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    SubjectRe: [PATCH 1/2] mm: fix bugs of mpol_rebind_nodemask()
    on 2010-4-30 2:03, David Rientjes wrote:
    > 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
    > cpuset.
    > 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. ]

    I updated it and remade a new patchset, could you review it for me?


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