->i_mtime = inode->i_ctime = CURRENT_TIME;
>  		inode->i_generation = get_seconds();
> @@ -1528,6 +1527,7 @@ shmem_get_inode(struct super_block *sb, 
>  			init_special_inode(inode, mode, dev);
>  			break;
>  		case S_IFREG:
> +			inode->i_mapping->a_ops = &shmem_aops;
>  			inode->i_op = &shmem_inode_operations;
>  			inode->i_fop = &shmem_file_operations;
>  			mpol_shared_policy_init(&info->policy,
> @@ -1929,6 +1929,7 @@ static int shmem_symlink(struct inode *d
>  			return error;
>  		}
>  		unlock_page(page);
> +		inode->i_mapping->a_ops = &shmem_aops;
>  		inode->i_op = &shmem_symlink_inode_operations;
>  		kaddr = kmap_atomic(page, KM_USER0);
>  		memcpy(kaddr, symname, len);
>