Re: fs compression

[Tom Marshall <tom@cyngn.com>]

As I mentioned earlier, I had a hack working for transparent file 
compression.  But it has a few problems.  The two major ones are (1) It 
reads the lower pages synchronously by waiting on pages with 
wait_on_page_locked(), destroying readahead, and (2) it is swapping out 
i_size to call the lower readpage.  So I'm ditching that and going 
another route.

My idea now is to create a wrapper inode that is passed back to the VFS 
layer.  The wrapper inode would have the correct i_size, blkbits, and 
implement address_space_operations.  The implementation of 
readpage/readpages would then call down into the lower filesystem's 
readpage/readpages to fetch compressed data.  When the required pages 
are available, it would decompress and populate its own requested pages.

But one thing I'm wrestling with is how to be asynchronously notified 
when the lower readpage/readpages complete.  The two ideas that come to 
mind are (1) plumbing a callback into mpage_end_io(), (2) allowing 
override of mpage_end_io() with a custom function, (3) creating kernel 
threads analogous to kblockd to wait for pending pages.

But again, I'm new at this, so maybe there's an easier or better way?