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authorAndrey Gruzdev <andrey.gruzdev@virtuozzo.com>2021-04-01 12:22:25 +0300
committerDr. David Alan Gilbert <dgilbert@redhat.com>2021-04-07 18:37:28 +0100
commiteeccb99c9d28484303f721e94c5084e9c29a3d03 (patch)
treebdd58066e9701094a592aa2343a8528f7a08a5b2 /migration
parent1a8e44a89f1976e06300393337f78d561f95b339 (diff)
migration: Pre-fault memory before starting background snasphot
This commit solves the issue with userfault_fd WP feature that background snapshot is based on. For any never poluated or discarded memory page, the UFFDIO_WRITEPROTECT ioctl() would skip updating PTE for that page, thereby loosing WP setting for it. So we need to pre-fault pages for each RAM block to be protected before making a userfault_fd wr-protect ioctl(). Fixes: 278e2f551a095b234de74dca9c214d5502a1f72c (migration: support UFFD write fault processing in ram_save_iterate()) Signed-off-by: Andrey Gruzdev <andrey.gruzdev@virtuozzo.com> Reported-by: David Hildenbrand <david@redhat.com> Reviewed-by: David Hildenbrand <david@redhat.com> Message-Id: <20210401092226.102804-4-andrey.gruzdev@virtuozzo.com> Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com> dgilbert: Bodged ifdef __linux__ on ram_write_tracking_prepare, should really go in a stub
Diffstat (limited to 'migration')
-rw-r--r--migration/migration.c8
-rw-r--r--migration/ram.c49
-rw-r--r--migration/ram.h1
3 files changed, 58 insertions, 0 deletions
diff --git a/migration/migration.c b/migration/migration.c
index be4729e7c8..8ca034136b 100644
--- a/migration/migration.c
+++ b/migration/migration.c
@@ -3827,6 +3827,14 @@ static void *bg_migration_thread(void *opaque)
update_iteration_initial_status(s);
+ /*
+ * Prepare for tracking memory writes with UFFD-WP - populate
+ * RAM pages before protecting.
+ */
+#ifdef __linux__
+ ram_write_tracking_prepare();
+#endif
+
qemu_savevm_state_header(s->to_dst_file);
qemu_savevm_state_setup(s->to_dst_file);
diff --git a/migration/ram.c b/migration/ram.c
index 40e78952ad..7e2bc0fdd3 100644
--- a/migration/ram.c
+++ b/migration/ram.c
@@ -1561,6 +1561,55 @@ out:
}
/*
+ * ram_block_populate_pages: populate memory in the RAM block by reading
+ * an integer from the beginning of each page.
+ *
+ * Since it's solely used for userfault_fd WP feature, here we just
+ * hardcode page size to qemu_real_host_page_size.
+ *
+ * @bs: RAM block to populate
+ */
+static void ram_block_populate_pages(RAMBlock *bs)
+{
+ char *ptr = (char *) bs->host;
+
+ for (ram_addr_t offset = 0; offset < bs->used_length;
+ offset += qemu_real_host_page_size) {
+ char tmp = *(ptr + offset);
+
+ /* Don't optimize the read out */
+ asm volatile("" : "+r" (tmp));
+ }
+}
+
+/*
+ * ram_write_tracking_prepare: prepare for UFFD-WP memory tracking
+ */
+void ram_write_tracking_prepare(void)
+{
+ RAMBlock *bs;
+
+ RCU_READ_LOCK_GUARD();
+
+ RAMBLOCK_FOREACH_NOT_IGNORED(bs) {
+ /* Nothing to do with read-only and MMIO-writable regions */
+ if (bs->mr->readonly || bs->mr->rom_device) {
+ continue;
+ }
+
+ /*
+ * Populate pages of the RAM block before enabling userfault_fd
+ * write protection.
+ *
+ * This stage is required since ioctl(UFFDIO_WRITEPROTECT) with
+ * UFFDIO_WRITEPROTECT_MODE_WP mode setting would silently skip
+ * pages with pte_none() entries in page table.
+ */
+ ram_block_populate_pages(bs);
+ }
+}
+
+/*
* ram_write_tracking_start: start UFFD-WP memory tracking
*
* Returns 0 for success or negative value in case of error
diff --git a/migration/ram.h b/migration/ram.h
index 6378bb3ebc..4833e9fd5b 100644
--- a/migration/ram.h
+++ b/migration/ram.h
@@ -82,6 +82,7 @@ void colo_incoming_start_dirty_log(void);
/* Background snapshot */
bool ram_write_tracking_available(void);
bool ram_write_tracking_compatible(void);
+void ram_write_tracking_prepare(void);
int ram_write_tracking_start(void);
void ram_write_tracking_stop(void);