The patch titled
fs/proc/vmcore.c: add hook to read_from_oldmem() to check for non-ram pages
has been added to the -mm tree. Its filename is
fs-proc-vmcorec-add-hook-to-read_from_oldmem-to-check-for-non-ram-pages.patch
Before you just go and hit "reply", please:
a) Consider who else should be cc'ed
b) Prefer to cc a suitable mailing list as well
c) Ideally: find the original patch on the mailing list and do a
reply-to-all to that, adding suitable additional cc's
*** Remember to use Documentation/SubmitChecklist when testing your code ***
See http://userweb.kernel.org/~akpm/stuff/added-to-mm.txt to find
out what to do about this
The current -mm tree may be found at http://userweb.kernel.org/~akpm/mmotm/
------------------------------------------------------
Subject: fs/proc/vmcore.c: add hook to read_from_oldmem() to check for non-ram pages
From: Olaf Hering <olaf@xxxxxxxxx>
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@xxxxxxxxx>
Cc: Alexey Dobriyan <adobriyan@xxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
fs/proc/vmcore.c | 52 ++++++++++++++++++++++++++++++++---
include/linux/crash_dump.h | 5 +++
2 files changed, 54 insertions(+), 3 deletions(-)
diff -puN fs/proc/vmcore.c~fs-proc-vmcorec-add-hook-to-read_from_oldmem-to-check-for-non-ram-pages fs/proc/vmcore.c
--- a/fs/proc/vmcore.c~fs-proc-vmcorec-add-hook-to-read_from_oldmem-to-check-for-non-ram-pages
+++ a/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;
diff -puN include/linux/crash_dump.h~fs-proc-vmcorec-add-hook-to-read_from_oldmem-to-check-for-non-ram-pages include/linux/crash_dump.h
--- a/include/linux/crash_dump.h~fs-proc-vmcorec-add-hook-to-read_from_oldmem-to-check-for-non-ram-pages
+++ a/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 */
_
Patches currently in -mm which might be from olaf@xxxxxxxxx are
linux-next.patch
fs-proc-vmcorec-add-hook-to-read_from_oldmem-to-check-for-non-ram-pages.patch
--
To unsubscribe from this list: send the line "unsubscribe mm-commits" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
