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SubjectRe: RFC: direct MTD support for SquashFS
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On Thu, Mar 18, 2010 at 4:38 PM, Ferenc Wagner <wferi@niif.hu> wrote:
>
> I could only compare apples to oranges before porting the patch to the
> LZMA variant.  So I refrain from that for a couple of days yet.  But
> meanwhile I started adding a pluggable backend framework to SquashFS,
> and would much appreciate some comments about the applicability of this
> idea.  The patch is (intended to be) a no-op, applies on top of current
> git (a3d3203e4bb40f253b1541e310dc0f9305be7c84).

This looks promising, making the backend pluggable (like the new
compressor framework) is far better and cleaner than scattering the
code full of #ifdef's. Far better than the previous patch :-)

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.

+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?
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...

Phillip
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