[PATCH] mm, page_alloc: capture page in task context only

[Hugh Dickins]

While stressing compaction, one run oopsed on NULL capc->cc in
__free_one_page()'s task_capc(zone): compact_zone_order() had been
interrupted, and a page was being freed in the return from interrupt.

Though you would not expect it from the source, both gccs I was using
(a 4.8.1 and a 7.5.0) had chosen to compile compact_zone_order() with
the ".cc = &cc" implemented by mov %rbx,-0xb0(%rbp) immediately before
callq compact_zone - long after the "current->capture_control = &capc".
An interrupt in between those finds capc->cc NULL (zeroed by an earlier
rep stos).

This could presumably be fixed by a barrier() before setting
current->capture_control in compact_zone_order(); but would also need
more care on return from compact_zone(), in order not to risk leaking
a page captured by interrupt just before capture_control is reset.

Maybe that is the preferable fix, but I felt safer for task_capc() to
exclude the rather surprising possibility of capture at interrupt time.

Fixes: 5e1f0f098b46 ("mm, compaction: capture a page under direct compaction")
Cc: stable@xxxxxxxxxxxxxxx # 5.1+
Signed-off-by: Hugh Dickins <hughd@xxxxxxxxxx>
---

 mm/page_alloc.c |    1 +
 1 file changed, 1 insertion(+)

--- 5.8-rc0/mm/page_alloc.c	2020-06-08 14:38:47.298625588 -0700
+++ linux/mm/page_alloc.c	2020-06-10 12:12:34.982950441 -0700
@@ -814,6 +814,7 @@ static inline struct capture_control *ta
 	struct capture_control *capc = current->capture_control;
 
 	return capc &&
+		in_task() &&
 		!(current->flags & PF_KTHREAD) &&
 		!capc->page &&
 		capc->cc->zone == zone &&