mm: protect free_pgtables with mmap_lock write lock in exit_mmap

oom-reaper and process_mrelease system call should protect against races
with exit_mmap which can destroy page tables while they walk the VMA
tree.  oom-reaper protects from that race by setting MMF_OOM_VICTIM and
by relying on exit_mmap to set MMF_OOM_SKIP before taking and releasing
mmap_write_lock.  process_mrelease has to elevate mm->mm_users to
prevent such race.

Both oom-reaper and process_mrelease hold mmap_read_lock when walking
the VMA tree.  The locking rules and mechanisms could be simpler if
exit_mmap takes mmap_write_lock while executing destructive operations
such as free_pgtables.

Change exit_mmap to hold the mmap_write_lock when calling unlock_range,
free_pgtables and remove_vma.  Note also that because oom-reaper checks
VM_LOCKED flag, unlock_range() should not be allowed to race with it.

Before this patch, remove_vma used to be called with no locks held,
however with fput being executed asynchronously and vm_ops->close not
being allowed to hold mmap_lock (it is called from __split_vma with
mmap_sem held for write), changing that should be fine.

In most cases this lock should be uncontended.  Previously, Kirill
reported ~4% regression caused by a similar change [1].  We reran the
same test and although the individual results are quite noisy, the
percentiles show lower regression with 1.6% being the worst case [2].
The change allows oom-reaper and process_mrelease to execute safely
under mmap_read_lock without worries that exit_mmap might destroy page
tables from under them.

[1] https://lore.kernel.org/all/20170725141723.ivukwhddk2voyhuc@node.shutemov.name/
[2] https://lore.kernel.org/all/CAJuCfpGC9-c9P40x7oy=jy5SphMcd0o0G_6U1-+JAziGKG6dGA@mail.gmail.com/

Link: https://lkml.kernel.org/r/20211209191325.3069345-1-surenb@google.comSigned-off-by: Suren Baghdasaryan <surenb@google.com>
Acked-by: Michal Hocko <mhocko@suse.com>
Cc: David Rientjes <rientjes@google.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Roman Gushchin <guro@fb.com>
Cc: Rik van Riel <riel@surriel.com>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Kirill A. Shutemov <kirill@shutemov.name>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Christian Brauner <christian@brauner.io>
Cc: Christoph Hellwig <hch@infradead.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: Jann Horn <jannh@google.com>
Cc: Shakeel Butt <shakeelb@google.com>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Christian Brauner <christian.brauner@ubuntu.com>
Cc: Florian Weimer <fweimer@redhat.com>
Cc: Jan Engelhardt <jengelh@inai.de>
Cc: Tim Murray <timmurray@google.com>
Cc: Jason Gunthorpe <jgg@nvidia.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/mmap.c

@@ -3149,25 +3149,27 @@ void exit_mmap(struct mm_struct *mm)
 		 * to mmu_notifier_release(mm) ensures mmu notifier callbacks in
 		 * __oom_reap_task_mm() will not block.
 		 *
-		 * This needs to be done before calling munlock_vma_pages_all(),
+		 * This needs to be done before calling unlock_range(),
 		 * which clears VM_LOCKED, otherwise the oom reaper cannot
 		 * reliably test it.
 		 */
 		(void)__oom_reap_task_mm(mm);
 
 		set_bit(MMF_OOM_SKIP, &mm->flags);
-		mmap_write_lock(mm);
-		mmap_write_unlock(mm);
 	}
 
+	mmap_write_lock(mm);
 	if (mm->locked_vm)
 		unlock_range(mm->mmap, ULONG_MAX);
 
 	arch_exit_mmap(mm);
 
 	vma = mm->mmap;
-	if (!vma)	/* Can happen if dup_mmap() received an OOM */
+	if (!vma) {
+		/* Can happen if dup_mmap() received an OOM */
+		mmap_write_unlock(mm);
 		return;
+	}
 
 	lru_add_drain();
 	flush_cache_mm(mm);
@@ -3178,16 +3180,14 @@ void exit_mmap(struct mm_struct *mm)
 	free_pgtables(&tlb, vma, FIRST_USER_ADDRESS, USER_PGTABLES_CEILING);
 	tlb_finish_mmu(&tlb);
 
-	/*
-	 * Walk the list again, actually closing and freeing it,
-	 * with preemption enabled, without holding any MM locks.
-	 */
+	/* Walk the list again, actually closing and freeing it. */
 	while (vma) {
 		if (vma->vm_flags & VM_ACCOUNT)
 			nr_accounted += vma_pages(vma);
 		vma = remove_vma(vma);
 		cond_resched();
 	}
+	mmap_write_unlock(mm);
 	vm_unacct_memory(nr_accounted);
 }