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    SubjectRe: [Bug #14894] pohmelfs: NULL pointer dereference
    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@oracle.com) 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


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