Commit 997c136f authored by Olaf Hering's avatar Olaf Hering Committed by Linus Torvalds
Browse files

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: default avatarOlaf Hering <olaf@aepfle.de>
Cc: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent 98bc93e5

fs/proc/vmcore.c

+49 −3
Original line number Diff line number Diff line
@@ -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 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;

include/linux/crash_dump.h

+5 −0
Original line number Diff line number Diff line
@@ -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 */

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