When unregistering uffd-wp, older kernels before commit f369b07c86143
("mm/uffd:reset write protection when unregister with wp-mode") won't
clear the uffd-wp PTE bit. When re-registering uffd-wp, the previous
uffd-wp PTE bits would trigger again. With above commit, the kernel will
clear the uffd-wp PTE bits when unregistering itself.

Consequently, we'll clear the uffd-wp PTE bits now twice -- whereby we
don't care about clearing them at all: a new background snapshot will
re-register uffd-wp and re-protect all memory either way.

So let's skip the manual clearing of uffd-wp. If ever relevant, we
could clear conditionally in uffd_unregister_memory() -- we just need a
way to figure out more recent kernels.

Signed-off-by: David Hildenbrand <david@redhat.com>
---
 migration/ram.c | 9 ---------
 1 file changed, 9 deletions(-)

diff --git a/migration/ram.c b/migration/ram.c
index 6e4e41952a..6a3dbee2c3 100644
--- a/migration/ram.c
+++ b/migration/ram.c
@@ -1918,12 +1918,6 @@ fail:
         if ((block->flags & RAM_UF_WRITEPROTECT) == 0) {
             continue;
         }
-        /*
-         * In case some memory block failed to be write-protected
-         * remove protection and unregister all succeeded RAM blocks
-         */
-        uffd_change_protection(rs->uffdio_fd, block->host, block->max_length,
-                false, false);
         uffd_unregister_memory(rs->uffdio_fd, block->host, block->max_length);
         /* Cleanup flags and remove reference */
         block->flags &= ~RAM_UF_WRITEPROTECT;
@@ -1949,9 +1943,6 @@ void ram_write_tracking_stop(void)
         if ((block->flags & RAM_UF_WRITEPROTECT) == 0) {
             continue;
         }
-        /* Remove protection and unregister all affected RAM blocks */
-        uffd_change_protection(rs->uffdio_fd, block->host, block->max_length,
-                false, false);
         uffd_unregister_memory(rs->uffdio_fd, block->host, block->max_length);
 
         trace_ram_write_tracking_ramblock_stop(block->idstr, block->page_size,
-- 
2.39.0

David Hildenbrand <david@redhat.com> wrote:
> When unregistering uffd-wp, older kernels before commit f369b07c86143
> ("mm/uffd:reset write protection when unregister with wp-mode") won't
> clear the uffd-wp PTE bit. When re-registering uffd-wp, the previous
> uffd-wp PTE bits would trigger again. With above commit, the kernel will
> clear the uffd-wp PTE bits when unregistering itself.
>
> Consequently, we'll clear the uffd-wp PTE bits now twice -- whereby we
> don't care about clearing them at all: a new background snapshot will
> re-register uffd-wp and re-protect all memory either way.
>
> So let's skip the manual clearing of uffd-wp. If ever relevant, we
> could clear conditionally in uffd_unregister_memory() -- we just need a
> way to figure out more recent kernels.
>
> Signed-off-by: David Hildenbrand <david@redhat.com>

Reviewed-by: Juan Quintela <quintela@redhat.com>

Fixing a bug by removing code O:-)