mm/munlock: delete munlock_vma_pages_all(), allow oomreap

munlock_vma_pages_range() will still be required, when munlocking but
not munmapping a set of pages; but when unmapping a pte, the mlock count
will be maintained in much the same way as it will be maintained when
mapping in the pte.  Which removes the need for munlock_vma_pages_all()
on mlocked vmas when munmapping or exiting: eliminating the catastrophic
contention on i_mmap_rwsem, and the need for page lock on the pages.

There is still a need to update locked_vm accounting according to the
munmapped vmas when munmapping: do that in detach_vmas_to_be_unmapped().
exit_mmap() does not need locked_vm updates, so delete unlock_range().

And wasn't I the one who forbade the OOM reaper to attack mlocked vmas,
because of the uncertainty in blocking on all those page locks?
No fear of that now, so permit the OOM reaper on mlocked vmas.

Signed-off-by: Hugh Dickins <hughd@google.com>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>

1 files changed, 2 insertions, 30 deletions

diff --git a/mm/mmap.c b/mm/mmap.c
index 1e8fdb0b51ed..64b5985b5295 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -2674,6 +2674,8 @@ detach_vmas_to_be_unmapped(struct mm_struct *mm, struct vm_area_struct *vma,
 	vma->vm_prev = NULL;
 	do {
 		vma_rb_erase(vma, &mm->mm_rb);
+		if (vma->vm_flags & VM_LOCKED)
+			mm->locked_vm -= vma_pages(vma);
 		mm->map_count--;
 		tail_vma = vma;
 		vma = vma->vm_next;
@@ -2778,22 +2780,6 @@ int split_vma(struct mm_struct *mm, struct vm_area_struct *vma,
 	return __split_vma(mm, vma, addr, new_below);
 }
 
-static inline void
-unlock_range(struct vm_area_struct *start, unsigned long limit)
-{
-	struct mm_struct *mm = start->vm_mm;
-	struct vm_area_struct *tmp = start;
-
-	while (tmp && tmp->vm_start < limit) {
-		if (tmp->vm_flags & VM_LOCKED) {
-			mm->locked_vm -= vma_pages(tmp);
-			munlock_vma_pages_all(tmp);
-		}
-
-		tmp = tmp->vm_next;
-	}
-}
-
 /* Munmap is split into 2 main parts -- this part which finds
  * what needs doing, and the areas themselves, which do the
  * work.  This now handles partial unmappings.
@@ -2874,12 +2860,6 @@ int __do_munmap(struct mm_struct *mm, unsigned long start, size_t len,
 			return error;
 	}
 
-	/*
-	 * unlock any mlock()ed ranges before detaching vmas
-	 */
-	if (mm->locked_vm)
-		unlock_range(vma, end);
-
 	/* Detach vmas from rbtree */
 	if (!detach_vmas_to_be_unmapped(mm, vma, prev, end))
 		downgrade = false;
@@ -3147,20 +3127,12 @@ void exit_mmap(struct mm_struct *mm)
 		 * Nothing can be holding mm->mmap_lock here and the above call
 		 * to mmu_notifier_release(mm) ensures mmu notifier callbacks in
 		 * __oom_reap_task_mm() will not block.
-		 *
-		 * 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);
-	if (mm->locked_vm)
-		unlock_range(mm->mmap, ULONG_MAX);
-
 	arch_exit_mmap(mm);
 
 	vma = mm->mmap;