[Ocfs2-devel] [PATCH 19/41] ocfs2: Integrate CoW in file write.

[Tao Ma]

Hi Joel,

Joel Becker wrote:
> On Tue, Aug 18, 2009 at 02:19:20PM +0800, Tao Ma wrote:
>> @@ -1674,6 +1692,7 @@ int ocfs2_write_begin_nolock(struct address_space *mapping,
>>  	struct ocfs2_alloc_context *meta_ac = NULL;
>>  	handle_t *handle;
>>  	struct ocfs2_extent_tree et;
>> +	unsigned int refcounted_cpos, cow_len;
>>  
>>  	ret = ocfs2_alloc_write_ctxt(&wc, osb, pos, len, di_bh);
>>  	if (ret) {
>> @@ -1701,12 +1720,36 @@ int ocfs2_write_begin_nolock(struct address_space *mapping,
>>  	}
>>  
>>  	ret = ocfs2_populate_write_desc(inode, wc, &clusters_to_alloc,
>> -					&extents_to_split);
>> -	if (ret) {
>> +					&extents_to_split, &refcounted_cpos);
>> +	if (ret && ret != -ETXTBSY) {
>>  		mlog_errno(ret);
>>  		goto out;
>>  	}
>>  
>> +	if (ret == -ETXTBSY) {
>> +		BUG_ON(refcounted_cpos == UINT_MAX);
>> +		cow_len = wc->w_clen - (refcounted_cpos - wc->w_cpos);
>> +
>> +		ret = ocfs2_refcount_cow(inode, di_bh,
>> +					 refcounted_cpos, cow_len);
>> +		if (ret) {
>> +			mlog_errno(ret);
>> +			goto out;
>> +		}
>> +
>> +		/* reinitialize write_desc and populate it again. */
>> +		ocfs2_clear_write_desc(wc);
>> +		ret = ocfs2_populate_write_desc(inode, wc, &clusters_to_alloc,
>> +						&extents_to_split,
>> +						&refcounted_cpos);
>> +		if (ret) {
>> +			mlog_errno(ret);
>> +			goto out;
>> +		}
>> +
>> +		BUG_ON(refcounted_cpos != UINT_MAX);
>> +	}
> 
> 	First see my comment on the ocfs2_refcount_cal_clusters() and a
> possible short CoW.
> 	Basically, ocfs2_refcount_cow() can do a short CoW in two
> situations.  First, the one I described about
> ocfs2_refcount_cal_clusters().  The second is if the write wants to
> cover three extents, the middle of which is not refcounted:
> 
>   |--refcounted--|--not refcounted--]--refcounted--|
> 
> ocfs2_refcount_cal_cow_clusters() will be called on the first extent.
> It will set up *cow_len to cover that first extent, but then it will hit
> the second extent and break out.  ocfs2_refcount_cow() will then replace
> the first extent.  We end up with:
> 
>   |--not refcounted--|--not refcounted--]--refcounted--|
> 
> 
> 	Now we call to ocfs2_prepare_write_desc() again.  It will see
> the first and second extents as not refcounted, but then it will hit
> the third extent and return -ETXTBSY.  refcounted_cpos will be set to
> the third extent.  Then in ocfs2_write_begin_nolock(), we'll hit:
> 
> 		BUG_ON(refcounted_cpos != UINT_MAX);
> 
> Am I missing something?
you are right.

ocfs2_write_begin normally only write one page and with my x86_64 box, 
we have PAGE_SIZE <= CLUSTER_SIZE, so we never hit this issue before.
So maybe I have to add a new function which will iterate from pos to 
pos+len to make sure all the refcounted extents are CoWed. Thanks.

Regards,
Tao