mm: skip retry when new limit is not below old one in page_counter_set_max

In page_counter_set_max, we want to make sure the new limit is not below
the concurrently-changing counter value.  We read the counter and check
that the limit is not below the counter before the swap.  After the swap,
we read the counter again and retry in case the counter is incremented as
this may violate the requirement.  Even though the page_counter_try_charge
can see the old limit, it is guaranteed that the counter is not above the
old limit after the increment.  So in case the new limit is not below the
old limit, the counter is guaranteed to be not above the new limit too.
We can skip the retry in this case to optimize a little bit.

Link: https://lkml.kernel.org/r/20220821154055.109635-1-minhquangbui99@gmail.com
Signed-off-by: Bui Quang Minh <minhquangbui99@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/mm/page_counter.c b/mm/page_counter.c
index eb156ff5d60303c9ce69977ff2bc7c3204fa1331..8a0cc24b60dd4ca50737a823bc0b94cbeec0ef19 100644 (file)
--- a/mm/page_counter.c
+++ b/mm/page_counter.c
@@ -193,7 +193,7 @@ int page_counter_set_max(struct page_counter *counter, unsigned long nr_pages)
 
                old = xchg(&counter->max, nr_pages);
 
-               if (page_counter_read(counter) <= usage)
+               if (page_counter_read(counter) <= usage || nr_pages >= old)
                        return 0;
 
                counter->max = old;