mm: don't miss the last page because of round-off error

I've noticed, that dying memory cgroups are often pinned in memory by a
single pagecache page.  Even under moderate memory pressure they sometimes
stayed in such state for a long time.  That looked strange.

My investigation showed that the problem is caused by applying the LRU
pressure balancing math:

  scan = div64_u64(scan * fraction[lru], denominator),

where

  denominator = fraction[anon] + fraction[file] + 1.

Because fraction[lru] is always less than denominator, if the initial scan
size is 1, the result is always 0.

This means the last page is not scanned and has
no chances to be reclaimed.

Fix this by rounding up the result of the division.

In practice this change significantly improves the speed of dying cgroups
reclaim.

[[email protected]: prevent double calculation of DIV64_U64_ROUND_UP() arguments]
Link: http://lkml.kernel.org/r/20180829213311.GA13501@castle
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Roman Gushchin <[email protected]>
Reviewed-by: Andrew Morton <[email protected]>
Cc: Johannes Weiner <[email protected]>
Cc: Michal Hocko <[email protected]>
Cc: Tejun Heo <[email protected]>
Cc: Rik van Riel <[email protected]>
Cc: Konstantin Khlebnikov <[email protected]>
Cc: Matthew Wilcox <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

diff --git a/include/linux/math64.h b/include/linux/math64.h
index 837f2f2d1d341ff56549d6b134149e1e92a49a69..bb2c84afb80c6c2246d233f7acab29c33bdb372e 100644 (file)
--- a/include/linux/math64.h
+++ b/include/linux/math64.h
@@ -281,4 +281,7 @@ static inline u64 mul_u64_u32_div(u64 a, u32 mul, u32 divisor)
 }
 #endif /* mul_u64_u32_div */
 
+#define DIV64_U64_ROUND_UP(ll, d)      \
+       ({ u64 _tmp = (d); div64_u64((ll) + _tmp - 1, _tmp); })
+
 #endif /* _LINUX_MATH64_H */

diff --git a/mm/vmscan.c b/mm/vmscan.c
index c5ef7240cbcbba05b4ef759b3c05d1ffdb163369..961401c46334cf815b1d92d7755314577aabd3e2 100644 (file)
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -2456,9 +2456,11 @@ out:
                        /*
                         * Scan types proportional to swappiness and
                         * their relative recent reclaim efficiency.
+                        * Make sure we don't miss the last page
+                        * because of a round-off error.
                         */
-                       scan = div64_u64(scan * fraction[file],
-                                        denominator);
+                       scan = DIV64_U64_ROUND_UP(scan * fraction[file],
+                                                 denominator);
                        break;
                case SCAN_FILE:
                case SCAN_ANON: