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SubjectRe: tmpfs: kernel BUG at mm/shmem.c:814
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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);
>




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