PageLRU and the other flags

[Matthew Wilcox <willy@infradead.org>]

I'm not quite familiar with this side of the MM yet, but it seems to
me like we're encoding four page states in three bits:

 - !PageLRU (not an LRU page)
 - PageLRU (on the inactive list)
 - PageLRU + PageActive (on the active list)
 - PageLRU + PageUnevictable (on the unevictable list)

Can we neaten this up into two bits?

 - 00 (not an LRU page)
 - 01 (active list)
 - 10 (inactive list)
 - 11 (unevictable)

People who free up page flags are always popular, right?

I don't think the missing combos are used:

!LRU + Active
!LRU + Active + Unevictable
!LRU + Unevictable
LRU + Active + Unevictable

It seems fairly straightforward to add these macros:

bool PageLRU(struct page *page)
{
	page->flags & PAGE_FLAGS_LRU_MASK;
}

bool PageActive(struct page *page)
{
	page->flags & PAGE_FLAGS_LRU_MASK == PAGE_FLAGS_ACTIVE;
}

bool PageInactive(struct page *page)
{
	page->flags & PAGE_FLAGS_LRU_MASK == PAGE_FLAGS_INACTIVE;
}

bool PageUnevictable(struct page *page)
{
	page->flags & PAGE_FLAGS_LRU_MASK == PAGE_FLAGS_UNEVICTABLE;
}

SetPageActive seems a little more tricky to do atomically.  A cmpxchg()
loop, perhaps?  If we changed the API to be PageUnevictableToActive(page)
then it's an atomic subtraction / addition (although it's now twelve
macro instead of six Set/Clear for three bits).  Then I'm not sure how
to do TestClearPageActive().  Maybe TryPageActiveToInactive()?  That'd be
another twelve macros if all those state transitions are possible.