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@xxxxxxxxxx>
> Link: https://linux-review.googlesource.com/id/I2e1f5b5b080ac9c4e0eb7f98768dba6fd7821693
That's that doing here?
> Cc: stable@xxxxxxxxxxxxxxx
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;
}
