[glibc/arm/morello/main] cheri: malloc: Set fixed mmap_threshold for capability narrowing

[glibc/arm/morello/main] cheri: malloc: Set fixed mmap_threshold for capability narrowing

[Szabolcs Nagy]

https://sourceware.org/git/gitweb.cgi?p=glibc.git;h=dc23cc80210799f8d8ab5b1ea2e00341e613a1a7

commit dc23cc80210799f8d8ab5b1ea2e00341e613a1a7
Author: Szabolcs Nagy <szabolcs.nagy@arm.com>
Date:   Thu Dec 8 16:32:56 2022 +0000

    cheri: malloc: Set fixed mmap_threshold for capability narrowing
    
    Capability narrowing requires extra alignment for large allocations,
    this can cause significant heap fragmentation so ensure large
    allocations use a single mmap and unmaped on free. The fragmentation
    is bug 14581.
    
    This patch disables dynamic mmap threshold when cap_narrowing is
    enabled and uses a threshold of 256 kbytes on morello (which means
    at most 64 byte alignment requirement on the heap). The dynamic
    threshold could quickly grow very big and stay there making the
    fragmentation issue common.

Diff:
---
 malloc/arena.c                     | 8 +++++++-
 sysdeps/aarch64/morello/libc-cap.h | 4 ++++
 2 files changed, 11 insertions(+), 1 deletion(-)

diff --git a/malloc/arena.c b/malloc/arena.c
index c63cb70f74..05df8c1510 100644
--- a/malloc/arena.c
+++ b/malloc/arena.c
@@ -331,7 +331,13 @@ ptmalloc_init (void)
   if (TUNABLE_GET_FULL (glibc, mem, cap_narrowing, int32_t, NULL) == 0)
     cap_narrowing_enabled = false;
   else
-    cap_narrowing_enabled = true;
+    {
+      cap_narrowing_enabled = true;
+# ifdef __CAP_MMAP_THRESHOLD
+      /* Default mmap threshold to avoid heap fragmentation.  */
+      do_set_mmap_threshold (__CAP_MMAP_THRESHOLD);
+# endif
+    }
 #endif
   cap_init ();
 
diff --git a/sysdeps/aarch64/morello/libc-cap.h b/sysdeps/aarch64/morello/libc-cap.h
index 3549a6db6d..9e6f66aa22 100644
--- a/sysdeps/aarch64/morello/libc-cap.h
+++ b/sysdeps/aarch64/morello/libc-cap.h
@@ -285,6 +285,10 @@ __libc_cap_map_del (void *p)
    allocations, i.e. __libc_cap_align (n) <= MALLOC_ALIGNMENT.  */
 #define __CAP_ALIGN_THRESHOLD 32759
 
+/* Set the mmap_threshold to this value when narrowing is enabled
+   to avoid heap fragmentation due to alignment requirements.  */
+#define __CAP_MMAP_THRESHOLD 262144
+
 /* Round up the allocation size so the allocated pointer bounds
    can be represented.  Note: this may be called before any
    checks on n, so it should work with all possible n values.  */