• Andrew Morton's avatar
    [PATCH] readahead fix · 00d6555e
    Andrew Morton authored
    Changes the way in which the readahead code locates the readahead
    setting for the underlying device.
    
    - struct block_device and struct address_space gain a *pointer* to the
      current readahead tunable.
    
    - The tunable lives in the request queue and is altered with the
      traditional ioctl.
    
    - The value gets *copied* into the struct file at open() time.  So a
      fcntl() mode to modify it per-fd is simple.
    
    - Filesystems which are not request_queue-backed get the address of the
      global `default_ra_pages'.  If we want, this can become a tunable.
    
    - Filesystems are at liberty to alter address_space.ra_pages to point
      at some other fs-private default at new_inode/read_inode/alloc_inode
      time.
    
    - The ra_pages pointer can become a structure pointer if, at some time
      in the future, high-level code needs more detailed information about
      device characteristics.
    
      In fact, it'll need to become a struct pointer for use by
      writeback: my current writeback code has the problem that multiple
      pdflush threads can get stuck on the same request queue.  That's a
      waste of resources.  I currently have a silly flag in the superblock
      to try to avoid this.
    
      The proper way to get this exclusion is for the high-level
      writeback code to be able to do a test-and-set against a
      per-request_queue flag.  That flag can live in a structure alongside
      ra_pages, conveniently accessible at the pagemap level.
    
    One thing still to-be-done is going into all callers of blk_init_queue
    and blk_queue_make_request and making sure that they're setting up a
    sensible default.  ATA wants 248 sectors, and floppy drives don't want
    128kbytes, I suspect.  Later.
    00d6555e
blkpg.c 8.33 KB