Re: find_get_swapcache_page() question

[Hugh Dickins]

On Mon, 27 Aug 2001, Marcelo Tosatti wrote:
> 
> Looking at find_get_swapcache_page(), I can't see _how_ we can find a
> page on the swapper pagecache table that is not a swapcache page.

I had a look at this a couple of weeks back.  If I remember rightly,
the page found by __find_page_nolock() cannot be other than a swapcache
page, and will always have PageSwapCache bit set... until pagecache_lock
is dropped on return to its only caller lookup_swap_cache().  In which the 
		if (!PageSwapCache(found))
			BUG();
		if (found->mapping != &swapper_space)
			BUG();
are not safe, since there may a concurrent remove_from_swap_cache(),
either from try_to_unuse() or from Rik's new vm_swap_full() deletion.
Those tests would be safe if the page were locked, but it's not.

I say find_get_swapcache_page() serves no purpose, should be deleted,
and find_get_page() used instead.  That was one of various things in
the swapoff patch I posted to linux-mm on 16 Aug, which I need to
finish off, cut into pieces and submit to Linus.

Hugh