[PATCH] allow mremap and MAP_ANONYMOUS for large allocations

[Eric Wong <e@80x24.org>]

Our use of file-backed mmap on Linux benefits small allocations
since our overhead is most noticeable there.  Allocations
requiring mmap on a normal allocator are large enough that the
overhead from mwrap makes no difference.

However, Linux mremap can provide a noticeable improvement when
growing arrays quickly, such as those used for open address hash
tables or giant strings in HTML rendering.

This reverts 2d31f30386fa848b93a72bc2e4329e966026ab45
(mymalloc: use file-backed DIRECT_MMAP, too, 2023-02-01)
---
 dlmalloc_c.h | 8 +++++---
 mymalloc.h   | 7 +++++--
 2 files changed, 10 insertions(+), 5 deletions(-)

diff --git a/dlmalloc_c.h b/dlmalloc_c.h
index 38a0846..e842a53 100644
--- a/dlmalloc_c.h
+++ b/dlmalloc_c.h
@@ -4319,9 +4319,11 @@ static int sys_trim(mstate m, size_t pad) {
               !has_segment_link(m, sp)) { /* can't shrink if pinned */
             size_t newsize = sp->size - extra;
             (void)newsize; /* placate people compiling -Wunused-variable */
-            /* Prefer mremap, fall back to munmap */
-            if ((CALL_MREMAP(sp->base, sp->size, newsize, 0) != MFAIL) ||
-                (CALL_MUNMAP(sp->base + newsize, extra) == 0)) {
+            /*
+             * dlmalloc preferred mremap, here, but mwrap uses a file-backed
+             * mmap for small allocations on Linux and we close the FD ASAP.
+             */
+            if (CALL_MUNMAP(sp->base + newsize, extra) == 0) {
               released = extra;
             }
           }
diff --git a/mymalloc.h b/mymalloc.h
index 773d3ce..44c5dd3 100644
--- a/mymalloc.h
+++ b/mymalloc.h
@@ -96,8 +96,11 @@ static void *my_mmap(size_t size)
 #define USE_LOCKS 0 /* we do our own global_mtx + ms_tsd */
 #if MWRAP_FILE_BACKED
 #	define MMAP(size) my_mmap(size)
-#	define DIRECT_MMAP(size) my_mmap(size)
-#	define HAVE_MREMAP 0
+/*
+ * we use anonymous mmap (via DIRECT_MMAP) and mremap since it benefits
+ * large allocations which require resizing (open-addressed hash tables,
+ * large string buffers, etc)
+ */
 #endif
 #include "dlmalloc_c.h"
 #undef ABORT /* conflicts with Perl */