A __split_vma is not a worthy event to report, and it's definitely not
a unmap so it would be incorrect to report unmap for the whole region
to the userfaultfd manager if a __split_vma fails.
So only call userfaultfd_unmap_prep after the __vma_splitting is over
and do_munmap cannot fail anymore.
Also add unlikely because it's better to optimize for the vast
majority of apps that aren't using userfaultfd in a non cooperative
way. Ideally we should also find a way to eliminate the branch
entirely if CONFIG_USERFAULTFD=n, but it would complicate things so
stick to unlikely for now.
Signed-off-by: Andrea Arcangeli <aarcange@xxxxxxxxxx>
---
mm/mmap.c | 22 +++++++++++++++-------
1 file changed, 15 insertions(+), 7 deletions(-)
diff --git a/mm/mmap.c b/mm/mmap.c
index f19efcf75418..9b0161f1adf3 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -2639,13 +2639,6 @@ int do_munmap(struct mm_struct *mm, unsigned long start, size_t len,
if (vma->vm_start >= end)
return 0;
- if (uf) {
- int error = userfaultfd_unmap_prep(vma, start, end, uf);
-
- if (error)
- return error;
- }
-
/*
* If we need to split any vma, do it now to save pain later.
*
@@ -2679,6 +2672,21 @@ int do_munmap(struct mm_struct *mm, unsigned long start, size_t len,
}
vma = prev ? prev->vm_next : mm->mmap;
+ if (unlikely(uf)) {
+ /*
+ * If userfaultfd_unmap_prep returns an error the vmas
+ * will remain splitted, but userland will get a
+ * highly unexpected error anyway. This is no
+ * different than the case where the first of the two
+ * __split_vma fails, but we don't undo the first
+ * split, despite we could. This is unlikely enough
+ * failure that it's not worth optimizing it for.
+ */
+ int error = userfaultfd_unmap_prep(vma, start, end, uf);
+ if (error)
+ return error;
+ }
+
/*
* unlock any mlock()ed ranges before detaching vmas
*/
--
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