Messages in this thread Patch in this message | | | Date | Wed, 19 Jan 2022 11:02:59 +0100 | From | Peter Zijlstra <> | Subject | Re: [PATCH] mm/mmzone.c: fix page_cpupid_xchg_last() to READ_ONCE() the page flags |
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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; }
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