The patch titled
Subject: mm/mempolicy: fix lock contention on mems_allowed
has been added to the -mm mm-unstable branch. Its filename is
mm-mempolicy-fix-lock-contention-on-mems_allowed.patch
This patch will shortly appear at
https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/mm-mempolicy-fix-lock-contention-on-mems_allowed.patch
This patch will later appear in the mm-unstable branch at
git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm
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------------------------------------------------------
From: Abel Wu <wuyun.abel@xxxxxxxxxxxxx>
Subject: mm/mempolicy: fix lock contention on mems_allowed
Date: Thu, 11 Aug 2022 20:41:57 +0800
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.
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).
Link: https://lkml.kernel.org/r/20220811124157.74888-1-wuyun.abel@xxxxxxxxxxxxx
Fixes: 78b132e9bae9 ("mm/mempolicy: remove or narrow the lock on current")
Signed-off-by: Abel Wu <wuyun.abel@xxxxxxxxxxxxx>
Cc: Vlastimil Babka <vbabka@xxxxxxx>, Michal Hocko <mhocko@xxxxxxxxxx>
Cc: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
Cc: Muchun Song <songmuchun@xxxxxxxxxxxxx>
Cc: Wei Yang <richard.weiyang@xxxxxxxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
mm/mempolicy.c | 4 +++-
1 file changed, 3 insertions(+), 1 deletion(-)
--- a/mm/mempolicy.c~mm-mempolicy-fix-lock-contention-on-mems_allowed
+++ a/mm/mempolicy.c
@@ -853,12 +853,14 @@ static long do_set_mempolicy(unsigned sh
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)
_
Patches currently in -mm which might be from wuyun.abel@xxxxxxxxxxxxx are
mm-mempolicy-fix-lock-contention-on-mems_allowed.patch
