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