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    SubjectRe: [mm 4.15-rc8] Random oopses under memory pressure.
    On Fri, Jan 19, 2018 at 02:53:25PM -0800, Linus Torvalds wrote:

    > It would probably be good to add the size too, just to explain why
    > it's potentially expensive.
    >
    > That said, apparently we do have hundreds of them, with just
    > cpufreq_frequency_table having a ton. Maybe some are hidden in macros
    > and removing one removes a lot.

    cpufreq_table_find_index_...(), mostly.

    > The real problem is that sometimes the subtraction is simply the right
    > thing to do, and there's no sane way to say "yeah, this is one of
    > those cases you shouldn't warn about".

    FWIW, the sizes of the most common ones are
    91 sizeof struct cpufreq_frequency_table = 12
    Almost all of those come from
    cpufreq_for_each_valid_entry(pos, table)
    if (....)
    return pos - table;
    and I wonder if we would be better off with something like
    #define cpufreq_for_each_valid_entry(pos, table, idx) \
    for (pos = table, idx = 0; pos->frequency != CPUFREQ_TABLE_END; pos++, idx++) \
    if (pos->frequency == CPUFREQ_ENTRY_INVALID) \
    continue; \
    else
    so that those loops would become
    cpufreq_for_each_valid_entry(pos, table, idx)
    if (....)
    return idx;
    36 sizeof struct Indirect = 24
    21 sizeof struct ips_scb = 216
    18 sizeof struct runlist_element = 24
    13 sizeof struct zone = 1728
    Some are from
    #define zone_idx(zone) ((zone) - (zone)->zone_pgdat->node_zones)
    but there's
    static inline int zone_id(const struct zone *zone)
    {
    struct pglist_data *pgdat = zone->zone_pgdat;

    return zone - pgdat->node_zones;
    }
    and a couple of places where we have
    for (zone = node_zones; zone - node_zones < MAX_NR_ZONES; ++zone) {
    Those bloody well ought to be
    for (zone = node_zones, end = node_zones + MAX_NR_ZONES; zone < end; zone++) {
    13 sizeof struct vring = 40
    11 sizeof struct usbhsh_device = 24
    10 sizeof struct xpc_partition = 888
    9 sizeof struct skge_element = 40
    9 sizeof struct lock_class = 400
    9 sizeof struct hstate = 29872
    That little horror comes from get_hstate_idx() and hstate_index(). Code generated
    for division is
    movabsq $-5542915600080909725, %rax
    sarq $4, %rdi
    imulq %rdi, %rax
    7 sizeof struct nvme_rdma_queue = 312
    7 sizeof struct iso_context = 208
    6 sizeof struct i915_power_well = 48
    6 sizeof struct hpet_dev = 168
    6 sizeof struct ext4_extent = 12
    6 sizeof struct esas2r_target = 120
    5 sizeof struct iio_chan_spec = 152
    5 sizeof struct hwspinlock = 96
    4 sizeof struct myri10ge_slice_state = 704
    4 sizeof struct ext4_extent_idx = 12
    Another interesting-looking one is struct vhost_net_virtqueue (18080 bytes)

    Note that those sizes are rather sensitive to lockdep, spinlock debugging, etc.

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