kernel: Preserve kernel threads' page directories when they differ from the regular kernel page directory

kernel/src/arch/x86_64/disk/ATA.cpp

@@ -750,12 +750,17 @@ Result<u64> ATADevice::read(u8* buf, usize offset, usize size) const
 
     ScopedKMutexLock<100>(m_drive->channel()->lock());
 
+    // FIXME: Don't always allocate this if we don't need it.
     auto* temp = TRY(make_array<u8>(block_size));
     auto guard = make_scope_guard([temp] { delete[] temp; });
 
     if (offset % block_size)
     {
+        // The size we need to read to round up to a block.
         usize extra_size = block_size - (offset % block_size);
+        // Maybe we don't even want enough to get to the next block?
+        if (extra_size > size) extra_size = size;
+
         TRY(m_drive->read_lba(offset / block_size, temp, 1));
         memcpy(buf, temp + (offset % block_size), extra_size);
         offset += extra_size;

kernel/src/thread/Scheduler.cpp

@@ -233,9 +233,11 @@ namespace Scheduler
         {
             switch_context(old_thread, new_thread, regs);
             if (!old_thread->is_kernel) old_thread->fp_data.save();
+            if (old_thread->is_kernel && MMU::get_page_directory() != MMU::kernel_page_directory())
+                old_thread->directory = MMU::get_page_directory();
+            if (new_thread->directory) MMU::switch_page_directory(new_thread->directory);
             if (!new_thread->is_kernel)
             {
-                MMU::switch_page_directory(new_thread->directory);
                 CPU::switch_kernel_stack(new_thread->kernel_stack.top());
                 new_thread->fp_data.restore();
             }