mm/mmzone.c: use try_cmpxchg() in page_cpupid_xchg_last()

This will let us avoid an additional read from page->flags when retrying
the compare-exchange on some architectures.

Link: https://lkml.kernel.org/r/20220120011200.1322836-1-pcc@google.com
Link: https://linux-review.googlesource.com/id/I2e1f5b5b080ac9c4e0eb7f98768dba6fd7821693
Signed-off-by: Peter Collingbourne <pcc@google.com>
Suggested-by: Peter Zijlstra <peterz@infradead.org>
Cc: Andrey Konovalov <andreyknvl@gmail.com>
Cc: Mel Gorman <mgorman@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/mm/mmzone.c b/mm/mmzone.c
index eb89d6e018e29b4231a486b821fac62f8b683b36..d8a9b0e1b52670727adc618fa18ce491fcd61b52 100644 (file)
--- 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;
 }