Messages in this thread | | | Date | Thu, 11 Aug 2022 16:11:21 +0200 | From | Michal Hocko <> | Subject | Re: [PATCH v2] mm/mempolicy: fix lock contention on mems_allowed |
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fix the lkml address (fat fingers, sorry)
On Thu 11-08-22 16:06:37, Michal Hocko wrote: > [Cc Wei Yang who is author of 78b132e9bae9] > > On Thu 11-08-22 20:41:57, Abel Wu wrote: > > The mems_allowed field can be modified by other tasks, so it isn't > > safe to access it with alloc_lock unlocked even in the current > > process context. > > > > Say there are two tasks: A from cpusetA is performing set_mempolicy(2), > > and B is changing cpusetA's cpuset.mems: > > > > A (set_mempolicy) B (echo xx > cpuset.mems) > > ------------------------------------------------------- > > pol = mpol_new(); > > update_tasks_nodemask(cpusetA) { > > foreach t in cpusetA { > > cpuset_change_task_nodemask(t) { > > mpol_set_nodemask(pol) { > > task_lock(t); // t could be A > > new = f(A->mems_allowed); > > update t->mems_allowed; > > pol.create(pol, new); > > task_unlock(t); > > } > > } > > } > > } > > task_lock(A); > > A->mempolicy = pol; > > task_unlock(A); > > > > In this case A's pol->nodes is computed by old mems_allowed, and could > > be inconsistent with A's new mems_allowed. > > Just to clarify. With an unfortunate timing and those two nodemasks > overlap the end user effect could be a premature OOM because some nodes > wouldn't be considered, right? > > > While it is different when replacing vmas' policy: the pol->nodes is > > gone wild only when current_cpuset_is_being_rebound(): > > > > A (mbind) B (echo xx > cpuset.mems) > > ------------------------------------------------------- > > pol = mpol_new(); > > mmap_write_lock(A->mm); > > cpuset_being_rebound = cpusetA; > > update_tasks_nodemask(cpusetA) { > > foreach t in cpusetA { > > cpuset_change_task_nodemask(t) { > > mpol_set_nodemask(pol) { > > task_lock(t); // t could be A > > mask = f(A->mems_allowed); > > update t->mems_allowed; > > pol.create(pol, mask); > > task_unlock(t); > > } > > } > > foreach v in A->mm { > > if (cpuset_being_rebound == cpusetA) > > pol.rebind(pol, cpuset.mems); > > v->vma_policy = pol; > > } > > mmap_write_unlock(A->mm); > > mmap_write_lock(t->mm); > > mpol_rebind_mm(t->mm); > > mmap_write_unlock(t->mm); > > } > > } > > cpuset_being_rebound = NULL; > > > > In this case, the cpuset.mems, which has already done updating, is > > finally used for calculating pol->nodes, rather than A->mems_allowed. > > So it is OK to call mpol_set_nodemask() with alloc_lock unlocked when > > doing mbind(2). > > > > Fixes: 78b132e9bae9 ("mm/mempolicy: remove or narrow the lock on current") > > Signed-off-by: Abel Wu <wuyun.abel@bytedance.com> > > The fix looks correct. > > > --- > > mm/mempolicy.c | 4 +++- > > 1 file changed, 3 insertions(+), 1 deletion(-) > > > > diff --git a/mm/mempolicy.c b/mm/mempolicy.c > > index d39b01fd52fe..61e4e6f5cfe8 100644 > > --- a/mm/mempolicy.c > > +++ b/mm/mempolicy.c > > @@ -855,12 +855,14 @@ static long do_set_mempolicy(unsigned short mode, unsigned short flags, > > goto out; > > } > > > > + task_lock(current); > > ret = mpol_set_nodemask(new, nodes, scratch); > > if (ret) { > > + task_unlock(current); > > mpol_put(new); > > goto out; > > } > > - task_lock(current); > > + > > old = current->mempolicy; > > current->mempolicy = new; > > if (new && new->mode == MPOL_INTERLEAVE) > > -- > > 2.31.1 > > -- > Michal Hocko > SUSE Labs
-- Michal Hocko SUSE Labs
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