On Thu, May 28, 2009 at 11:44:34AM +0200, Jan Kara wrote:
> On Wed 27-05-09 21:05:59, Aneesh Kumar K.V wrote:
> > On Wed, May 27, 2009 at 03:01:05PM +0200, Jan Kara wrote:
> > > When we do delayed allocation of some buffer, we want to signal to VFS that
> > > the buffer is new (set buffer_new) so that it properly zeros out everything.
> > > But we don't have the buffer mapped yet so we cannot really unmap underlying
> > > metadata in this state. Make VFS avoid doing unmapping of metadata when the
> > > buffer is not yet mapped.
> > >
> ...
> > > @@ -2683,7 +2685,7 @@ int nobh_write_begin(struct file *file, struct address_space *mapping,
> > > goto failed;
> > > if (!buffer_mapped(bh))
> > > is_mapped_to_disk = 0;
> > > - if (buffer_new(bh))
> > > + if (buffer_new(bh) && buffer_mapped(bh))
> > > unmap_underlying_metadata(bh->b_bdev, bh->b_blocknr);
> > > if (PageUptodate(page)) {
> > > set_buffer_uptodate(bh);
> >
> > Both xfs and ext4 return mapped delay buffer_head when we do a get_block
> > with delayed allocation in write_begin phase.
> Yeah, I knew about ext4 doing this. Thanks for pointing this out. I
> wanted to trigger a separate discussion about this and similar problems -
> the current state of buffer bits is quite messy (I think Ted complained
> about this as well recently) and we should somehow clean it up.
> In this particular case: What's the point in returning the buffer mapped?
> It does not make any sence logically (block *does not* have any physical
> location assigned) and technically you have to map it to some fake block
> and later remap it correctly when you do block allocation. So maybe I'm
> missing some good reason but from what I can see, it just does not make
> sence...
Marking it mapped make sure we don't do multiple get_block calls for
every write. For each write in write_begin path we do a get_block
call if the buffer is not mapped. (__block_prepare_write have more
details.)
-aneesh
--
To unsubscribe from this list: send the line "unsubscribe linux-ext4" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
