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    SubjectRe: [PATCH] Double check memmap is actually valid with a memmap has unexpected holes
    Hi Mel,

    On Tue, May 05, 2009 at 09:29:44AM +0100, Mel Gorman wrote:
    > pfn_valid() is meant to be able to tell if a given PFN has valid memmap
    > associated with it or not. In FLATMEM, it is expected that holes always
    > have valid memmap as long as there is valid PFNs either side of the hole.
    > In SPARSEMEM, it is assumed that a valid section has a memmap for the
    > entire section.
    >
    > However, ARM and maybe other embedded architectures in the future free
    > memmap backing holes to save memory on the assumption the memmap is never
    > used. The page_zone() linkages are then broken even though pfn_valid()
    > returns true. A walker of the full memmap in this case must do additional
    > check to ensure the memmap they are looking at is sane by making sure the
    > zone and PFN linkages are still valid. This is expensive, but walkers of
    > the full memmap are extremely rare.
    >
    > This was caught before for FLATMEM and hacked around but it hits again
    > for SPARSEMEM because the page_zone() linkages can look ok where the PFN
    > linkages are totally screwed. This looks like a hatchet job but the reality
    > is that any clean solution would end up consuming all the memory saved
    > by punching these unexpected holes in the memmap. For example, we tried
    > marking the memmap within the section invalid but the section size exceeds
    > the size of the hole in most cases so pfn_valid() starts returning false
    > where valid memmap exists. Shrinking the size of the section would increase
    > memory consumption offsetting the gains.
    >
    > This patch identifies when an architecture is punching unexpected holes
    > in the memmap that the memory model cannot automatically detect. When set,
    > walkers of the full memmap must call memmap_valid_within() for each PFN and
    > passing in what it expects the page and zone to be for that PFN. If it finds
    > the linkages to be broken, it assumes the memmap is invalid for that PFN.
    >
    > Signed-off-by: Mel Gorman <mel@csn.ul.ie>

    I think we also need to fix up show_mem(). Attached is a
    compile-tested patch, please have a look. I am not sure about memory
    hotplug issues but on a quick glance the vmstat stuff seems to be
    optimistic as well.

    ---
    From: Johannes Weiner <hannes@cmpxchg.org>
    Subject: lib: adjust show_mem() to support memmap holes

    Some architectures free the backing of mem_map holes. pfn_valid() is
    not able to report this properly, so a stronger check is needed if the
    caller is about to use the page descriptor derived from a pfn.

    Change the node walking to zone walking and use memmap_valid_within()
    to check for holes. This is reliable as it additionally checks for
    page_zone() and page_to_pfn() coherency.

    Not-yet-signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
    ---
    lib/show_mem.c | 21 +++++++++------------
    1 files changed, 9 insertions(+), 12 deletions(-)

    diff --git a/lib/show_mem.c b/lib/show_mem.c
    index 238e72a..ed3c3ec 100644
    --- a/lib/show_mem.c
    +++ b/lib/show_mem.c
    @@ -11,29 +11,27 @@

    void show_mem(void)
    {
    - pg_data_t *pgdat;
    unsigned long total = 0, reserved = 0, shared = 0,
    nonshared = 0, highmem = 0;
    + struct zone *zone;

    printk(KERN_INFO "Mem-Info:\n");
    show_free_areas();

    - for_each_online_pgdat(pgdat) {
    - unsigned long i, flags;
    + for_each_populated_zone(zone) {
    + unsigned long start = zone->zone_start_pfn;
    + unsigned long end = start + zone->spanned_pages;
    + unsigned long pfn;

    - pgdat_resize_lock(pgdat, &flags);
    - for (i = 0; i < pgdat->node_spanned_pages; i++) {
    - struct page *page;
    - unsigned long pfn = pgdat->node_start_pfn + i;
    + for (pfn = start; pfn < end; pfn++) {
    + struct page *page = pfn_to_page(pfn);

    - if (unlikely(!(i % MAX_ORDER_NR_PAGES)))
    + if (unlikely(!(pfn % MAX_ORDER_NR_PAGES)))
    touch_nmi_watchdog();

    - if (!pfn_valid(pfn))
    + if (!memmap_valid_within(pfn, page, zone))
    continue;

    - page = pfn_to_page(pfn);
    -
    if (PageHighMem(page))
    highmem++;

    @@ -46,7 +44,6 @@ void show_mem(void)

    total++;
    }
    - pgdat_resize_unlock(pgdat, &flags);
    }

    printk(KERN_INFO "%lu pages RAM\n", total);
    --
    1.6.2.1.135.gde769


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