Re: "mm: fix lazyfree BUG_ON check in try_to_unmap_one()" build error

[Minchan Kim <minchan@kernel.org>]

Hi Sergey,

On Thu, Mar 09, 2017 at 01:29:08PM +0900, Sergey Senozhatsky wrote:
> Hello Minchan,
> 
> /* I can't https://marc.info/?l=linux-kernel&m=148886631303107 thread
>    in my mail box for some reason so the Reply-To message-id may be wrong. */
> 
> 
> 
> commit "mm: fix lazyfree BUG_ON check in try_to_unmap_one()"
> (mmotm fd07630cbf59bead90046dd3e5cfd891e58e6987)
> 
> 
> 	if (VM_WARN_ON_ONCE(PageSwapBacked(page) !=
> 			PageSwapCache(page))) {
> 	...
> 	}
> 
> 
> does not compile on !CONFIG_DEBUG_VM configs, because VM_WARN_ONCE() is
> 
> 	#define BUILD_BUG_ON_INVALID(e) ((void)(sizeof((__force long)(e))))
> 
> 
> 
> In file included from ./include/linux/mmdebug.h:4:0,
>                  from ./include/linux/mm.h:8,
>                  from mm/rmap.c:48:
> mm/rmap.c: In function ‘try_to_unmap_one’:
> ./include/linux/bug.h:45:33: error: void value not ignored as it ought to be
>  #define BUILD_BUG_ON_INVALID(e) ((void)(sizeof((__force long)(e))))
>                                  ^
> ./include/linux/mmdebug.h:49:31: note: in expansion of macro ‘BUILD_BUG_ON_INVALID’
>  #define VM_WARN_ON_ONCE(cond) BUILD_BUG_ON_INVALID(cond)
>                                ^~~~~~~~~~~~~~~~~~~~
> mm/rmap.c:1416:8: note: in expansion of macro ‘VM_WARN_ON_ONCE’
>     if (VM_WARN_ON_ONCE(PageSwapBacked(page) !=
>         ^~~~~~~~~~~~~~~
> 
> 	-ss
> 

Thanks for the report, Sergey!
If others are not against, I want to go this.

>From 38b10e560d066c2cef8f9d028e14008cefdaa3e0 Mon Sep 17 00:00:00 2001
From: Minchan Kim <minchan@kernel.org>
Date: Thu, 9 Mar 2017 14:58:23 +0900
Subject: [PATCH] mm: do not use VM_WARN_ON_ONCE as if condition

Sergey reported VM_WARN_ON_ONCE returns void with !CONFIG_DEBUG_VM
so we cannot use it as if's condition unlike WARN_ON.

This patch fixes it.

Signed-off-by: Minchan Kim <minchan@kernel.org>
---
 mm/rmap.c | 5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)

diff --git a/mm/rmap.c b/mm/rmap.c
index 1d82057144ba..7d24bb93445b 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -1413,12 +1413,11 @@ static int try_to_unmap_one(struct page *page, struct vm_area_struct *vma,
 			 * Store the swap location in the pte.
 			 * See handle_pte_fault() ...
 			 */
-			if (VM_WARN_ON_ONCE(PageSwapBacked(page) !=
-						PageSwapCache(page))) {
+			if (unlikely(PageSwapBacked(page) != PageSwapCache(page))) {
+				WARN_ON_ONCE(1);
 				ret = SWAP_FAIL;
 				page_vma_mapped_walk_done(&pvmw);
 				break;
-
 			}
 
 			/* MADV_FREE page check */
-- 
2.7.4