Re: [PATCH] ext4: serialize unaligned asynchronous DIO

[Ted Ts'o <tytso@mit.edu>]

On Thu, Jan 13, 2011 at 04:23:14PM -0600, Eric Sandeen wrote:
> Mingming suggested that perhaps we can track outstanding
> conversions, and wait on that instead so that non-sparse
> files won't be affected, but I've had trouble making that
> work so far, and would like to get the corruption hole
> plugged ASAP.  Perhaps adding a prink_once() warning of
> the perf degradation on this path would be useful?

Yeah, I think a printk_once(), or maybe better yet, a warning
ext4_msg() ratelimited to once a day, is the way to go.  I'd print the
inode number and process name that did the offending async DIO, so it
can help out the system administrator.

I've looked over the rest of the patch, and it seems good.  Just one
question:

> +static int
> +ext4_unaligned_aio(struct inode *inode, const struct iovec *iov,
> +		unsigned long nr_segs, loff_t pos)
> +{
> +	struct super_block *sb = inode->i_sb;
> +	int blockmask = sb->s_blocksize - 1;
> +	size_t count = iov_length(iov, nr_segs);
> +	loff_t final_size = pos + count;
> +
> +	if (pos >= inode->i_size)
> +		return 0;

Why is it ok if the write is extended the file?  Are you depending on
some other lock (i_data_sem, perhaps?) to serialize the write in that
case?  If so, could you please add a comment to that effect?

       	      	    	       	     	     - Ted