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    SubjectRe: RFC: direct MTD support for SquashFS
    Date
    Ferenc Wagner <wferi@niif.hu> writes:

    > Phillip Lougher <phillip.lougher@gmail.com> writes:
    >
    >> On Thu, Mar 18, 2010 at 4:38 PM, Ferenc Wagner <wferi@niif.hu> wrote:
    >>
    >> +static int fill_bdev_super(struct super_block *sb, void *data, int silent)
    >> +{
    >> + struct squashfs_sb_info *msblk;
    >> + struct squashfs_bdev *bdev;
    >> + int err = squashfs_fill_super2(sb, data, silent, &squashfs_bdev_ops);
    >> + if (err)
    >> + return err;
    >> +
    >> + bdev = kzalloc(sizeof(*bdev), GFP_KERNEL);
    >> + if (!bdev)
    >> + return -ENOMEM;
    >> +
    >> + bdev->devblksize = sb_min_blocksize(sb, BLOCK_SIZE);
    >> + bdev->devblksize_log2 = ffz(~bdev->devblksize);
    >> +
    >> + msblk = sb->s_fs_info;
    >> + msblk->backend_data = bdev;
    >> + return 0;
    >> +}
    >>
    >> This function looks rather 'back-to-front' to me. I'm assuming that
    >> squashfs_fill_super2() will be the current fill superblock function?
    >
    > Yes, with the extra parameter added.
    >
    >> This function wants to read data off the filesystem through the
    >> backend, and yet the backend (bdev, mblk->backend_data) hasn't been
    >> initialised when it's called...
    >
    > It can't be, because msblk = sb->s_fs_info is allocated by
    > squashfs_fill_super(). Now it will be passed the ops, so after
    > allocating msblk it can also fill out the ops. After that it can read,
    > and squashfs_read_data() will call the init, read and free operations of
    > the backend.

    And here we indeed have a rather fundamental problem. This isn't
    specific to the discussed plugin system at all. Even in the current
    code, to set msblk->block_size squashfs_fill_super() calls
    squashfs_read_table() to read the superblock, which in turn calls
    squashfs_read_data(), which uses msblk->block_size to allocate enough
    buffer heads, but msblk->block_size just can't be set at this point.
    msblk->bytes_used is preset with a dummy value to make the read
    possible, but msblk->block_size is not. Fortunately, one buffer head is
    allocated each time nevertheless. I wonder what a correct solution
    would look lke..
    --
    Regards,
    Feri.


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