fs/proc/vmcore.c: add hook to read_from_oldmem() to check for non-ram pages

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>
Cc: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 49 insertions, 3 deletions

diff --git a/fs/proc/vmcore.c b/fs/proc/vmcore.c
index 74802bc5ded9..cd99bf557650 100644
--- a/fs/proc/vmcore.c
+++ b/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 *buf, size_t count,
                 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;