[PATCH] readahead fix
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.
Showing
Please register or sign in to comment