RFC: direct MTD support for SquashFS

[Ferenc Wagner]

Phillip Lougher <phillip.lougher at gmail.com> writes:

> A couple of specific comments...
>
> +/* A backend is initialized for each SquashFS block read operation,
> + * making further sequential reads possible from the block.
> + */
> +static void *bdev_init(struct squashfs_sb_info *msblk, u64 index,
> size_t length)
> +{
> +	struct squashfs_bdev *bdev = msblk->backend_data;
> +	struct buffer_head *bh;
> +
> +	bh = kcalloc((msblk->block_size >> bdev->devblksize_log2) + 1,
> +			sizeof(*bh), GFP_KERNEL);
>
> You should alloc against the larger of msblk->block_size and
> METADATA_SIZE (8 Kbytes).  Block_size could be 4 Kbytes only.

I plugged in a max().  Couldn't that trailing +1 be converted into a +2
like this?

bh = kcalloc((max(msblk->block_size, METADATA_SIZE) + 2) >> bdev->devblksize_log2

> +static int fill_bdev_super(struct super_block *sb, void *data, int silent)
>
> This function looks rather 'back-to-front' to me.  I'm assuming that
> squashfs_fill_super2() will be the current fill superblock function?
> 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...

I solved it by introducing a callback function for adding the backend.
That may be overkill, but it seems to give the most shared code.

The attached patch series survived some testing here.  My only doubt:
the current backend interface necessitates a memory copy from the buffer
heads.  This is no problem for mtd and lzma which copy the data anyway,
but makes this code less efficient in the bdev+zlib case.

I've got one more patch, which I forgot to export, to pull out the
common logic from the backend init functions back into squashfs_read_data().
With the bdev backend, that entails reading the first block twice in a
row most of the time.  This again could be worked around by extending
the backend interface, but I'm not sure if it's worth it.

How does this look like now?
-- 
Regards,
Feri.