[PATCH v3 13/13] migration/ram: Tolerate partially changed mappings in postcopy code

[David Hildenbrand]

When we partially change mappings (esp., mmap over parts of an existing
mmap like qemu_ram_remap() does) where we have a userfaultfd handler
registered, the handler will implicitly be unregistered from the parts that
changed.

Trying to place pages onto mappings where there is no longer a handler
registered will fail. Let's make sure that any waiter is woken up - we
have to do that manually.

Let's also document how UFFDIO_UNREGISTER will handle this scenario.

This is mainly a preparation for RAM blocks with resizable allcoations,
where the mapping of the invalid RAM range will change. The source will
keep sending pages that are outside of the new (shrunk) RAM size. We have
to treat these pages like they would have been migrated, but can
essentially simply drop the content (ignore the placement error).

Keep printing a warning when we hit EINVAL, to avoid hiding other
(programming) issues. ENOENT is unique.

Cc: "Dr. David Alan Gilbert" <address@hidden>
Cc: Juan Quintela <address@hidden>
Cc: Peter Xu <address@hidden>
Cc: Andrea Arcangeli <address@hidden>
Signed-off-by: David Hildenbrand <address@hidden>
---
 migration/postcopy-ram.c | 39 +++++++++++++++++++++++++++++++++++++++
 1 file changed, 39 insertions(+)

diff --git a/migration/postcopy-ram.c b/migration/postcopy-ram.c
index c68caf4e42..f39c6304de 100644
--- a/migration/postcopy-ram.c
+++ b/migration/postcopy-ram.c
@@ -506,6 +506,14 @@ static int cleanup_range(RAMBlock *rb, void *opaque)
     range_struct.start = (uintptr_t)host_addr;
     range_struct.len = length;
 
+    /*
+     * In case the mapping was partially changed since we enabled userfault
+     * (e.g., via qemu_ram_remap()), the userfaultfd handler was already 
removed
+     * for the mappings that changed. Unregistering will, however, still work
+     * and ignore mappings without a registered handler. There could only be
+     * an issue if we would suddenly encounter a mapping that's incompatible
+     * with UFFD - which cannot happen within a single RAM block.
+     */
     if (ioctl(mis->userfault_fd, UFFDIO_UNREGISTER, &range_struct)) {
         error_report("%s: userfault unregister %s", __func__, strerror(errno));
 
@@ -1180,6 +1188,17 @@ int postcopy_ram_incoming_setup(MigrationIncomingState 
*mis)
     return 0;
 }
 
+static int qemu_ufd_wake_ioctl(int userfault_fd, void *host_addr,
+                               uint64_t pagesize)
+{
+    struct uffdio_range range = {
+        .start = (uint64_t)(uintptr_t)host_addr,
+        .len = pagesize,
+    };
+
+    return ioctl(userfault_fd, UFFDIO_WAKE, &range);
+}
+
 static int qemu_ufd_copy_ioctl(int userfault_fd, void *host_addr,
                                void *from_addr, uint64_t pagesize, RAMBlock 
*rb)
 {
@@ -1198,6 +1217,26 @@ static int qemu_ufd_copy_ioctl(int userfault_fd, void 
*host_addr,
         zero_struct.mode = 0;
         ret = ioctl(userfault_fd, UFFDIO_ZEROPAGE, &zero_struct);
     }
+
+    /*
+     * When the mapping gets partially changed (e.g., qemu_ram_remap()) before
+     * we try to place a page, the userfaultfd handler will be removed for the
+     * changed mappings and placing pages will fail. We can safely ignore this,
+     * because mappings that changed on the destination don't need data from 
the
+     * source (e.g., qemu_ram_remap()). Wake up any waiter waiting for that 
page
+     * (unlikely but possible). Waking up waiters is always possible, even
+     * without a registered userfaultfd handler.
+     *
+     * Old kernels report EINVAL, new kernels report ENOENT in case there is
+     * no longer a userfaultfd handler for a mapping.
+     */
+    if (ret && (errno == ENOENT || errno == EINVAL)) {
+        if (errno == EINVAL) {
+            warn_report("%s: Failed to place page %p. Waking up any waiters.",
+                         __func__, host_addr);
+        }
+        ret = qemu_ufd_wake_ioctl(userfault_fd, host_addr, pagesize);
+    }
     if (!ret) {
         ramblock_recv_bitmap_set_range(rb, host_addr,
                                        pagesize / qemu_target_page_size());
-- 
2.24.1