mm: remove the now-unnecessary mmget_still_valid() hack

The preceding patches have ensured that core dumping properly takes the
mmap_lock.  Thanks to that, we can now remove mmget_still_valid() and all
its users.

Signed-off-by: Jann Horn <jannh@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Acked-by: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: "Eric W . Biederman" <ebiederm@xmission.com>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Hugh Dickins <hughd@google.com>
Link: http://lkml.kernel.org/r/20200827114932.3572699-8-jannh@google.com
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

2 files changed, 9 insertions, 37 deletions

diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
index 846d43df3fdf..217aa2705d5d 100644
--- a/fs/proc/task_mmu.c
+++ b/fs/proc/task_mmu.c
@@ -1244,24 +1244,6 @@ static ssize_t clear_refs_write(struct file *file, const char __user *buf,
 					count = -EINTR;
 					goto out_mm;
 				}
-				/*
-				 * Avoid to modify vma->vm_flags
-				 * without locked ops while the
-				 * coredump reads the vm_flags.
-				 */
-				if (!mmget_still_valid(mm)) {
-					/*
-					 * Silently return "count"
-					 * like if get_task_mm()
-					 * failed. FIXME: should this
-					 * function have returned
-					 * -ESRCH if get_task_mm()
-					 * failed like if
-					 * get_proc_task() fails?
-					 */
-					mmap_write_unlock(mm);
-					goto out_mm;
-				}
 				for (vma = mm->mmap; vma; vma = vma->vm_next) {
 					vma->vm_flags &= ~VM_SOFTDIRTY;
 					vma_set_page_prot(vma);
diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
index 0e4a3837da52..000b457ad087 100644
--- a/fs/userfaultfd.c
+++ b/fs/userfaultfd.c
@@ -601,8 +601,6 @@ static void userfaultfd_event_wait_completion(struct userfaultfd_ctx *ctx,
 
 		/* the various vma->vm_userfaultfd_ctx still points to it */
 		mmap_write_lock(mm);
-		/* no task can run (and in turn coredump) yet */
-		VM_WARN_ON(!mmget_still_valid(mm));
 		for (vma = mm->mmap; vma; vma = vma->vm_next)
 			if (vma->vm_userfaultfd_ctx.ctx == release_new_ctx) {
 				vma->vm_userfaultfd_ctx = NULL_VM_UFFD_CTX;
@@ -842,7 +840,6 @@ static int userfaultfd_release(struct inode *inode, struct file *file)
 	/* len == 0 means wake all */
 	struct userfaultfd_wake_range range = { .len = 0, };
 	unsigned long new_flags;
-	bool still_valid;
 
 	WRITE_ONCE(ctx->released, true);
 
@@ -858,7 +855,6 @@ static int userfaultfd_release(struct inode *inode, struct file *file)
 	 * taking the mmap_lock for writing.
 	 */
 	mmap_write_lock(mm);
-	still_valid = mmget_still_valid(mm);
 	prev = NULL;
 	for (vma = mm->mmap; vma; vma = vma->vm_next) {
 		cond_resched();
@@ -869,17 +865,15 @@ static int userfaultfd_release(struct inode *inode, struct file *file)
 			continue;
 		}
 		new_flags = vma->vm_flags & ~(VM_UFFD_MISSING | VM_UFFD_WP);
-		if (still_valid) {
-			prev = vma_merge(mm, prev, vma->vm_start, vma->vm_end,
-					 new_flags, vma->anon_vma,
-					 vma->vm_file, vma->vm_pgoff,
-					 vma_policy(vma),
-					 NULL_VM_UFFD_CTX);
-			if (prev)
-				vma = prev;
-			else
-				prev = vma;
-		}
+		prev = vma_merge(mm, prev, vma->vm_start, vma->vm_end,
+				 new_flags, vma->anon_vma,
+				 vma->vm_file, vma->vm_pgoff,
+				 vma_policy(vma),
+				 NULL_VM_UFFD_CTX);
+		if (prev)
+			vma = prev;
+		else
+			prev = vma;
 		vma->vm_flags = new_flags;
 		vma->vm_userfaultfd_ctx = NULL_VM_UFFD_CTX;
 	}
@@ -1309,8 +1303,6 @@ static int userfaultfd_register(struct userfaultfd_ctx *ctx,
 		goto out;
 
 	mmap_write_lock(mm);
-	if (!mmget_still_valid(mm))
-		goto out_unlock;
 	vma = find_vma_prev(mm, start, &prev);
 	if (!vma)
 		goto out_unlock;
@@ -1511,8 +1503,6 @@ static int userfaultfd_unregister(struct userfaultfd_ctx *ctx,
 		goto out;
 
 	mmap_write_lock(mm);
-	if (!mmget_still_valid(mm))
-		goto out_unlock;
 	vma = find_vma_prev(mm, start, &prev);
 	if (!vma)
 		goto out_unlock;