Re: [PATCH] ext4: serialize unaligned asynchronous DIO

[Eric Sandeen <sandeen@redhat.com>]

On 1/13/11 10:15 PM, Ted Ts'o wrote:
> 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'll add something like that.

> 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?

We only have this problem if we are going down the unwritten extent
route, which only happens for writes inside i_size:

        if (rw == WRITE && final_size <= inode->i_size) {
                /*
                 * We could direct write to holes and fallocate.
		   ...

I can add a comment, good point.  In fact I'll liberally add a few
comments, sorry, I usually do that but didn't tidy this patch up
prior to sending.  :)

-Eric
 
>        	      	    	       	     	     - Ted