[PATCH v2] add hook to read_from_oldmem() to check for non-ram pages

Olaf Hering
2011-05-06T13:20:31Z

The balloon driver in a Xen guest frees guest pages and marks them as
mmio. When the kernel crashes and the crash kernel attempts to read the
oldmem via /proc/vmcore a read from ballooned pages will generate 100%
load in dom0 because Xen asks qemu-dm for the page content. Since the
reads come in as 8byte requests each ballooned page is tried 512 times.

With this change a hook can be registered which checks wether the given
pfn is really ram. The hook has to return a value > 0 for ram pages, a
value < 0 on error (because the hypercall is not known) and 0 for
non-ram pages.

This will reduce the time to read /proc/vmcore. Without this change a
512M guest with 128M crashkernel region needs 200 seconds to read it,
with this change it takes just 2 seconds.

Signed-off-by: Olaf Hering <olaf@aepfle.de>

---
v2:
remove refcounting, called function has to take care of module refcounting
add comments to pfn_is_ram()
register_oldmem_pfn_is_ram() returns -EBUSY if a function is already registerd
move exports close to exported function
update prototypes to include 'pfn'

fs/proc/vmcore.c | 52 ++++++++++++++++++++++++++++++++++++++++++---
include/linux/crash_dump.h | 5 ++++
2 files changed, 54 insertions(+), 3 deletions(-)

Index: linux-2.6.39-rc6/fs/proc/vmcore.c
===================================================================
--- linux-2.6.39-rc6.orig/fs/proc/vmcore.c
+++ linux-2.6.39-rc6/fs/proc/vmcore.c
@@ -35,6 +35,46 @@ static u64 vmcore_size;

static struct proc_dir_entry *proc_vmcore = NULL;

+/*
+ * Returns > 0 for RAM pages, 0 for non-RAM pages, < 0 on error
+ * The called function has to take care of module refcounting.
+ */
+static int (*oldmem_pfn_is_ram)(unsigned long pfn);
+
+int register_oldmem_pfn_is_ram(int (*fn)(unsigned long pfn))
+{
+ if (oldmem_pfn_is_ram)
+ return -EBUSY;
+ oldmem_pfn_is_ram = fn;
+ return 0;
+}
+EXPORT_SYMBOL_GPL(register_oldmem_pfn_is_ram);
+
+void unregister_oldmem_pfn_is_ram(void)
+{
+ oldmem_pfn_is_ram = NULL;
+ wmb();
+}
+EXPORT_SYMBOL_GPL(unregister_oldmem_pfn_is_ram);
+
+static int pfn_is_ram(unsigned long pfn)
+{
+ int (*fn)(unsigned long pfn);
+ /* pfn is ram unless fn() checks pagetype */
+ int ret = 1;
+
+ /*
+ * Ask hypervisor if the pfn is really ram.
+ * A ballooned page contains no data and reading from such a page
+ * will cause high load in the hypervisor.
+ */
+ fn = oldmem_pfn_is_ram;
+ if (fn)
+ ret = fn(pfn);
+
+ return ret;
+}
+
/* Reads a page from the oldmem device from given offset. */
static ssize_t read_from_oldmem(char *buf, size_t count,
u64 *ppos, int userbuf)
@@ -55,9 +95,15 @@ static ssize_t read_from_oldmem(char *bu
else
nr_bytes = count;

- tmp = copy_oldmem_page(pfn, buf, nr_bytes, offset, userbuf);
- if (tmp < 0)
- return tmp;
+ /* If pfn is not ram, return zeros for sparse dump files */
+ if (pfn_is_ram(pfn) == 0)
+ memset(buf, 0, nr_bytes);
+ else {
+ tmp = copy_oldmem_page(pfn, buf, nr_bytes,
+ offset, userbuf);
+ if (tmp < 0)
+ return tmp;
+ }
*ppos += nr_bytes;
count -= nr_bytes;
buf += nr_bytes;
Index: linux-2.6.39-rc6/include/linux/crash_dump.h
===================================================================
--- linux-2.6.39-rc6.orig/include/linux/crash_dump.h
+++ linux-2.6.39-rc6/include/linux/crash_dump.h
@@ -66,6 +66,11 @@ static inline void vmcore_unusable(void)
if (is_kdump_kernel())
elfcorehdr_addr = ELFCORE_ADDR_ERR;
}
+
+#define HAVE_OLDMEM_PFN_IS_RAM 1
+extern int register_oldmem_pfn_is_ram(int (*fn)(unsigned long pfn));
+extern void unregister_oldmem_pfn_is_ram(void);
+
#else /* !CONFIG_CRASH_DUMP */
static inline int is_kdump_kernel(void) { return 0; }
#endif /* CONFIG_CRASH_DUMP */