Re: [PATCH] mm/mmzone.c: fix page_cpupid_xchg_last() to READ_ONCE() the page flags

[Peter Zijlstra <peterz@infradead.org>]

On Tue, Jan 18, 2022 at 03:05:39PM -0800, Peter Collingbourne wrote:
> After submitting a patch with a compare-exchange loop similar to this
> one to set the KASAN tag in the page flags, Andrey Konovalov pointed
> out that we should be using READ_ONCE() to read the page flags. Fix
> it here.

What does it actually fix? If it manages to split the read and read
garbage the cmpxchg will fail and we go another round, no harm done.

> Fixes: 75980e97dacc ("mm: fold page->_last_nid into page->flags where possible")

As per the above argument, I don't think this rates a Fixes tag, there
is no actual fix.

> Signed-off-by: Peter Collingbourne <pcc@google.com>
> Link: https://linux-review.googlesource.com/id/I2e1f5b5b080ac9c4e0eb7f98768dba6fd7821693

That's that doing here?

> Cc: stable@vger.kernel.org

That's massively over-selling things.

> ---
>  mm/mmzone.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/mm/mmzone.c b/mm/mmzone.c
> index eb89d6e018e2..f84b84b0d3fc 100644
> --- a/mm/mmzone.c
> +++ b/mm/mmzone.c
> @@ -90,7 +90,7 @@ int page_cpupid_xchg_last(struct page *page, int cpupid)
>  	int last_cpupid;
>  
>  	do {
> -		old_flags = flags = page->flags;
> +		old_flags = flags = READ_ONCE(page->flags);
>  		last_cpupid = page_cpupid_last(page);
>  
>  		flags &= ~(LAST_CPUPID_MASK << LAST_CPUPID_PGSHIFT);

I think that if you want to touch that code, something like the below
makes more sense...

diff --git a/mm/mmzone.c b/mm/mmzone.c
index eb89d6e018e2..ed9f4bcdc9ee 100644
--- a/mm/mmzone.c
+++ b/mm/mmzone.c
@@ -89,13 +89,14 @@ int page_cpupid_xchg_last(struct page *page, int cpupid)
 	unsigned long old_flags, flags;
 	int last_cpupid;
 
+	old_flags = READ_ONCE(page->flags);
 	do {
-		old_flags = flags = page->flags;
-		last_cpupid = page_cpupid_last(page);
+		flags = old_flags;
+		last_cpupid = (flags >> LAST_CPUPID_PGSHIFT) & LAST_CPUPID_MASK;
 
 		flags &= ~(LAST_CPUPID_MASK << LAST_CPUPID_PGSHIFT);
 		flags |= (cpupid & LAST_CPUPID_MASK) << LAST_CPUPID_PGSHIFT;
-	} while (unlikely(cmpxchg(&page->flags, old_flags, flags) != old_flags));
+	} while (unlikely(!try_cmpxchg(&page->flags, &old_flags, flags)));
 
 	return last_cpupid;
 }