Kernel: Don't remap all regions from Region::remap_vmobject_page()

When handling a page fault, we only need to remap the faulting region in
the current process. There's no need to traverse *all* regions that map
the same VMObject and remap them cross-process as well.

Those other regions will get remapped lazily by their own page fault
handlers eventually. Or maybe they won't and we avoided some work. :^)

Kernel/Memory/Region.cpp

@@ -237,14 +237,16 @@ bool Region::map_individual_page_impl(size_t page_index)
     return true;
 }
 
-bool Region::do_remap_vmobject_page(size_t page_index, bool with_flush)
+bool Region::remap_vmobject_page(size_t page_index, bool with_flush)
 {
-    if (!m_page_directory)
-        return true; // not an error, region may have not yet mapped it
-    if (!translate_vmobject_page(page_index))
-        return true; // not an error, region doesn't map this page
     SpinlockLocker page_lock(m_page_directory->get_lock());
-    SpinlockLocker lock(s_mm_lock);
+    SpinlockLocker mm_lock(s_mm_lock);
+    SpinlockLocker lock(m_vmobject->m_lock);
+
+    // NOTE: `page_index` is a VMObject page index, so first we convert it to a Region page index.
+    if (!translate_vmobject_page(page_index))
+        return false;
+
     VERIFY(physical_page(page_index));
     bool success = map_individual_page_impl(page_index);
     if (with_flush)
@@ -252,18 +254,6 @@ bool Region::do_remap_vmobject_page(size_t page_index, bool with_flush)
     return success;
 }
 
-bool Region::remap_vmobject_page(size_t page_index, bool with_flush)
-{
-    auto& vmobject = this->vmobject();
-    bool success = true;
-    SpinlockLocker lock(vmobject.m_lock);
-    vmobject.for_each_region([&](auto& region) {
-        if (!region.do_remap_vmobject_page(page_index, with_flush))
-            success = false;
-    });
-    return success;
-}
-
 void Region::unmap(ShouldFlushTLB should_flush_tlb)
 {
     if (!m_page_directory)

Kernel/Memory/Region.h

@@ -200,7 +200,6 @@ private:
     Region(VirtualRange const&, NonnullRefPtr<VMObject>, size_t offset_in_vmobject, OwnPtr<KString>, Region::Access access, Cacheable, bool shared);
 
     [[nodiscard]] bool remap_vmobject_page(size_t page_index, bool with_flush = true);
-    [[nodiscard]] bool do_remap_vmobject_page(size_t page_index, bool with_flush = true);
 
     void set_access_bit(Access access, bool b)
     {