Re: [PATCH 1/1] mm: compaction: avoid fast_isolate_around() to set pageblock_skip on reserved pages

[Andrea Arcangeli <aarcange@redhat.com>]

Hi Mel,

On Thu, Nov 26, 2020 at 10:47:20AM +0000, Mel Gorman wrote:
> Agreed. This thread has a lot of different directions in it at this
> point so what I'd hope for is first, a patch that initialises holes with
> zone/node linkages within a 1<<(MAX_ORDER-1) alignment. If there is a
> hole, it would be expected the pages are PageReserved. Second, a fix to
> fast_isolate that forces PFNs returned to always be within the stated
> zone boundaries.

The last two patches should resolve the struct page
initialization
https://git.kernel.org/pub/scm/linux/kernel/git/andrea/aa.git/ and the
VM_BUG_ON_PAGE never happened again as expected.

So I looked back to see how the "distance" logic is accurate. I added
those checks and I get negative hits:

diff --git a/mm/compaction.c b/mm/compaction.c
index cc1a7f600a86..844a90b0acdf 100644
--- a/mm/compaction.c
+++ b/mm/compaction.c
@@ -1331,6 +1331,12 @@ fast_isolate_freepages(struct compact_control *cc)
 	low_pfn = pageblock_start_pfn(cc->free_pfn - (distance >> 2));
 	min_pfn = pageblock_start_pfn(cc->free_pfn - (distance >> 1));
 
+	WARN_ON_ONCE((long) low_pfn < 0);
+	WARN_ON_ONCE((long) min_pfn < 0);
+	if ((long) low_pfn < 0)
+		return cc->free_pfn;
+	if ((long) min_pfn < 0)
+		return cc->free_pfn;
 	if (WARN_ON_ONCE(min_pfn > low_pfn))
 		low_pfn = min_pfn;

Both warn-on-once triggers, so it goes negative. While it appears not
kernel crashing since pfn_valid won't succeed on negative entries and
they'll always be higher than any pfn in the freelist, is this sign
that there's further room for improvement here?

Thanks,
Andrea