Re: [PATCH v14 049/138] mm/memcg: Add folio_lruvec_relock_irq() and folio_lruvec_relock_irqsave()

From: Mel Gorman
Date: Thu Jul 29 2021 - 04:36:53 EST

On Thu, Jul 15, 2021 at 04:35:35AM +0100, Matthew Wilcox (Oracle) wrote:
> These are the folio equivalents of relock_page_lruvec_irq() and
> folio_lruvec_relock_irqsave(). Also convert page_matches_lruvec()
> to folio_matches_lruvec().
> Signed-off-by: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx>
> Reviewed-by: Christoph Hellwig <hch@xxxxxx>

When build testing what you had in your for-next branch, I got a new
warning for powerpc defconfig

In file included from ./include/linux/mmzone.h:8,
from ./include/linux/gfp.h:6,
from ./include/linux/mm.h:10,
from mm/swap.c:17:
mm/swap.c: In function 'release_pages':
./include/linux/spinlock.h:290:3: warning: 'flags' may be used uninitialized in this function [-Wmaybe-uninitialized]
290 | _raw_spin_unlock_irqrestore(lock, flags); \
| ^~~~~~~~~~~~~~~~~~~~~~~~~~~
mm/swap.c:906:16: note: 'flags' was declared here
906 | unsigned long flags;
| ^~~~~

I'm fairly sure it's a false positive and the compiler just cannot figure
out that flags are only accessed when lruvec is !NULL and once lruvec is
!NULL, flags are valid

diff --git a/mm/swap.c b/mm/swap.c
index 6f382abeccf9..96a23af8d1c7 100644
--- a/mm/swap.c
+++ b/mm/swap.c
@@ -903,7 +903,7 @@ void release_pages(struct page **pages, int nr)
int i;
struct lruvec *lruvec = NULL;
- unsigned long flags;
+ unsigned long flags = 0;
unsigned int lock_batch;

for (i = 0; i < nr; i++) {