[PATCH] userfaultfd: reset err to be 0 when move_pages_ptes succeeded

[Foxie Flakey <foxieflakey@gmail.com>]

An fix for edge case can occur if move_pages_ptes return -EAGAIN, later
when checked and it is EAGAIN, outer loop would retry again on same page
and succeeded but the err isn't reset so the outer loop would think need
to retry again so it goes back again and move pages again. On third attempt
move_pages_ptes will fail because it already moved and returns an error
that is not EAGAIN when outer loop checks again it sees non EAGAIN so it
dont retry and break out of loop. When loop is terminated it did not update
the "moved" variable from successful 2nd iteration.

That behaviour manifested into this at userspace

Source:      [ .. unmapped  .. ][ .. mapped    ..]
Destination: [ .. mapped    .. ][ .. unmapped  ..]
                          ^     ^
                          \     Kernel moved this far in actuality
                           What is reported to userspace on struct
                           uffdio_move's move field

When the previous behaviour is
Source:      [ .. unmapped  .. ][ .. mapped    ..]
Destination: [ .. mapped    .. ][ .. unmapped  ..]
                                ^
                                Reported to user space via uffdio_move's
                                move field

Fixes: 50944692052b ("userfaultfd: opportunistic TLB-flush batching for present pages in MOVE")
Signed-off-by: Foxie Flakey <foxieflakey@gmail.com>
---
 mm/userfaultfd.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
index c3adedaaf7d5..595e7e232f90 100644
--- a/mm/userfaultfd.c
+++ b/mm/userfaultfd.c
@@ -2069,10 +2069,12 @@ static ssize_t move_pages(struct userfaultfd_ctx *ctx, unsigned long dst_start,
 			ret = move_pages_ptes(mm, dst_pmd, src_pmd,
 					      dst_vma, src_vma, dst_addr,
 					      src_addr, src_end - src_addr, mode);
-			if (ret < 0)
+			if (ret < 0) {
 				err = ret;
-			else
+			} else {
+				err = 0;
 				step_size = ret;
+			}
 		}

 		cond_resched();

base-commit: 62cc90241548d5570ee68e01aaba6506964e9811
-- 
2.55.0