Kernel: Allocate shared memory regions immediately

Lazily committed shared memory was not working in situations where one
process would write to the memory and another would only read from it.

Since the reading process would never cause a write fault in the shared
region, we'd never notice that the writing process had added real
physical pages to the VMObject. This happened because the lazily
committed pages were marked "present" in the page table.

This patch solves the issue by always allocating shared memory up front
and not trying to be clever about it.

3 files changed, 8 insertions, 2 deletions

diff --git a/Kernel/Syscalls/mmap.cpp b/Kernel/Syscalls/mmap.cpp
index b1ccb31b8e..56e585b934 100644
--- a/Kernel/Syscalls/mmap.cpp
+++ b/Kernel/Syscalls/mmap.cpp
@@ -141,7 +141,7 @@ void* Process::sys$mmap(Userspace<const Syscall::SC_mmap_params*> user_params)
     }
 
     if (map_anonymous) {
-        auto strategy = map_noreserve ? AllocationStrategy::None : AllocationStrategy::Reserve;
+        auto strategy = map_shared ? AllocationStrategy::AllocateNow : (map_noreserve ? AllocationStrategy::None : AllocationStrategy::Reserve);
         region = allocate_region(range.value(), !name.is_null() ? name : "mmap", prot, strategy);
         if (!region && (!map_fixed && addr != 0))
             region = allocate_region(allocate_range({}, size), !name.is_null() ? name : "mmap", prot, strategy);
diff --git a/Kernel/Syscalls/shbuf.cpp b/Kernel/Syscalls/shbuf.cpp
index 50b72f2e1c..4d744e0587 100644
--- a/Kernel/Syscalls/shbuf.cpp
+++ b/Kernel/Syscalls/shbuf.cpp
@@ -52,7 +52,7 @@ int Process::sys$shbuf_create(int size, void** buffer)
         return -EINVAL;
     size = PAGE_ROUND_UP(size);
 
-    auto vmobject = AnonymousVMObject::create_with_size(size, AllocationStrategy::Reserve);
+    auto vmobject = AnonymousVMObject::create_with_size(size, AllocationStrategy::AllocateNow);
     if (!vmobject)
         return -ENOMEM;
 
diff --git a/Kernel/VM/Region.cpp b/Kernel/VM/Region.cpp
index 62ef18995e..52b29aad26 100644
--- a/Kernel/VM/Region.cpp
+++ b/Kernel/VM/Region.cpp
@@ -282,6 +282,12 @@ bool Region::map_individual_page_impl(size_t page_index)
     if (!page || (!is_readable() && !is_writable())) {
         pte->clear();
     } else {
+        if (is_shared()) {
+            // Shared memory should not be lazily populated!
+            ASSERT(!page->is_shared_zero_page());
+            ASSERT(!page->is_lazy_committed_page());
+        }
+
         pte->set_cache_disabled(!m_cacheable);
         pte->set_physical_page_base(page->paddr().get());
         pte->set_present(true);