[PATCH] malloc: Remove dynamic mmap/trim threshold [BZ #30769]

[Wilco Dijkstra]

Whenever a large mmap is released the mmap and trim thresholds are updated.
As a result these thresholds grow ever larger which means huge allocations
are always served by arenas rather than mmap.  The thresholds can end up as
large as an arena, which completely stops all trimming of the top block.
Remove the code completely - the default thresholds seem way too low for
modern 64-bit targets, but they can be increased separately.

Passes regress - OK for commit?

---

diff --git a/malloc/malloc.c b/malloc/malloc.c
index 6a888b0eb7de53ae7b814275e86d2bd2f06b5e53..8b7a414ab8d2c0f84ba7be7449a7909d1930d325 100644
--- a/malloc/malloc.c
+++ b/malloc/malloc.c
@@ -1770,10 +1770,6 @@ struct malloc_par
   int n_mmaps;
   int n_mmaps_max;
   int max_n_mmaps;
-  /* the mmap_threshold is dynamic, until the user sets
-     it manually, at which point we need to disable any
-     dynamic behavior. */
-  int no_dyn_threshold;
 
   /* Statistics */
   INTERNAL_SIZE_T mmapped_mem;
@@ -4291,18 +4287,6 @@ _int_free_chunk (mstate av, mchunkptr p, INTERNAL_SIZE_T size, int have_lock)
     /* Preserve errno in case munmap sets it.  */
     int err = errno;
 
-    /* See if the dynamic brk/mmap threshold needs adjusting.
-       Dumped fake mmapped chunks do not affect the threshold.  */
-    if (!mp_.no_dyn_threshold
-        && chunksize_nomask (p) > mp_.mmap_threshold
-        && chunksize_nomask (p) <= DEFAULT_MMAP_THRESHOLD_MAX)
-      {
-        mp_.mmap_threshold = chunksize (p);
-        mp_.trim_threshold = 2 * mp_.mmap_threshold;
-        LIBC_PROBE (memory_mallopt_free_dyn_thresholds, 2,
-		    mp_.mmap_threshold, mp_.trim_threshold);
-      }
-
     munmap_chunk (p);
 
     __set_errno (err);
@@ -4901,40 +4885,32 @@ __malloc_stats (void)
 static __always_inline int
 do_set_trim_threshold (size_t value)
 {
-  LIBC_PROBE (memory_mallopt_trim_threshold, 3, value, mp_.trim_threshold,
-	      mp_.no_dyn_threshold);
+  LIBC_PROBE (memory_mallopt_trim_threshold, 2, value, mp_.trim_threshold);
   mp_.trim_threshold = value;
-  mp_.no_dyn_threshold = 1;
   return 1;
 }
 
 static __always_inline int
 do_set_top_pad (size_t value)
 {
-  LIBC_PROBE (memory_mallopt_top_pad, 3, value, mp_.top_pad,
-	      mp_.no_dyn_threshold);
+  LIBC_PROBE (memory_mallopt_top_pad, 2, value, mp_.top_pad);
   mp_.top_pad = value;
-  mp_.no_dyn_threshold = 1;
   return 1;
 }
 
 static __always_inline int
 do_set_mmap_threshold (size_t value)
 {
-  LIBC_PROBE (memory_mallopt_mmap_threshold, 3, value, mp_.mmap_threshold,
-	      mp_.no_dyn_threshold);
+  LIBC_PROBE (memory_mallopt_mmap_threshold, 2, value, mp_.mmap_threshold);
   mp_.mmap_threshold = value;
-  mp_.no_dyn_threshold = 1;
   return 1;
 }
 
 static __always_inline int
 do_set_mmaps_max (int32_t value)
 {
-  LIBC_PROBE (memory_mallopt_mmap_max, 3, value, mp_.n_mmaps_max,
-	      mp_.no_dyn_threshold);
+  LIBC_PROBE (memory_mallopt_mmap_max, 2, value, mp_.n_mmaps_max);
   mp_.n_mmaps_max = value;
-  mp_.no_dyn_threshold = 1;
   return 1;
 }