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

[Vlastimil Babka]

On 6/10/20 10:48 PM, Hugh Dickins wrote:
> 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).

Ugh, nasty. Same here with gcc 10.

> 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.

I was hoping a WRITE_ONCE(current->capture_control) would be enough,
but apparently it's not (I tried).

> 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>

Acked-by: Vlastimil Babka <vbabka@xxxxxxx>

But perhaps I would also make sure that we don't expose the half initialized
capture_control and run into this problem again later. It's not like this is a
fast path where barriers hurt. Something like this then? (with added comments)

diff --git a/mm/compaction.c b/mm/compaction.c
index fd988b7e5f2b..c89e26817278 100644
--- a/mm/compaction.c
+++ b/mm/compaction.c
@@ -2316,15 +2316,17 @@ static enum compact_result compact_zone_order(struct zone *zone, int order,
 		.page = NULL,
 	};
 
-	current->capture_control = &capc;
+	barrier();
+
+	WRITE_ONCE(current->capture_control, &capc);
 
 	ret = compact_zone(&cc, &capc);
 
 	VM_BUG_ON(!list_empty(&cc.freepages));
 	VM_BUG_ON(!list_empty(&cc.migratepages));
 
-	*capture = capc.page;
-	current->capture_control = NULL;
+	WRITE_ONCE(current->capture_control, NULL);
+	*capture = READ_ONCE(capc.page);
 
 	return ret;
 }