Kernel: Add MAP_NORESERVE support to mmap

Rather than lazily committing regions by default, we now commit
the entire region unless MAP_NORESERVE is specified.

This solves random crashes in low-memory situations where e.g. the
malloc heap allocated memory, but using pages that haven't been
used before triggers a crash when no more physical memory is available.

Use this flag to create large regions without actually committing
the backing memory. madvise() can be used to commit arbitrary areas
of such regions after creating them.

1 files changed, 11 insertions, 2 deletions

diff --git a/Kernel/VM/Region.cpp b/Kernel/VM/Region.cpp
index 22d49c60a1..5f40c88b8e 100644
--- a/Kernel/VM/Region.cpp
+++ b/Kernel/VM/Region.cpp
@@ -187,13 +187,18 @@ auto Region::set_volatile(VirtualAddress vaddr, size_t size, bool is_volatile, b
             // Attempt to remap the page range. We want to make sure we have
             // enough memory, if not we need to inform the caller of that
             // fact
-            if (!remap_page_range(first_page_index, last_page_index - first_page_index))
+            if (!remap_page_range(first_page_index, last_page_index - first_page_index, true))
                 return SetVolatileError::OutOfMemory;
         }
     }
     return SetVolatileError::Success;
 }
 
+bool Region::can_commit() const
+{
+    return vmobject().is_anonymous() || vmobject().is_purgeable();
+}
+
 bool Region::commit()
 {
     ScopedSpinLock lock(s_mm_lock);
@@ -339,7 +344,7 @@ bool Region::map_individual_page_impl(size_t page_index)
     return true;
 }
 
-bool Region::remap_page_range(size_t page_index, size_t page_count)
+bool Region::remap_page_range(size_t page_index, size_t page_count, bool do_commit)
 {
     bool success = true;
     ScopedSpinLock lock(s_mm_lock);
@@ -347,6 +352,10 @@ bool Region::remap_page_range(size_t page_index, size_t page_count)
     ScopedSpinLock page_lock(m_page_directory->get_lock());
     size_t index = page_index;
     while (index < page_index + page_count) {
+        if (do_commit && !commit(index)) {
+            success = false;
+            break;
+        }
         if (!map_individual_page_impl(index)) {
             success = false;
             break;