ANDROID: mm: assert that mmap_lock is taken exclusively in vm_write_begin

vm_write_{begin|end} has to be called when mmap_lock is taken
exlusively. Add an assert statement in vm_write_begin to enforce
that. free_pgtables can free page tables without exclusive mmap_lock
if the vma was isolated, therefore avoid assertions in such cases.

Bug: 257443051
Change-Id: Ie81aefe025c743cda6f66717d2f08f4d78a55608
Signed-off-by: Suren Baghdasaryan <surenb@google.com>

include/linux/mm.h

@@ -1758,6 +1758,12 @@ int generic_access_phys(struct vm_area_struct *vma, unsigned long addr,
 #ifdef CONFIG_SPECULATIVE_PAGE_FAULT
 static inline void vm_write_begin(struct vm_area_struct *vma)
 {
+        /*
+         * Isolated vma might be freed without exclusive mmap_lock but
+         * speculative page fault handler still needs to know it was changed.
+         */
+        if (!RB_EMPTY_NODE(&vma->vm_rb))
+	       mmap_assert_write_locked(vma->vm_mm);
 	/*
 	 * The reads never spins and preemption
 	 * disablement is not required.