Separate out the uffd bits so it clear's what's happening.

Don't bother setting vrm->mmap_locked after unlocking, because after this
we are done anyway.

The only time we drop the mmap lock is on VMA shrink, at which point
vrm->new_len will be < vrm->old_len and the operation will not be
performed anyway, so move this code out of the if (vrm->mmap_locked)
block.

All addresses returned by mremap() are page-aligned, so the
offset_in_page() check on ret seems only to be incorrectly trying to
detect whether an error occurred - explicitly check for this.

No functional change intended.

Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Reviewed-by: Vlastimil Babka <vbabka@suse.cz>
---
 mm/mremap.c | 22 +++++++++++++---------
 1 file changed, 13 insertions(+), 9 deletions(-)

diff --git a/mm/mremap.c b/mm/mremap.c
index 60eb0ac8634b..53447761e55d 100644
--- a/mm/mremap.c
+++ b/mm/mremap.c
@@ -1729,6 +1729,15 @@ static int check_prep_vma(struct vma_remap_struct *vrm)
 	return 0;
 }
 
+static void notify_uffd(struct vma_remap_struct *vrm, unsigned long to)
+{
+	struct mm_struct *mm = current->mm;
+
+	userfaultfd_unmap_complete(mm, vrm->uf_unmap_early);
+	mremap_userfaultfd_complete(vrm->uf, vrm->addr, to, vrm->old_len);
+	userfaultfd_unmap_complete(mm, vrm->uf_unmap);
+}
+
 static unsigned long do_mremap(struct vma_remap_struct *vrm)
 {
 	struct mm_struct *mm = current->mm;
@@ -1754,18 +1763,13 @@ static unsigned long do_mremap(struct vma_remap_struct *vrm)
 	res = vrm_implies_new_addr(vrm) ? mremap_to(vrm) : mremap_at(vrm);
 
 out:
-	if (vrm->mmap_locked) {
+	if (vrm->mmap_locked)
 		mmap_write_unlock(mm);
-		vrm->mmap_locked = false;
-
-		if (!offset_in_page(res) && vrm->mlocked && vrm->new_len > vrm->old_len)
-			mm_populate(vrm->new_addr + vrm->old_len, vrm->delta);
-	}
 
-	userfaultfd_unmap_complete(mm, vrm->uf_unmap_early);
-	mremap_userfaultfd_complete(vrm->uf, vrm->addr, res, vrm->old_len);
-	userfaultfd_unmap_complete(mm, vrm->uf_unmap);
+	if (!IS_ERR_VALUE(res) && vrm->mlocked && vrm->new_len > vrm->old_len)
+		mm_populate(vrm->new_addr + vrm->old_len, vrm->delta);
 
+	notify_uffd(vrm, res);
 	return res;
 }
 
-- 
2.50.1