linux-kernel - Re: [PATCH v14 049/138] mm/memcg: Add folio_lruvec_relock_irq() and folio_lruvec_relock

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Date:   Thu, 29 Jul 2021 14:39:02 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Mel Gorman <mgorman@...hsingularity.net>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        linux-fsdevel@...r.kernel.org, Christoph Hellwig <hch@....de>
Subject: Re: [PATCH v14 049/138] mm/memcg: Add folio_lruvec_relock_irq() and
 folio_lruvec_relock_irqsave()

On Thu, Jul 29, 2021 at 09:36:44AM +0100, Mel Gorman wrote:
> 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@...radead.org>
> > Reviewed-by: Christoph Hellwig <hch@....de>
> 
> 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

Yes, I read it over carefully and I can't see a way in which this
can happen.  Weird that this change made the compiler unable to figure
that out.  Pushed out a new for-next with your patch included.  Thanks!