[PATCH] mempool: simplify alloc

Mempool is pretty clever.  Looks too clever for its own good :) It
shouldn't really know so much about page reclaim internals.

- don't guess about what effective page reclaim might involve.

- don't randomly flush out all dirty data if some unlikely thing
  happens (alloc returns NULL). page reclaim can (sort of :P) handle
  it.

I think the main motivation is trying to avoid pool->lock at all costs.
However the first allocation is attempted with __GFP_WAIT cleared, so it
will be 'can_try_harder' if it hits the page allocator.  So if allocation
still fails, then we can probably afford to hit the pool->lock - and what's
the alternative?  Try page reclaim and hit zone->lru_lock?

A nice upshot is that we don't need to do any fancy memory barriers or do
(intentionally) racy access to pool-> fields outside the lock.
Signed-off-by: Nick Piggin <nickpiggin@yahoo.com.au>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

mm/mempool.c

@@ -198,36 +198,22 @@ void * mempool_alloc(mempool_t *pool, unsigned int __nocast gfp_mask)
 	void *element;
 	unsigned long flags;
 	DEFINE_WAIT(wait);
-	int gfp_nowait;
+	int gfp_temp;
+
+	might_sleep_if(gfp_mask & __GFP_WAIT);
 
 	gfp_mask |= __GFP_NOMEMALLOC;	/* don't allocate emergency reserves */
 	gfp_mask |= __GFP_NORETRY;	/* don't loop in __alloc_pages */
 	gfp_mask |= __GFP_NOWARN;	/* failures are OK */
-	gfp_nowait = gfp_mask & ~(__GFP_WAIT | __GFP_IO);
 
-	might_sleep_if(gfp_mask & __GFP_WAIT);
+	gfp_temp = gfp_mask & ~(__GFP_WAIT|__GFP_IO);
+
 repeat_alloc:
-	element = pool->alloc(gfp_nowait, pool->pool_data);
+
+	element = pool->alloc(gfp_temp, pool->pool_data);
 	if (likely(element != NULL))
 		return element;
 
-	/*
-	 * If the pool is less than 50% full and we can perform effective
-	 * page reclaim then try harder to allocate an element.
-	 */
-	mb();
-	if ((gfp_mask & __GFP_FS) && (gfp_mask != gfp_nowait) &&
-				(pool->curr_nr <= pool->min_nr/2)) {
-		element = pool->alloc(gfp_mask, pool->pool_data);
-		if (likely(element != NULL))
-			return element;
-	}
-
-	/*
-	 * Kick the VM at this point.
-	 */
-	wakeup_bdflush(0);
-
 	spin_lock_irqsave(&pool->lock, flags);
 	if (likely(pool->curr_nr)) {
 		element = remove_element(pool);
@@ -240,6 +226,8 @@ void * mempool_alloc(mempool_t *pool, unsigned int __nocast gfp_mask)
 	if (!(gfp_mask & __GFP_WAIT))
 		return NULL;
 
+	/* Now start performing page reclaim */
+	gfp_temp = gfp_mask;
 	prepare_to_wait(&pool->wait, &wait, TASK_UNINTERRUPTIBLE);
 	mb();
 	if (!pool->curr_nr)