• Olaf Hering's avatar
    fs/proc/vmcore.c: add hook to read_from_oldmem() to check for non-ram pages · 997c136f
    Olaf Hering authored
    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>
    997c136f
vmcore.c 17.1 KB