[PATCH v2 3/4] migration/postcopy: Detect file system on dest host

[Peter Xu <peterx@redhat.com>]

Postcopy requires the memory support userfaultfd to work.  Right now we
check it but it's a bit too late (when switching to postcopy migration).

Do that early right at enabling of postcopy.

Note that this is still only a best effort because ramblocks can be
dynamically created.  We can add check in hostmem creations and fail if
postcopy enabled, but maybe that's too aggressive.

Still, we have chance to fail the most obvious where we know there's an
existing unsupported ramblock.

Signed-off-by: Peter Xu <peterx@redhat.com>
---
 migration/postcopy-ram.c | 34 ++++++++++++++++++++++++++++++----
 1 file changed, 30 insertions(+), 4 deletions(-)

diff --git a/migration/postcopy-ram.c b/migration/postcopy-ram.c
index 93f39f8e06..bbb8af61ae 100644
--- a/migration/postcopy-ram.c
+++ b/migration/postcopy-ram.c
@@ -36,6 +36,7 @@
 #include "yank_functions.h"
 #include "tls.h"
 #include "qemu/userfaultfd.h"
+#include "qemu/mmap-alloc.h"
 
 /* Arbitrary limit on size of each discard command,
  * keeps them around ~200 bytes
@@ -336,11 +337,12 @@ static bool ufd_check_and_apply(int ufd, MigrationIncomingState *mis)
 
 /* Callback from postcopy_ram_supported_by_host block iterator.
  */
-static int test_ramblock_postcopiable(RAMBlock *rb, void *opaque)
+static int test_ramblock_postcopiable(RAMBlock *rb)
 {
     const char *block_name = qemu_ram_get_idstr(rb);
     ram_addr_t length = qemu_ram_get_used_length(rb);
     size_t pagesize = qemu_ram_pagesize(rb);
+    QemuFsType fs;
 
     if (length % pagesize) {
         error_report("Postcopy requires RAM blocks to be a page size multiple,"
@@ -348,6 +350,15 @@ static int test_ramblock_postcopiable(RAMBlock *rb, void *opaque)
                      "page size of 0x%zx", block_name, length, pagesize);
         return 1;
     }
+
+    if (rb->fd >= 0) {
+        fs = qemu_fd_getfs(rb->fd);
+        if (fs != QEMU_FS_TYPE_TMPFS && fs != QEMU_FS_TYPE_HUGETLBFS) {
+            error_report("Host backend files need to be TMPFS or HUGETLBFS only");
+            return 1;
+        }
+    }
+
     return 0;
 }
 
@@ -366,6 +377,7 @@ bool postcopy_ram_supported_by_host(MigrationIncomingState *mis)
     struct uffdio_range range_struct;
     uint64_t feature_mask;
     Error *local_err = NULL;
+    RAMBlock *block;
 
     if (qemu_target_page_size() > pagesize) {
         error_report("Target page size bigger than host page size");
@@ -390,9 +402,23 @@ bool postcopy_ram_supported_by_host(MigrationIncomingState *mis)
         goto out;
     }
 
-    /* We don't support postcopy with shared RAM yet */
-    if (foreach_not_ignored_block(test_ramblock_postcopiable, NULL)) {
-        goto out;
+    /*
+     * We don't support postcopy with some type of ramblocks.
+     *
+     * NOTE: we explicitly ignored ramblock_is_ignored() instead we checked
+     * all possible ramblocks.  This is because this function can be called
+     * when creating the migration object, during the phase RAM_MIGRATABLE
+     * is not even properly set for all the ramblocks.
+     *
+     * A side effect of this is we'll also check against RAM_SHARED
+     * ramblocks even if migrate_ignore_shared() is set (in which case
+     * we'll never migrate RAM_SHARED at all), but normally this shouldn't
+     * affect in reality, or we can revisit.
+     */
+    RAMBLOCK_FOREACH(block) {
+        if (test_ramblock_postcopiable(block)) {
+            goto out;
+        }
     }
 
     /*
-- 
2.39.1