Re: [Bug #14894] pohmelfs: NULL pointer dereference

Evgeniy Polyakov
2010-02-02T20:57:43Z

Hi.

Sorry, it took really long to catch all the things around, but it is
better than never.

History says that commit which introduced per-bdi writeback broke
pohmelfs, since it does not have backing store device and did not fill
appropriate superblock entry, which lead to crash on umoount.

Jens provided a patch below, which I tested to fix the problem.
There is a problem though, when all writes happend before umount,
but did net yet written back to storage, get lost. I can not say whether
this happens because of per-bdi writeback (it should not I think), but
that's what happens.

Albeit being a bug, it will not be fixed :)
The reason is simple, new backing storage for POHMELFS (a distributed
hash table called elliptics network) is essentially ready and I start
working on porting POHMELFS to it, which means all its bugs will be
fixed there^W^Werased and written from scratch.

Jens, thanks a lot for you patch, Greg please pull it up.
If you did not find it or can not extract from the reply, I will resend.

Thanks everyone!

On Wed, Dec 30, 2009 at 10:14:34AM +0100, Jens Axboe (jens.axboe-QHcLZuEGTsvQT0dZR+AlfA@public.gmane.org) wrote:
> Since pohmelfs isn't tied to a single block device, it needs to setup a
> backing dev like nfs/btrfs/etc do. Here's a completely untested patch.
>
> diff --git a/drivers/staging/pohmelfs/inode.c b/drivers/staging/pohmelfs/inode.c
> index f69b778..cd25811 100644
> --- a/drivers/staging/pohmelfs/inode.c
> +++ b/drivers/staging/pohmelfs/inode.c
> @@ -36,6 +36,7 @@
> #define POHMELFS_MAGIC_NUM 0x504f482e
>
> static struct kmem_cache *pohmelfs_inode_cache;
> +static atomic_t psb_bdi_num = ATOMIC_INIT(0);
>
> /*
> * Removes inode from all trees, drops local name cache and removes all queued
> @@ -1331,6 +1332,8 @@ static void pohmelfs_put_super(struct super_block *sb)
> pohmelfs_crypto_exit(psb);
> pohmelfs_state_exit(psb);
>
> + bdi_destroy(&psb->bdi);
> +
> kfree(psb);
> sb->s_fs_info = NULL;
> }
> @@ -1815,11 +1818,22 @@ static int pohmelfs_fill_super(struct super_block *sb, void *data, int silent)
> if (!psb)
> goto err_out_exit;
>
> + err = bdi_init(&psb->bdi);
> + if (err)
> + goto err_out_free_sb;
> +
> + err = bdi_register(&psb->bdi, NULL, "pfs-%d", atomic_inc_return(&psb_bdi_num));
> + if (err) {
> + bdi_destroy(&psb->bdi);
> + goto err_out_free_sb;
> + }
> +
> sb->s_fs_info = psb;
> sb->s_op = &pohmelfs_sb_ops;
> sb->s_magic = POHMELFS_MAGIC_NUM;
> sb->s_maxbytes = MAX_LFS_FILESIZE;
> sb->s_blocksize = PAGE_SIZE;
> + sb->s_bdi = &psb->bdi;
>
> psb->sb = sb;
>
> @@ -1863,11 +1877,11 @@ static int pohmelfs_fill_super(struct super_block *sb, void *data, int silent)
>
> err = pohmelfs_parse_options((char *) data, psb, 0);
> if (err)
> - goto err_out_free_sb;
> + goto err_out_free_bdi;
>
> err = pohmelfs_copy_crypto(psb);
> if (err)
> - goto err_out_free_sb;
> + goto err_out_free_bdi;
>
> err = pohmelfs_state_init(psb);
> if (err)
> @@ -1916,6 +1930,8 @@ err_out_state_exit:
> err_out_free_strings:
> kfree(psb->cipher_string);
> kfree(psb->hash_string);
> +err_out_free_bdi:
> + bdi_destroy(&psb->bdi);
> err_out_free_sb:
> kfree(psb);
> err_out_exit:
> diff --git a/drivers/staging/pohmelfs/netfs.h b/drivers/staging/pohmelfs/netfs.h
> index 623a07d..01cba00 100644
> --- a/drivers/staging/pohmelfs/netfs.h
> +++ b/drivers/staging/pohmelfs/netfs.h
> @@ -18,6 +18,7 @@
>
> #include <linux/types.h>
> #include <linux/connector.h>
> +#include <linux/backing-dev.h>
>
> #define POHMELFS_CN_IDX 5
> #define POHMELFS_CN_VAL 0
> @@ -624,6 +625,8 @@ struct pohmelfs_sb {
>
> struct super_block *sb;
>
> + struct backing_dev_info bdi;
> +
> /*
> * Algorithm strings.
> */
>
> --
> Jens Axboe

--
Evgeniy Polyakov