[CRIU] [PATCH 4/5] kdat: Cache kdat object into /run/criu.kdat (v2)

[Pavel Emelyanov]

Doing kerndat checks on every criu start is way too slow. We
need some way to speed this checks up on a particular box.

As suggested by Andre, Dima and Mike let's try to keep the
collected kdat bits into some tmpfs file. Keeping it on
tmpfs would invaludate this cache on every machine reboot.

There have been many suggestions how to generate this file,
my proposal is to create it once the kdat object is filled
by criu, w/o any explicit command. Optionally we can add
'criu kdat --save|--drop' actions to manage this file.

v2:
* don't ignore return code of write() (some glibcs complain)
* unlink tmp file in case rename failed

Signed-off-by: Pavel Emelyanov <xemul at virtuozzo.com>
---
 criu/include/kerndat.h |  6 ++++
 criu/include/magic.h   |  7 +++++
 criu/kerndat.c         | 85 +++++++++++++++++++++++++++++++++++++++++++++++++-
 3 files changed, 97 insertions(+), 1 deletion(-)

diff --git a/criu/include/kerndat.h b/criu/include/kerndat.h
index 6b621c6..b1722eb 100644
--- a/criu/include/kerndat.h
+++ b/criu/include/kerndat.h
@@ -29,6 +29,12 @@ enum loginuid_func {
 };
 
 struct kerndat_s {
+	/*
+	 * XXX: This structure is written AS IS into a tmpfs file,
+	 * so any changes in it should result in changed KDAT_MAGIC
+	 * value to avoid loading of bad kdat bits.
+	 */
+	u32 magic;
 	dev_t shmem_dev;
 	int last_cap;
 	u64 zero_page_pfn;
diff --git a/criu/include/magic.h b/criu/include/magic.h
index 059d005..2297930 100644
--- a/criu/include/magic.h
+++ b/criu/include/magic.h
@@ -116,4 +116,11 @@
 #define STATS_MAGIC		0x57093306 /* Ostashkov */
 #define IRMAP_CACHE_MAGIC	0x57004059 /* Ivanovo */
 
+/*
+ * Magic for kerndat_s structure. When changing, keep the
+ * old magics to avoid magic re-use.
+ */
+
+#define KDAT_MAGIC_1		0x57023458 /* Torzhok */
+
 #endif /* __CR_MAGIC_H__ */
diff --git a/criu/kerndat.c b/criu/kerndat.c
index 87a7b62..3aca1e4 100644
--- a/criu/kerndat.c
+++ b/criu/kerndat.c
@@ -650,10 +650,93 @@ close:
 	bclose(&f);
 	return ret;
 }
+
+#define KERNDAT_CACHE_FILE	"/run/criu.kdat"
+#define KERNDAT_CACHE_FILE_TMP	"/run/.criu.kdat"
+
+static int kerndat_try_load_cache(void)
+{
+	int fd, ret;
+
+	fd = open(KERNDAT_CACHE_FILE, O_RDONLY);
+	if (fd < 0) {
+		if (errno == ENOENT)
+			return 1;
+
+		pr_perror("Can't load %s", KERNDAT_CACHE_FILE);
+		return -1;
+	}
+
+	ret = read(fd, &kdat, sizeof(kdat));
+	if (ret < 0) {
+		pr_perror("Can'r read kdat cache");
+		return -1;
+	}
+
+	close(fd);
+
+	if (ret != sizeof(kdat) || kdat.magic != KDAT_MAGIC_1) {
+		pr_warn("Stale %s file\n", KERNDAT_CACHE_FILE);
+		unlink(KERNDAT_CACHE_FILE);
+		return 1;
+	}
+
+	pr_info("Loaded criu.kdat cache\n");
+	return 0;
+}
+
+static int kerndat_save_cache(void)
+{
+	int fd, ret;
+	struct statfs s;
+
+	fd = open(KERNDAT_CACHE_FILE_TMP, O_CREAT | O_EXCL | O_WRONLY, 0600);
+	if (fd < 0) {
+		/*
+		 * It can happen that we race with some other criu
+		 * instance. That's OK, just ignore this error and
+		 * proceed.
+		 */
+		if (errno == EEXIST)
+			return 0;
+
+		pr_perror("Can't save %s", KERNDAT_CACHE_FILE_TMP);
+		return -1;
+	}
+
+	if (fstatfs(fd, &s) < 0) {
+		pr_perror("Can't statfs %s", KERNDAT_CACHE_FILE_TMP);
+		return -1;
+	}
+
+	if (s.f_type != TMPFS_MAGIC) {
+		pr_warn("Can't keep criu.kdat on tempfs\n");
+		close(fd);
+		return 0;
+	}
+
+	kdat.magic = KDAT_MAGIC_1;
+	ret = write(fd, &kdat, sizeof(kdat));
+	if (ret != sizeof(kdat))
+		pr_warn("Kerndat cache writing failed.\n");
+	close(fd);
+
+	ret = rename(KERNDAT_CACHE_FILE_TMP, KERNDAT_CACHE_FILE);
+	if (ret < 0) {
+		pr_perror("Can't save %s", KERNDAT_CACHE_FILE);
+		unlink(KERNDAT_CACHE_FILE_TMP);
+	}
+	return ret;
+}
+
 int kerndat_init(void)
 {
 	int ret;
 
+	ret = kerndat_try_load_cache();
+	if (ret <= 0)
+		return ret;
+
 	ret = check_pagemap();
 	if (!ret)
 		ret = kerndat_get_shmemdev();
@@ -691,5 +774,5 @@ int kerndat_init(void)
 	kerndat_lsm();
 	kerndat_mmap_min_addr();
 
-	return ret;
+	return kerndat_save_cache();
 }
-- 
2.5.5