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

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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 <[email protected]>
Cc: Alexey Dobriyan <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>
This commit is contained in:
Olaf Hering 2011-05-26 16:25:54 -07:00 • committed by Linus Torvalds
commit 997c136f51
2 changed files with 54 additions and 3 deletions
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52
fs/proc/vmcore.c
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@ -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;