[PATCH] mm, compaction: move high_pfn to the for loop scope.

[Rokudo Yan]

In fast_isolate_freepages, high_pfn will be used if a prefered one(PFN >= low_fn) not found. But the high_pfn
is not reset before searching an free area, so when it was used as freepage, it may from another free area searched before.
And move_freelist_head(freelist, freepage) will have unexpected behavior(eg. corrupt the MOVABLE freelist)

Unable to handle kernel paging request at virtual address dead000000000200
Mem abort info:
  ESR = 0x96000044
  Exception class = DABT (current EL), IL = 32 bits
  SET = 0, FnV = 0
  EA = 0, S1PTW = 0
Data abort info:
  ISV = 0, ISS = 0x00000044
  CM = 0, WnR = 1
[dead000000000200] address between user and kernel address ranges

-000|list_cut_before(inline)
-000|move_freelist_head(inline)
-000|fast_isolate_freepages(inline)
-000|isolate_freepages(inline)
-000|compaction_alloc(?, ?)
-001|unmap_and_move(inline)
-001|migrate_pages([NSD:0xFFFFFF80088CBBD0] from = 0xFFFFFF80088CBD88, [NSD:0xFFFFFF80088CBBC8] get_new_p
-002|__read_once_size(inline)
-002|static_key_count(inline)
-002|static_key_false(inline)
-002|trace_mm_compaction_migratepages(inline)
-002|compact_zone(?, [NSD:0xFFFFFF80088CBCB0] capc = 0x0)
-003|kcompactd_do_work(inline)
-003|kcompactd([X19] p = 0xFFFFFF93227FBC40)
-004|kthread([X20] _create = 0xFFFFFFE1AFB26380)
-005|ret_from_fork(asm)
---|end of frame

The issue was reported on an smart phone product with 6GB ram and 3GB zram as swap device.

This patch fixes the issue by reset high_pfn before searching each free area, which ensure
freepage and freelist match when call move_freelist_head in fast_isolate_freepages().

Fixes: 5a811889de10f1eb ("mm, compaction: use free lists to quickly locate a migration target")

Signed-off-by: Rokudo Yan <wu-yan@xxxxxxx>
---
 mm/compaction.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/mm/compaction.c b/mm/compaction.c
index cc1a7f600a86..75f0e550b18f 100644
--- a/mm/compaction.c
+++ b/mm/compaction.c
@@ -1303,7 +1303,7 @@ fast_isolate_freepages(struct compact_control *cc)
 {
 	unsigned int limit = min(1U, freelist_scan_limit(cc) >> 1);
 	unsigned int nr_scanned = 0;
-	unsigned long low_pfn, min_pfn, high_pfn = 0, highest = 0;
+	unsigned long low_pfn, min_pfn, highest = 0;
 	unsigned long nr_isolated = 0;
 	unsigned long distance;
 	struct page *page = NULL;
@@ -1348,6 +1348,7 @@ fast_isolate_freepages(struct compact_control *cc)
 		struct page *freepage;
 		unsigned long flags;
 		unsigned int order_scanned = 0;
+		unsigned long high_pfn = 0;
 
 		if (!area->nr_free)
 			continue;
-- 
2.25.1