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    SubjectRe: [PATCH 1/2] mm: fix bugs of mpol_rebind_nodemask()
    On Thu, 29 Apr 2010, Miao Xie wrote:

    > > That's been the behavior for at least three years so changing it from
    > > under the applications isn't acceptable, see
    > > Documentation/vm/numa_memory_policy.txt regarding mempolicy rebinds and
    > > the two flags that are defined that can be used to adjust the behavior.
    > Is the flags what you said MPOL_F_STATIC_NODES and MPOL_F_RELATIVE_NODES?
    > But the codes that I changed isn't under MPOL_F_STATIC_NODES or MPOL_F_RELATIVE_NODES.
    > The documentation doesn't say what we should do if either of these two flags is not set.

    MPOL_F_STATIC_NODES and MPOL_F_RELATIVE_NODES allow you to adjust the
    behavior of the rebind: the former requires specific nodes to be assigned
    to the mempolicy and could suppress the rebind completely, if necessary;
    the latter ensures the mempolicy nodemask has a certain weight as nodes
    are assigned in a round-robin manner. The behavior that you're referring
    to is provided via MPOL_F_RELATIVE_NODES, which guarantees whatever weight
    is passed via set_mempolicy() will be preserved when mems are added to a

    Regardless of whether the behavior is documented when either flag is
    passed, we can't change the long-standing default behavior that people use
    when their cpuset mems are rebound: we can only extend the functionality
    and the behavior you're seeking is already available with a
    MPOL_F_RELATIVE_NODES flag modifier.

    > Furthermore, in order to fix no node to alloc memory, when we want to update mempolicy
    > and mems_allowed, we expand the set of nodes first (set all the newly nodes) and
    > shrink the set of nodes lazily(clean disallowed nodes).

    That's a cpuset implementation choice, not a mempolicy one; mempolicies
    have nothing to do with an empty current->mems_allowed.

    > But remap() breaks the expanding, so if we don't remove remap(), the problem can't be
    > fixed. Otherwise, cpuset has to do the rebinding by itself and the code is ugly.
    > Like this:
    > static void cpuset_change_task_nodemask(struct task_struct *tsk, nodemask_t *newmems)
    > {
    > nodemask_t tmp;
    > ...
    > /* expand the set of nodes */
    > if (!mpol_store_user_nodemask(tsk->mempolicy)) {
    > nodes_remap(tmp, ...);
    > nodes_or(tsk->mempolicy->v.nodes, tsk->mempolicy->v.nodes, tmp);
    > }
    > ...
    > /* shrink the set of nodes */
    > if (!mpol_store_user_nodemask(tsk->mempolicy))
    > tsk->mempolicy->v.nodes = tmp;
    > }

    I don't see why this is even necessary, the mempolicy code could simply
    return numa_node_id() when nodes_empty(current->mempolicy->v.nodes) to
    close the race.

    [ Your pseudo-code is also lacking task_lock(tsk), which is required to
    safely dereference tsk->mempolicy, and this is only available so far in
    -mm since the oom killer rewrite. ]

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