Only use do and dont dump on pagemap

The do and dont dump calls were costings a lot during start up of snmalloc.  This reduces the times they are called to only be for the pagemap.

src/snmalloc/ds/pagemap.h

@@ -68,6 +68,10 @@ namespace snmalloc
       auto page_end = pointer_align_up<OS_PAGE_SIZE, char>(last);
       size_t using_size = pointer_diff(page_start, page_end);
       PAL::template notify_using<NoZero>(page_start, using_size);
+      if constexpr (pal_supports<CoreDump, PAL>)
+      {
+        PAL::notify_do_dump(page_start, using_size);
+      }
     }
 
     constexpr FlatPagemap() = default;
@@ -192,6 +196,12 @@ namespace snmalloc
 
       auto new_body_untyped = PAL::reserve(request_size);
 
+      if constexpr (pal_supports<CoreDump, PAL>)
+      {
+        // Pagemap should not be in core dump except where it is non-zero.
+        PAL::notify_do_not_dump(new_body_untyped, request_size);
+      }
+
       if (new_body_untyped == nullptr)
       {
         PAL::error("Failed to initialise snmalloc.");

src/snmalloc/pal/pal_consts.h

@@ -55,6 +55,12 @@ namespace snmalloc
      * This Pal provides a millisecond time source
      */
     Time = (1 << 5),
+
+    /**
+     * This Pal provides selective core dumps, so
+     * modify which parts get dumped.
+     */
+    CoreDump = (1 << 6),
   };
 
   /**

src/snmalloc/pal/pal_freebsd.h

@@ -43,7 +43,8 @@ namespace snmalloc
      * field is declared explicitly to remind anyone modifying this class to
      * add new features that they should add any required feature flags.
      */
-    static constexpr uint64_t pal_features = PALBSD_Aligned::pal_features;
+    static constexpr uint64_t pal_features =
+      PALBSD_Aligned::pal_features | CoreDump;
 
     /**
      * FreeBSD uses atypically small address spaces on its 64 bit RISC machines.
@@ -68,10 +69,8 @@ namespace snmalloc
     /**
      * Notify platform that we will not be using these pages.
      *
-     * We use the `MADV_FREE` and `NADV_NOCORE` flags to `madvise`.  The first
-     * allows the system to discard the page and replace it with a CoW mapping
-     * of the zero page.  The second prevents this mapping from appearing in
-     * core files.
+     * We use the `MADV_FREE` flag to `madvise`. This allows the system to
+     * discard the page and replace it with a CoW mapping of the zero page.
      */
     static void notify_not_using(void* p, size_t size) noexcept
     {
@@ -80,7 +79,6 @@ namespace snmalloc
       if constexpr (DEBUG)
         memset(p, 0x5a, size);
 
-      madvise(p, size, MADV_NOCORE);
       madvise(p, size, MADV_FREE);
 
       if constexpr (mitigations(pal_enforce_access))
@@ -90,28 +88,19 @@ namespace snmalloc
     }
 
     /**
-     * Notify platform that we will be using these pages for reading.
-     *
-     * This is used only for pages full of zeroes and so we exclude them from
-     * core dumps.
+     * Notify platform that these pages should be included in a core dump.
      */
-    static void notify_using_readonly(void* p, size_t size) noexcept
+    static void notify_do_dump(void* p, size_t size) noexcept
     {
-      PALBSD_Aligned<PALFreeBSD>::notify_using_readonly(p, size);
-      madvise(p, size, MADV_NOCORE);
+      madvise(p, size, MADV_CORE);
     }
 
     /**
-     * Notify platform that we will be using these pages.
-     *
-     * We may have previously marked this memory as not being included in core
-     * files, so mark it for inclusion again.
+     * Notify platform that these pages should not be included in a core dump.
      */
-    template<ZeroMem zero_mem>
-    static void notify_using(void* p, size_t size) noexcept
+    static void notify_do_not_dump(void* p, size_t size) noexcept
     {
-      PALBSD_Aligned<PALFreeBSD>::notify_using<zero_mem>(p, size);
-      madvise(p, size, MADV_CORE);
+      madvise(p, size, MADV_NOCORE);
     }
 
 #  if defined(__CHERI_PURE_CAPABILITY__)

src/snmalloc/pal/pal_linux.h

@@ -27,7 +27,8 @@ namespace snmalloc
      *
      * We always make sure that linux has entropy support.
      */
-    static constexpr uint64_t pal_features = PALPOSIX::pal_features | Entropy;
+    static constexpr uint64_t pal_features =
+      PALPOSIX::pal_features | Entropy | CoreDump;
 
     static constexpr size_t page_size =
       Aal::aal_name == PowerPC ? 0x10000 : PALPOSIX::page_size;
@@ -58,7 +59,6 @@ namespace snmalloc
       void* p = PALPOSIX<PALLinux>::reserve(size);
       if (p)
       {
-        madvise(p, size, MADV_DONTDUMP);
 #  ifdef SNMALLOC_PAGEID
 #    ifndef PR_SET_VMA
 #      define PR_SET_VMA 0x53564d41
@@ -125,7 +125,6 @@ namespace snmalloc
       if constexpr (DEBUG)
         memset(p, 0x5a, size);
 
-      madvise(p, size, MADV_DONTDUMP);
       madvise(p, size, madvise_free_flags);
 
       if constexpr (mitigations(pal_enforce_access))
@@ -135,25 +134,19 @@ namespace snmalloc
     }
 
     /**
-     * Notify platform that we will be using these pages for reading.
-     *
-     * This is used only for pages full of zeroes and so we exclude them from
-     * core dumps.
+     * Notify platform that these pages should be included in a core dump.
      */
-    static void notify_using_readonly(void* p, size_t size) noexcept
+    static void notify_do_dump(void* p, size_t size) noexcept
     {
-      PALPOSIX<PALLinux>::notify_using_readonly(p, size);
-      madvise(p, size, MADV_DONTDUMP);
+      madvise(p, size, MADV_DODUMP);
     }
 
     /**
-     * Notify platform that we will be using these pages.
+     * Notify platform that these pages should not be included in a core dump.
      */
-    template<ZeroMem zero_mem>
-    static void notify_using(void* p, size_t size) noexcept
+    static void notify_do_not_dump(void* p, size_t size) noexcept
     {
-      PALPOSIX<PALLinux>::notify_using<zero_mem>(p, size);
-      madvise(p, size, MADV_DODUMP);
+      madvise(p, size, MADV_DONTDUMP);
     }
 
     static uint64_t get_entropy64()