Re: [PATCH] 2.6.11-mm1 "nobh" support for ext3 writeback mode

[Badari Pulavarty <pbadari@us.ibm.com>]

Andrew Morton wrote:
> Badari Pulavarty <pbadari@us.ibm.com> wrote:
> 
>> 	iblock = index << (PAGE_CACHE_SHIFT - inode->i_sb->s_blocksize_bits);
> 
> 
> It would still be nice to add a comment in here...
> 
> 
>>+	if (test_opt(inode->i_sb, NOBH) && !page_has_buffers(page)) {
>>+		if (!PageUptodate(page)) {
>>+			struct buffer_head map_bh;
>>+			bh = &map_bh;
>>+			bh->b_state = 0;
>>+			clear_buffer_mapped(bh);
>>+			ext3_get_block(inode, iblock, bh, 0);
>>+			if (!buffer_mapped(bh)) 
>>+				goto unlock;
>>+			err = -EIO;
>>+			set_bh_page(bh, page, 0);
>>+			bh->b_this_page = 0;
>>+			bh->b_size = 1 << inode->i_blkbits;
>>+			ll_rw_block(READ, 1, &bh);
>>+			wait_on_buffer(bh);
>>+			if (!buffer_uptodate(bh))
>>+				goto unlock;
>>+			SetPageMappedToDisk(page);
>>+		}
>>+		kaddr = kmap_atomic(page, KM_USER0);
>>+		memset(kaddr + offset, 0, length);
>>+		flush_dcache_page(page);
>>+		kunmap_atomic(kaddr, KM_USER0);
>>+		set_page_dirty(page);
>>+		err = 0;
>>+		goto unlock;
>>+	}
>>+	
>> 	if (!page_has_buffers(page))
>> 		create_empty_buffers(page, blocksize, 0);
> 
> 
> Given that we're about to go add buffers to the page, why not do that
> first, and use the page's own buffer_head rather than cooking up a local
> one?  Then we can simply use sb_bread().
> 
> 

Only reason for cooking up the local one is not to attach the buffer to
the page forever. That will end up forcing other routines (like 
writepage/writepages) go thro "confused" code. I was hoping not to
attach buffers if they are not really needed.

But again, doing it the way you suggested will make the code more
readable.

Thanks,
Badari