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    Subject[PATCH 5.16 0945/1039] ipv4: avoid quadratic behavior in netns dismantle
    Date
    From: Eric Dumazet <edumazet@google.com>

    commit d07418afea8f1d9896aaf9dc5ae47ac4f45b220c upstream.

    net/ipv4/fib_semantics.c uses an hash table of 256 slots,
    keyed by device ifindexes: fib_info_devhash[DEVINDEX_HASHSIZE]

    Problem is that with network namespaces, devices tend
    to use the same ifindex.

    lo device for instance has a fixed ifindex of one,
    for all network namespaces.

    This means that hosts with thousands of netns spend
    a lot of time looking at some hash buckets with thousands
    of elements, notably at netns dismantle.

    Simply add a per netns perturbation (net_hash_mix())
    to spread elements more uniformely.

    Also change fib_devindex_hashfn() to use more entropy.

    Fixes: aa79e66eee5d ("net: Make ifindex generation per-net namespace")
    Signed-off-by: Eric Dumazet <edumazet@google.com>
    Reviewed-by: David Ahern <dsahern@kernel.org>
    Signed-off-by: Jakub Kicinski <kuba@kernel.org>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
    ---
    net/ipv4/fib_semantics.c | 36 +++++++++++++++++-------------------
    1 file changed, 17 insertions(+), 19 deletions(-)

    --- a/net/ipv4/fib_semantics.c
    +++ b/net/ipv4/fib_semantics.c
    @@ -29,6 +29,7 @@
    #include <linux/init.h>
    #include <linux/slab.h>
    #include <linux/netlink.h>
    +#include <linux/hash.h>

    #include <net/arp.h>
    #include <net/ip.h>
    @@ -319,11 +320,15 @@ static inline int nh_comp(struct fib_inf

    static inline unsigned int fib_devindex_hashfn(unsigned int val)
    {
    - unsigned int mask = DEVINDEX_HASHSIZE - 1;
    + return hash_32(val, DEVINDEX_HASHBITS);
    +}
    +
    +static struct hlist_head *
    +fib_info_devhash_bucket(const struct net_device *dev)
    +{
    + u32 val = net_hash_mix(dev_net(dev)) ^ dev->ifindex;

    - return (val ^
    - (val >> DEVINDEX_HASHBITS) ^
    - (val >> (DEVINDEX_HASHBITS * 2))) & mask;
    + return &fib_info_devhash[fib_devindex_hashfn(val)];
    }

    static unsigned int fib_info_hashfn_1(int init_val, u8 protocol, u8 scope,
    @@ -433,12 +438,11 @@ int ip_fib_check_default(__be32 gw, stru
    {
    struct hlist_head *head;
    struct fib_nh *nh;
    - unsigned int hash;

    spin_lock(&fib_info_lock);

    - hash = fib_devindex_hashfn(dev->ifindex);
    - head = &fib_info_devhash[hash];
    + head = fib_info_devhash_bucket(dev);
    +
    hlist_for_each_entry(nh, head, nh_hash) {
    if (nh->fib_nh_dev == dev &&
    nh->fib_nh_gw4 == gw &&
    @@ -1607,12 +1611,10 @@ link_it:
    } else {
    change_nexthops(fi) {
    struct hlist_head *head;
    - unsigned int hash;

    if (!nexthop_nh->fib_nh_dev)
    continue;
    - hash = fib_devindex_hashfn(nexthop_nh->fib_nh_dev->ifindex);
    - head = &fib_info_devhash[hash];
    + head = fib_info_devhash_bucket(nexthop_nh->fib_nh_dev);
    hlist_add_head(&nexthop_nh->nh_hash, head);
    } endfor_nexthops(fi)
    }
    @@ -1964,8 +1966,7 @@ void fib_nhc_update_mtu(struct fib_nh_co

    void fib_sync_mtu(struct net_device *dev, u32 orig_mtu)
    {
    - unsigned int hash = fib_devindex_hashfn(dev->ifindex);
    - struct hlist_head *head = &fib_info_devhash[hash];
    + struct hlist_head *head = fib_info_devhash_bucket(dev);
    struct fib_nh *nh;

    hlist_for_each_entry(nh, head, nh_hash) {
    @@ -1984,12 +1985,11 @@ void fib_sync_mtu(struct net_device *dev
    */
    int fib_sync_down_dev(struct net_device *dev, unsigned long event, bool force)
    {
    - int ret = 0;
    - int scope = RT_SCOPE_NOWHERE;
    + struct hlist_head *head = fib_info_devhash_bucket(dev);
    struct fib_info *prev_fi = NULL;
    - unsigned int hash = fib_devindex_hashfn(dev->ifindex);
    - struct hlist_head *head = &fib_info_devhash[hash];
    + int scope = RT_SCOPE_NOWHERE;
    struct fib_nh *nh;
    + int ret = 0;

    if (force)
    scope = -1;
    @@ -2134,7 +2134,6 @@ out:
    int fib_sync_up(struct net_device *dev, unsigned char nh_flags)
    {
    struct fib_info *prev_fi;
    - unsigned int hash;
    struct hlist_head *head;
    struct fib_nh *nh;
    int ret;
    @@ -2150,8 +2149,7 @@ int fib_sync_up(struct net_device *dev,
    }

    prev_fi = NULL;
    - hash = fib_devindex_hashfn(dev->ifindex);
    - head = &fib_info_devhash[hash];
    + head = fib_info_devhash_bucket(dev);
    ret = 0;

    hlist_for_each_entry(nh, head, nh_hash) {

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