linux-kernel - Re: RFC: direct MTD support for SquashFS

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Message-ID: <87d3z1unm9.fsf@tac.ki.iif.hu>
Date:	Fri, 19 Mar 2010 02:05:50 +0100
From:	Ferenc Wagner <wferi@...f.hu>
To:	Phillip Lougher <phillip.lougher@...il.com>
Cc:	linux-fsdevel@...r.kernel.org, linux-mtd@...ts.infradead.org,
	linux-kernel@...r.kernel.org, linux-embedded@...r.kernel.org
Subject: Re: RFC: direct MTD support for SquashFS

Ferenc Wagner <wferi@...f.hu> writes:

> Phillip Lougher <phillip.lougher@...il.com> writes:
>
>> On Thu, Mar 18, 2010 at 4:38 PM, Ferenc Wagner <wferi@...f.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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