Re: [PATCH 2/2] readahead: improve sequential read detection

[Oleg Nesterov <oleg@tv-sign.ru>]

Ram wrote:
>
> On Wed, 2005-03-02 at 11:08, Oleg Nesterov wrote:
> >
> >  out:
> > - return newsize;
> > + return ra->prev_page + 1;
>
> This change introduces one key behavioural change in
> page_cache_readahead(). Instead of returning the number-of-pages
> successfully read, it now returns the next-page-index which is yet to be
> read. Was this essential?

The problem is that with this change:
+       if (offset == ra->prev_page && --req_size)
+               ++offset;
we can't just return newsize.

Because the caller of page_cache_readahead(offset, req_size) expects
that returned value is the number-of-pages successfully read from
this original offset.

Consider do_generic_mapping_read() reading two pages at offset 10,
with ra->prev_page == 10.

1st page, index == 10:
	ret_size = page_cache_readahead(10, 2);		// returns 1
	next_index += ret_size;				// next_index == 11

2nd page, index == 11:
	if (index == next_index)			// Yes
		page_cache_readahead(11, 1);		// BOGUS!

It can be solved without behavioural change of course, but it will be
more complex.

> At least, a comment towards this effect at the top of the function is
> worth adding.

Yes, it's my fault. I should have added comment in changelog at least.

Oleg.