• Mel Gorman's avatar
    [ARM] Skip memory holes in FLATMEM when reading /proc/pagetypeinfo · e80d6a24
    Mel Gorman authored
    Ordinarily, memory holes in flatmem still have a valid memmap and is safe
    to use. However, an architecture (ARM) frees up the memmap backing memory
    holes on the assumption it is never used. /proc/pagetypeinfo reads the
    whole range of pages in a zone believing that the memmap is valid and that
    pfn_valid will return false if it is not. On ARM, freeing the memmap breaks
    the page->zone linkages even though pfn_valid() returns true and the kernel
    can oops shortly afterwards due to accessing a bogus struct zone *.
    This patch lets architectures say when FLATMEM can have holes in the
    memmap. Rather than an expensive check for valid memory, /proc/pagetypeinfo
    will confirm that the page linkages are still valid by checking page->zone
    is still the expected zone. The lookup of page_zone is safe as there is a
    limited range of memory that is accessed when calling page_zone.  Even if
    page_zone happens to return the correct zone, the impact is that the counters
    in /proc/pagetypeinfo are slightly off but fragmentation monitoring is
    unlikely to be relevant on an embedded system.
    Reported-by: default avatarH Hartley Sweeten <hsweeten@visionengravers.com>
    Signed-off-by: default avatarMel Gorman <mel@csn.ul.ie>
    Tested-by: default avatarH Hartley Sweeten <hsweeten@visionengravers.com>
    Signed-off-by: default avatarRussell King <rmk+kernel@arm.linux.org.uk>
Kconfig 30.7 KB