On Thu 28-05-09 15:45:54, Aneesh Kumar K.V wrote: > 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 > > sense... > > 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.) Thanks for explanation. But you can always immediately return from get_block() when you see that buffer has buffer_delay() set if you do not want to do an allocation from there (e.g. in ext3 I want to do an allocation in such case for compatibility reasons but that's a separate story). Alternatively, if the get_block() call + immediate return bothers you, we could introduce something like AOP_FLAG_DELALLOC and make __block_prepare_write call get_block only if buffer is !mapped and !delay when this flag is set. IMO that would be probably the best. What do you think? Honza -- Jan Kara <jack@xxxxxxx> SUSE Labs, CR -- 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