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    SubjectRe: [RFC-PATCH 2/4] mm: Add __rcu_alloc_page_lockless() func.
    On Thu, Sep 24, 2020 at 01:19:07PM +0200, Peter Zijlstra wrote:
    > On Thu, Sep 24, 2020 at 10:16:14AM +0200, Uladzislau Rezki wrote:
    > > The key point is "enough". We need pages to make a) fast progress b) support
    > > single argument of kvfree_rcu(one_arg). Not vice versa. That "enough" depends
    > > on scheduler latency and vague pre-allocated number of pages, it might
    > > be not enough what would require to refill it more and more or we can overshoot
    > > that would lead to memory overhead. So we have here timing issues and
    > > not accurate model. IMHO.
    >
    > I'm firmly opposed to the single argument kvfree_rcu() idea, that's
    > requiring memory to free memory.
    >
    Hmm.. The problem is there is a demand in it:

    Please have a look at places in the kernel where people do not
    embed the rcu_head into their stuctures and do like:

    <snip>
    synchronize_rcu();
    kfree(p);
    <snip>

    <snip>
    urezki@pc638:~/data/coding/linux-rcu.git$ find ./ -name "*.c" | xargs grep -C 1 -rn "synchronize_rcu" | grep kfree
    ./fs/nfs/sysfs.c-113- kfree(old);
    ./fs/ext4/super.c-1708- kfree(old_qname);
    ./kernel/trace/ftrace.c-5079- kfree(direct);
    ./kernel/trace/ftrace.c-5156- kfree(direct);
    ./kernel/trace/trace_probe.c-1087- kfree(link);
    ./kernel/module.c-3910- kfree(mod->args);
    ./net/core/sysctl_net_core.c-143- kfree(cur);
    ./arch/x86/mm/mmio-mod.c-314- kfree(found_trace);
    ./drivers/mfd/dln2.c-183- kfree(i);
    ./drivers/block/drbd/drbd_state.c-2074- kfree(old_conf);
    ./drivers/block/drbd/drbd_nl.c-1689- kfree(old_disk_conf);
    ./drivers/block/drbd/drbd_nl.c-2522- kfree(old_net_conf);
    ./drivers/block/drbd/drbd_nl.c-2935- kfree(old_disk_conf);
    ./drivers/block/drbd/drbd_receiver.c-3805- kfree(old_net_conf);
    ./drivers/block/drbd/drbd_receiver.c-4177- kfree(old_disk_conf);
    ./drivers/ipack/carriers/tpci200.c-189- kfree(slot_irq);
    ./drivers/crypto/nx/nx-842-pseries.c-1010- kfree(old_devdata);
    ./drivers/net/ethernet/myricom/myri10ge/myri10ge.c-3583- kfree(mgp->ss);
    ./drivers/net/ethernet/mellanox/mlx5/core/fpga/tls.c:286: synchronize_rcu(); /* before kfree(flow) */
    ./drivers/net/ethernet/mellanox/mlxsw/core.c-1574- kfree(rxl_item);
    ./drivers/net/ethernet/chelsio/cxgb4/cxgb4_main.c-6642- kfree(adapter->mbox_log);
    ./drivers/net/ethernet/chelsio/cxgb4/cxgb4_main.c-6644- kfree(adapter);
    ./drivers/infiniband/hw/hfi1/sdma.c-1337- kfree(dd->per_sdma);
    ./drivers/infiniband/core/device.c:2164: * synchronize_rcu before the netdev is kfreed, so we
    ./drivers/misc/vmw_vmci/vmci_context.c-692- kfree(notifier);
    ./drivers/misc/vmw_vmci/vmci_event.c-213- kfree(s);
    ./drivers/staging/fwserial/fwserial.c-2122- kfree(peer);
    urezki@pc638:~/data/coding/linux-rcu.git$
    <snip>

    --
    Vlad Rezki

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