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Subject[PATCH] mm, compaction: make sure we isolate a valid freepage when high_pfn is used
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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().

Link: http://lkml.kernel.org/r/1558711908-15688-1-git-send-email-suzuki.poulose@arm.com
Fixes: 5a811889de10f1eb ("mm, compaction: use free lists to quickly locate a migration target")
---
mm/compaction.c | 1 +
1 file changed, 1 insertion(+)

diff --git a/mm/compaction.c b/mm/compaction.c
index cc1a7f600a86..6754229f4213 100644
--- a/mm/compaction.c
+++ b/mm/compaction.c
@@ -1352,6 +1352,7 @@ fast_isolate_freepages(struct compact_control *cc)
if (!area->nr_free)
continue;

+ high_pfn = 0;
spin_lock_irqsave(&cc->zone->lock, flags);
freelist = &area->free_list[MIGRATE_MOVABLE];
list_for_each_entry_reverse(freepage, freelist, lru) {
--
2.25.1
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