On Mon, Sep 18, 2006 at 05:34:42PM +0400, Vladimir V. Saveliev wrote: > Can we put it that way: if a filesystem can use page_cache directly - it has > to set f_op->aio_read to generic_file_aio_read > and set f_op->read to NULL. If the filesystem wants to try to screw things up > a bit - > it implements f_op->read and its f_op->read is called by sys_read and > sys_readv > regardless to whether it has aio_read or not?
Not entirely. The idea I had in my mind was the following: - real filesystems should always implement the aio_ methods and must support async and vectored I/O - drivers or simple synthetic filesystems can implement just ->read and ->write and stay out of all the complexity. In the end I would like to enforce and invariant where a fileesystem or driver implements either the aio_ or normal methods, but we can't do that yet as there are far too many places calling thos directly. Thus we have do_sync_read and do_sync_write that are used as read and write methods for those more complex filesystems. (Anyone's got an idea how to enforce that people never set ->read and ->write to anything else when they implement the aio methods?) > > why does this matter for reiser4? > > > > reiser4 reads some files via generic page cache routines. In that case > reiser4' read calls do_sync_read. > Therefore, it has to define f_op->aio_read. > OTOH, there are files for which reiser4' read does not call do_sync_read. > > In case of readv, f_op->aio_read is called directly (if it is defined), which > may result in that reiser4' aio_read is called for files for which > reiser4' read would never call do_sync_read. > To avoid the problem we have to implement reiser4_aio_read which either calls > generic_file_aio_read or does something very similar to do_loop_readv_writev. In that case reiserfs should only implement aio_read and aio_write methods and use do_loop_readv_writev which we should export for a beginning. Longer term you should try to implement full blown aio and vector support even for those odd files (or find a way to migrate the pagecache). Do files change from odd to normal while they're instanciated? Otherwise you could just declare to sets of file_operations, once that uses the pagecache and one that doesn't and decide at inode instanciation time which one to use.
