Re: [PATCH 4/4] ext3: Implement delayed allocation on page_mkwrite time

[Andrew Morton <akpm@linux-foundation.org>]

On Mon,  2 May 2011 22:56:56 +0200
Jan Kara <jack@suse.cz> wrote:

> So far, ext3 was allocating necessary blocks for mmapped writes when
> writepage() was called. There are several issues with this. The worst
> being that user is allowed to arbitrarily exceed disk quotas because
> writepage() is called from flusher thread context (which is root) and thus
> quota limits are ignored. Another bad consequence is that data is just lost
> if we find there's no space on the filesystem during ->writepage() time.
> 
> We solve these issues by implementing block reservation in page_mkwrite()
> callback. We don't want to really allocate blocks on page_mkwrite() time
> because for random writes via mmap (as seen for example with applications using
> BerkeleyDB) it results in much more fragmented files and thus much worse
> performance. So we allocate indirect blocks and reserve space for data block in
> page_mkwrite() and do the allocation of data block from writepage().

Yes, instantiating the metadata and accounting the data is a good
approach.  The file layout will be a bit suboptimal, but surely that
will be a minor thing.

But boy, it's a complicated patch!  Are we really sure that we want to
make changes this extensive to our antiquated old fs?  Or do we just
say "yeah, it's broken with quotas - use ext4"?