[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>

diff --git a/mm/mempool.c b/mm/mempool.c
index d691b5cb8022c640a1b4fc7538cccfce24970003..e9a0a6337b21f66d72b765d79993cd6e74c40c64 100644 (file)
--- a/mm/mempool.c
+++ b/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 @@ repeat_alloc:
        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)