On Tue, Nov 24, 2015 at 04:27:56PM +0100, Vlastimil Babka wrote:

On 11/13/2015 03:23 AM, Joonsoo Kim wrote:

quoted

Now, we have tracepoint in test_pages_isolated() to notify
pfn which cannot be isolated. But, in alloc_contig_range(),
some error path doesn't call test_pages_isolated() so it's still
hard to know exact pfn that causes allocation failure.

This patch change this situation by calling test_pages_isolated()
in almost error path. In allocation failure case, some overhead
is added by this change, but, allocation failure is really rare
event so it would not matter.

In fatal signal pending case, we don't call test_pages_isolated()
because this failure is intentional one.

Signed-off-by: Joonsoo Kim <redacted>
---
 mm/page_alloc.c | 10 +++++++---
 1 file changed, 7 insertions(+), 3 deletions(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index d89960d..e78d78f 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c

@@ -6756,8 +6756,12 @@ int alloc_contig_range(unsigned long start, unsigned long end,
 	if (ret)
 		return ret;

+	/*
+	 * In case of -EBUSY, we'd like to know which page causes problem.
+	 * So, just fall through. We will check it in test_pages_isolated().
+	 */
 	ret = __alloc_contig_migrate_range(&cc, start, end);
-	if (ret)
+	if (ret && ret != -EBUSY)
 		goto done;

 	/*

@@ -6784,8 +6788,8 @@ int alloc_contig_range(unsigned long start, unsigned long end,
 	outer_start = start;
 	while (!PageBuddy(pfn_to_page(outer_start))) {
 		if (++order >= MAX_ORDER) {
-			ret = -EBUSY;
-			goto done;
+			outer_start = start;
+			break;
 		}
 		outer_start &= ~0UL << order;
 	}

Ugh isn't this crazy loop broken? Shouldn't it test that the buddy
it finds has order high enough? e.g.:
  buddy = pfn_to_page(outer_start)
  outer_start + (1UL << page_order(buddy)) > start

Otherwise you might end up with something like:
- at "start" there's a page that CMA failed to freed
- at "start-1" there's another non-buddy page
- at "start-3" there's an order-1 buddy, so you set outer_start to start-3
- test_pages_isolated() will complain (via the new tracepoint) about
pfn of start-1, but actually you would like it to complain about pfn
of "start"?

So the loop has been broken before your patch, but it didn't matter,
just potentially wasted some time by picking bogus outer_start. But
now your tracepoint will give you weird results.

Good catch. I will fix it.

Thanks.

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