Optimize segment memory allocation further

If the page from the segmem pool is too big, do not just trim it, but
rather trade it for a smaller page if that is sufficient.

Ref #60

src/fellow_cache.c

@@ -4971,21 +4971,36 @@ static size_t fellow_busy_seg_memalloc(struct fellow_busy *fbo,
 	    fbo_segmem_get(fbo->segmem, fbo->fc->tune));
 	AN(mem.ptr);
 	sz = (size_t)1 << mem.bits;
-	if (sz >= fds->seg.size) {
-		fcs->alloc.ptr = mem.ptr;
-		fcs->alloc.size = sz;
-		buddy_trim1_ptr_extent(fbo->fc->membuddy, &fcs->alloc,
-		    fds->seg.size);
+
+	/*
+	 * if the fbo_segmem pool gave us too large/small a page,
+	 * trade it in for a new allocation
+	 */
+	if (log2up(fds->seg.size) < mem.bits ||
+	    (cram == 0 && sz < fds->seg.size)) {
+		struct buddy_reqs *reqs =
+		    BUDDY_REQS_STK(fbo->fc->membuddy, 1);
+
+		BUDDY_REQS_PRI(reqs, FEP_SPCPRI);
+		AN(buddy_req_extent(reqs, fds->seg.size, 0));
+		buddy_return1_ptr_page(fbo->fc->membuddy, &mem);
+		AN(buddy_alloc_wait(reqs));
+		fcs->alloc = buddy_get_ptr_extent(reqs, 0);
+		buddy_alloc_wait_done(reqs);
 	}
 	else {
-		buddy_return1_ptr_page(fbo->fc->membuddy, &mem);
-		fcs->alloc = buddy_alloc1_ptr_extent_wait(fbo->fc->membuddy,
-		    FEP_SPCPRI, fds->seg.size, cram);
+		fcs->alloc.ptr = mem.ptr;
+		fcs->alloc.size = sz;
 	}
 
 	if (fcs->alloc.ptr == NULL)
 		return (0);
 
+	if (fds->seg.size < fcs->alloc.size) {
+		buddy_trim1_ptr_extent(fbo->fc->membuddy, &fcs->alloc,
+		    fds->seg.size);
+	}
+
 	// cram needs to be chosen correctly
 	assert(fcs->alloc.size >= MIN_FELLOW_BLOCK);
 	assert((fcs->alloc.size & FELLOW_BLOCK_ALIGN) == 0);