Re: [Patch 2/2] Retry based aio read - filesystem read changes

[Andrew Morton]

Suparna Bhattacharya <[e-mail]> wrote:
>
> +extern int FASTCALL(aio_wait_on_page_bit(struct page *page, int bit_nr));
> +static inline int aio_wait_on_page_locked(struct page *page)

Oh boy.

There are soooo many more places where we can block:

- write() -> down(&inode->i_sem)

- read()/write() -> read indirect block -> wait_on_buffer()

- read()/write() -> read bitmap block -> wait_on_buffer()

- write() -> allocate block -> mark_buffer_dirty() ->
	balance_dirty_pages() -> writer throttling

- write() -> allocate block -> journal_get_write_access()

- read()/write() -> update_a/b/ctime() -> journal_get_write_access()

- ditto for other journalling filesystems

- read()/write() -> anything -> get_request_wait()
  (This one can be avoided by polling the backing_dev_info congestion
   flags)

- read()/write() -> page allocator -> blk_congestion_wait()

- write() -> balance_dirty_pages() -> writer throttling

- probably others.

Now, I assume that what you're looking for here is an 80% solution, but it
seems that a lot more changes will be needed to get even that far.

And given that a single kernel thread per spindle can easily keep that
spindle saturated all the time, one does wonder "why try to do it this way at
all"?