x86: Fix step size adjustment during initial memory mapping

The old scheme can lead to failure in certain cases - the
problem is that after bumping step_size the next (non-final)
iteration is only guaranteed to make available a memory block
the size of what step_size was before. E.g. for a memory block
[0,3004600000) we'd have:

iter start end step amount
1 3004400000 30045fffff 2M 2M
2 3004000000 30043fffff 64M 4M
3 3000000000 3003ffffff 2G 64M
4 2000000000 2fffffffff 64G 64G

Yet to map 64G with 4k pages (as happens e.g. under PV Xen) we
need slightly over 128M, but the first three iterations made
only about 70M available.

The condition (new_mapped_ram_size > mapped_ram_size) for
bumping step_size is just not suitable. Instead we want to bump
it when we know we have enough memory available to cover a block
of the new step_size. And rather than making that condition more
complicated than needed, simply adjust step_size by the largest
possible factor we know we can cover at that point - which is
shifting it left by one less than the difference between page
table level shifts. (Interestingly the original STEP_SIZE_SHIFT
definition had a comment hinting at that having been the
intention, just that it should have been PUD_SHIFT-PMD_SHIFT-1
instead of (PUD_SHIFT-PMD_SHIFT)/2, and of course for non-PAE
32-bit we can't really use these two constants as they're equal
there.)

Furthermore the comment in get_new_step_size() didn't get
updated when the bottom-down mapping logic got added. Yet while
an overflow (flushing step_size to zero) of the shift doesn't
matter for the top-down method, it does for bottom-up because
round_up(x, 0) = 0, and an upper range boundary of zero can't
really work well.

Signed-off-by: Jan Beulich <[e-mail]>
Acked-by: Yinghai Lu <[e-mail]>
Link: http://lkml.kernel.org/r/[e-mail]
Signed-off-by: Ingo Molnar <[e-mail]>

arch/x86/mm/init.c

···
 static unsigned long __init get_new_step_size(unsigned long step_size)
 {
 	/*
-	 * Explain why we shift by 5 and why we don't have to worry about
-	 * 'step_size << 5' overflowing:
-	 *
-	 * initial mapped size is PMD_SIZE (2M).
+	 * Initial mapped size is PMD_SIZE (2M).
 	 * We can not set step_size to be PUD_SIZE (1G) yet.
 	 * In worse case, when we cross the 1G boundary, and
 	 * PG_LEVEL_2M is not set, we will need 1+1+512 pages (2M + 8k)
-	 * to map 1G range with PTE. Use 5 as shift for now.
+	 * to map 1G range with PTE. Hence we use one less than the
+	 * difference of page table level shifts.
 	 *
-	 * Don't need to worry about overflow, on 32bit, when step_size
-	 * is 0, round_down() returns 0 for start, and that turns it
-	 * into 0x100000000ULL.
+	 * Don't need to worry about overflow in the top-down case, on 32bit,
+	 * when step_size is 0, round_down() returns 0 for start, and that
+	 * turns it into 0x100000000ULL.
+	 * In the bottom-up case, round_up(x, 0) returns 0 though too, which
+	 * needs to be taken into consideration by the code below.
 	 */
-	return step_size << 5;
+	return step_size << (PMD_SHIFT - PAGE_SHIFT - 1);
 }
 
 /**
···
 	unsigned long step_size;
 	unsigned long addr;
 	unsigned long mapped_ram_size = 0;
-	unsigned long new_mapped_ram_size;
 
 	/* xen has big range in reserved near end of ram, skip it at first.*/
 	addr = memblock_find_in_range(map_start, map_end, PMD_SIZE, PMD_SIZE);
···
 				start = map_start;
 		} else
 			start = map_start;
-		new_mapped_ram_size = init_range_memory_mapping(start,
+		mapped_ram_size += init_range_memory_mapping(start,
 							last_start);
 		last_start = start;
 		min_pfn_mapped = last_start >> PAGE_SHIFT;
-		/* only increase step_size after big range get mapped */
-		if (new_mapped_ram_size > mapped_ram_size)
+		if (mapped_ram_size >= step_size)
 			step_size = get_new_step_size(step_size);
-		mapped_ram_size += new_mapped_ram_size;
 	}
 
 	if (real_end < map_end)
···
 static void __init memory_map_bottom_up(unsigned long map_start,
 					unsigned long map_end)
 {
-	unsigned long next, new_mapped_ram_size, start;
+	unsigned long next, start;
 	unsigned long mapped_ram_size = 0;
 	/* step_size need to be small so pgt_buf from BRK could cover it */
 	unsigned long step_size = PMD_SIZE;
···
 	 * for page table.
 	 */
 	while (start < map_end) {
-		if (map_end - start > step_size) {
+		if (step_size && map_end - start > step_size) {
 			next = round_up(start + 1, step_size);
 			if (next > map_end)
 				next = map_end;
-		} else
+		} else {
 			next = map_end;
+		}
 
-		new_mapped_ram_size = init_range_memory_mapping(start, next);
+		mapped_ram_size += init_range_memory_mapping(start, next);
 		start = next;
 
-		if (new_mapped_ram_size > mapped_ram_size)
+		if (mapped_ram_size >= step_size)
 			step_size = get_new_step_size(step_size);
-		mapped_ram_size += new_mapped_ram_size;
 	}
 }