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

[Badari Pulavarty]

Andrew Morton wrote:
> Badari Pulavarty <pbadari@xxxxxxxxxx> 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
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