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Commit 997c136f518c5debd63847e78e2a8694f56dcf90

Authored by Olaf Hering
Committed by Linus Torvalds
1 parent 98bc93e505
Exists in master and in 7 other branches imx_3.0.35_4.1.0, imx_3.10.17_1.0.1_ga, imx_3.10.53_1.1.0_ga, imx_3.14.28_1.0.0_ga, smarc-imx_3.10.53_1.1.0_ga, smarc-imx_3.14.28_1.0.0_ga, smarc-l5.0.0_1.0.0-ga

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>

Showing 2 changed files with 54 additions and 3 deletions Side-by-side Diff

... ... @@ -35,6 +35,46 @@
35 35  
36 36 static struct proc_dir_entry *proc_vmcore = NULL;
37 37  
  38 +/*
  39 + * Returns > 0 for RAM pages, 0 for non-RAM pages, < 0 on error
  40 + * The called function has to take care of module refcounting.
  41 + */
  42 +static int (*oldmem_pfn_is_ram)(unsigned long pfn);
  43 +
  44 +int register_oldmem_pfn_is_ram(int (*fn)(unsigned long pfn))
  45 +{
  46 + if (oldmem_pfn_is_ram)
  47 + return -EBUSY;
  48 + oldmem_pfn_is_ram = fn;
  49 + return 0;
  50 +}
  51 +EXPORT_SYMBOL_GPL(register_oldmem_pfn_is_ram);
  52 +
  53 +void unregister_oldmem_pfn_is_ram(void)
  54 +{
  55 + oldmem_pfn_is_ram = NULL;
  56 + wmb();
  57 +}
  58 +EXPORT_SYMBOL_GPL(unregister_oldmem_pfn_is_ram);
  59 +
  60 +static int pfn_is_ram(unsigned long pfn)
  61 +{
  62 + int (*fn)(unsigned long pfn);
  63 + /* pfn is ram unless fn() checks pagetype */
  64 + int ret = 1;
  65 +
  66 + /*
  67 + * Ask hypervisor if the pfn is really ram.
  68 + * A ballooned page contains no data and reading from such a page
  69 + * will cause high load in the hypervisor.
  70 + */
  71 + fn = oldmem_pfn_is_ram;
  72 + if (fn)
  73 + ret = fn(pfn);
  74 +
  75 + return ret;
  76 +}
  77 +
38 78 /* Reads a page from the oldmem device from given offset. */
39 79 static ssize_t read_from_oldmem(char *buf, size_t count,
40 80 u64 *ppos, int userbuf)
... ... @@ -55,9 +95,15 @@
55 95 else
56 96 nr_bytes = count;
57 97  
58   - tmp = copy_oldmem_page(pfn, buf, nr_bytes, offset, userbuf);
59   - if (tmp < 0)
60   - return tmp;
  98 + /* If pfn is not ram, return zeros for sparse dump files */
  99 + if (pfn_is_ram(pfn) == 0)
  100 + memset(buf, 0, nr_bytes);
  101 + else {
  102 + tmp = copy_oldmem_page(pfn, buf, nr_bytes,
  103 + offset, userbuf);
  104 + if (tmp < 0)
  105 + return tmp;
  106 + }
61 107 *ppos += nr_bytes;
62 108 count -= nr_bytes;
63 109 buf += nr_bytes;
include/linux/crash_dump.h
Diff comments   View file @ 997c136
... ... @@ -66,6 +66,11 @@
66 66 if (is_kdump_kernel())
67 67 elfcorehdr_addr = ELFCORE_ADDR_ERR;
68 68 }
  69 +
  70 +#define HAVE_OLDMEM_PFN_IS_RAM 1
  71 +extern int register_oldmem_pfn_is_ram(int (*fn)(unsigned long pfn));
  72 +extern void unregister_oldmem_pfn_is_ram(void);
  73 +
69 74 #else /* !CONFIG_CRASH_DUMP */
70 75 static inline int is_kdump_kernel(void) { return 0; }
71 76 #endif /* CONFIG_CRASH_DUMP */