mm/compaction.c: correct zone boundary handling when isolating pages from a pageblock

syzbot reported the following error from a tree with a head commit of baf76f0c58ae ("slip: make slhc_free() silently accept an error pointer")

  BUG: unable to handle kernel paging request at ffffea0003348000
  #PF error: [normal kernel read fault]
  PGD 12c3f9067 P4D 12c3f9067 PUD 12c3f8067 PMD 0
  Oops: 0000 [#1] PREEMPT SMP KASAN
  CPU: 1 PID: 28916 Comm: syz-executor.2 Not tainted 5.1.0-rc6+ #89
  Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
  RIP: 0010:constant_test_bit arch/x86/include/asm/bitops.h:314 [inline]
  RIP: 0010:PageCompound include/linux/page-flags.h:186 [inline]
  RIP: 0010:isolate_freepages_block+0x1c0/0xd40 mm/compaction.c:579
  Code: 01 d8 ff 4d 85 ed 0f 84 ef 07 00 00 e8 29 00 d8 ff 4c 89 e0 83 85 38 ff
  ff ff 01 48 c1 e8 03 42 80 3c 38 00 0f 85 31 0a 00 00 <4d> 8b 2c 24 31 ff 49
  c1 ed 10 41 83 e5 01 44 89 ee e8 3a 01 d8 ff
  RSP: 0018:ffff88802b31eab8 EFLAGS: 00010246
  RAX: 1ffffd4000669000 RBX: 00000000000cd200 RCX: ffffc9000a235000
  RDX: 000000000001ca5e RSI: ffffffff81988cc7 RDI: 0000000000000001
  RBP: ffff88802b31ebd8 R08: ffff88805af700c0 R09: 0000000000000000
  R10: 0000000000000000 R11: 0000000000000000 R12: ffffea0003348000
  R13: 0000000000000000 R14: ffff88802b31f030 R15: dffffc0000000000
  FS:  00007f61648dc700(0000) GS:ffff8880ae900000(0000) knlGS:0000000000000000
  CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
  CR2: ffffea0003348000 CR3: 0000000037c64000 CR4: 00000000001426e0
  Call Trace:
   fast_isolate_around mm/compaction.c:1243 [inline]
   fast_isolate_freepages mm/compaction.c:1418 [inline]
   isolate_freepages mm/compaction.c:1438 [inline]
   compaction_alloc+0x1aee/0x22e0 mm/compaction.c:1550

There is no reproducer and it is difficult to hit -- 1 crash every few days. The issue is very similar to the fix in commit 6b0868c820ff ("mm/compaction.c: correct zone boundary handling when resetting pageblock skip hints"). When isolating free pages around a target pageblock, the boundary handling is off by one and can stray into the next pageblock. Triggering the syzbot error requires that the end of pageblock is section or zone aligned, and that the next section is unpopulated.

A more subtle consequence of the bug is that pageblocks were being improperly used as migration targets which potentially hurts fragmentation avoidance in the long-term one page at a time.

A debugging patch revealed that it's definitely possible to stray outside of a pageblock which is not intended. While syzbot cannot be used to verify this patch, it was confirmed that the debugging warning no longer triggers with this patch applied. It has also been confirmed that the THP allocation stress tests are not degraded by this patch.

---
 mm/compaction.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

index 3319e0872d01..444029da4e9d 100644
@@ -1228,7 +1228,7 @@ fast_isolate_around(struct compact_control *cc, unsigned long pfn, unsigned long
 
 	/* Pageblock boundaries */
 	start_pfn = pageblock_start_pfn(pfn);
-	end_pfn = min(start_pfn + pageblock_nr_pages, zone_end_pfn(cc->zone));
+	end_pfn = min(pageblock_end_pfn(pfn), zone_end_pfn(cc->zone)) - 1;
 
 	/* Scan before */
 	if (start_pfn != pfn) {

@@ -1239,7 +1239,7 @@ fast_isolate_around(struct compact_control *cc, unsigned long pfn, unsigned long
 
 	/* Scan after */
 	start_pfn = pfn + nr_isolated;
-	if (start_pfn != end_pfn)
+	if (start_pfn < end_pfn)
 		isolate_freepages_block(cc, &start_pfn, end_pfn, &cc->freepages, 1, false);
 
 	/* Skip this pageblock in the future as it's full or nearly full */