Re: rw_swap_page() and swapin readahead

["Stephen C. Tweedie" <sct@redhat.com>]

Hi,

On Wed, 25 Nov 1998 23:59:17 +0100 (CET), Rik van Riel
<H.H.vanRiel@phys.uu.nl> said:

> On Wed, 25 Nov 1998, Stephen C. Tweedie wrote:
>> 
>> The PG_free_after bit is there only to mark that increment, so that
>> the page count is decremented again (asynchronously) once the IO is
>> complete and no sooner. 

> Then what does the PG_decr_after do? It seems like there
> are two flags to do the same thing... I'm curious :)

It's in fs/buffer.c, after_unlock_page:

	if (test_and_clear_bit(PG_decr_after, &page->flags))
		atomic_dec(&nr_async_pages);
	if (test_and_clear_bit(PG_swap_unlock_after, &page->flags))
		swap_after_unlock_page(page->offset);
	if (test_and_clear_bit(PG_free_after, &page->flags))
		__free_page(page);

So PG_free_after causes us to decrement the page count after IO;
PG_decr_after decrements the global async page IO count (the one which
stops us doing runaway async swapout), andd PG_swap_unlock_after unlocks
the swap map after the IO.

--Stephen
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