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    Subject[PATCH AUTOSEL 4.14 19/19] PM / hibernate: memory_bm_find_bit(): Tighten node optimisation
    Date
    From: Andy Whitcroft <apw@canonical.com>

    [ Upstream commit da6043fe85eb5ec621e34a92540735dcebbea134 ]

    When looking for a bit by number we make use of the cached result from the
    preceding lookup to speed up operation. Firstly we check if the requested
    pfn is within the cached zone and if not lookup the new zone. We then
    check if the offset for that pfn falls within the existing cached node.
    This happens regardless of whether the node is within the zone we are
    now scanning. With certain memory layouts it is possible for this to
    false trigger creating a temporary alias for the pfn to a different bit.
    This leads the hibernation code to free memory which it was never allocated
    with the expected fallout.

    Ensure the zone we are scanning matches the cached zone before considering
    the cached node.

    Deep thanks go to Andrea for many, many, many hours of hacking and testing
    that went into cornering this bug.

    Reported-by: Andrea Righi <andrea.righi@canonical.com>
    Tested-by: Andrea Righi <andrea.righi@canonical.com>
    Signed-off-by: Andy Whitcroft <apw@canonical.com>
    Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
    Signed-off-by: Sasha Levin <sashal@kernel.org>
    ---
    kernel/power/snapshot.c | 9 ++++++++-
    1 file changed, 8 insertions(+), 1 deletion(-)

    diff --git a/kernel/power/snapshot.c b/kernel/power/snapshot.c
    index 0972a8e09d082..ff2aabb70de93 100644
    --- a/kernel/power/snapshot.c
    +++ b/kernel/power/snapshot.c
    @@ -734,8 +734,15 @@ static int memory_bm_find_bit(struct memory_bitmap *bm, unsigned long pfn,
    * We have found the zone. Now walk the radix tree to find the leaf node
    * for our PFN.
    */
    +
    + /*
    + * If the zone we wish to scan is the the current zone and the
    + * pfn falls into the current node then we do not need to walk
    + * the tree.
    + */
    node = bm->cur.node;
    - if (((pfn - zone->start_pfn) & ~BM_BLOCK_MASK) == bm->cur.node_pfn)
    + if (zone == bm->cur.zone &&
    + ((pfn - zone->start_pfn) & ~BM_BLOCK_MASK) == bm->cur.node_pfn)
    goto node_found;

    node = zone->rtree;
    --
    2.20.1
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