Currently page_cache_readahead() treats ra->size == 0 (first read)
and ra->size == -1 (ra_off was called) separately, but does exactly
the same in both cases.

With this patch we may assume that the reading starts in 'ra_off()'
state, so we don't need to consider the first read as a special case.


file_ra_state_init() sets
	ra->prev_page = -1;
	ra->size      =  0;

When the page_cache_readahead() is called for the first time it sets
ra->size to nonzero value either via get_init_ra_size() or ra_off().
So ra->size == 0 implies that ra->prev_page == -1. I am ignoring the
case when readahead is disabled via ra->ra_pages == 0.


page_cache_readahead detects sub-page sized reads:
	if (offset == ra->prev_page && req_size == 1 && ra->size != 0)

But if offset == ra->prev_page, then ra->size == 0 can happen only if
offset == -1, so there is no need to check ra->size here. If application
starts reading 16Tb file from the last page then readahead can't help.


First offset==0 read or first sequential detection:
	if ((ra->size == 0 && offset == 0) || (ra->size == -1 && sequential)
could be changed to:
	if ((ra->size == 0 && sequential) || (ra->size == -1 && sequential)
which means:
	if (sequential && (ra->size == 0 || ra->size == -1))


Random case detection:
	if (!sequential || (ra->size == 0))
But if sequential == 1, then ra->size can't be 0, this case is already handled
before.


Now we have:

	if (offset == ra->prev_page && req_size == 1)
		/* sub-page reads */

	if (sequential && (ra->size == 0 || ra->size == -1))
		/* first offset==0 read or first sequential */

	if (!sequential)
		/* random case */

Now ->size is checked only in one place, so ra_off() can set ra->size = 0,
and we can just test ->size against 0.
Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>