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SubjectRe: [PATCH mmotm] mm: vmscan: fix shrinker_rwsem in free_shrinker_info()
On Tue, Mar 30, 2021 at 4:44 PM Hugh Dickins <hughd@google.com> wrote:
>
> Lockdep warns mm/vmscan.c: suspicious rcu_dereference_protected() usage!
> when free_shrinker_info() is called from mem_cgroup_css_free(): there it
> is called with no locking, whereas alloc_shrinker_info() calls it with
> down_write of shrinker_rwsem - which seems appropriate. Rearrange that
> so free_shrinker_info() can manage the shrinker_rwsem for itself.
>
> Link: https://lkml.kernel.org/r/20210317140615.GB28839@xsang-OptiPlex-9020
> Reported-by: kernel test robot <oliver.sang@intel.com>
> Signed-off-by: Hugh Dickins <hughd@google.com>
> Cc: Yang Shi <shy828301@gmail.com>
> ---
> Sorry, I've made no attempt to work out precisely where in the series
> the locking went missing, nor tried to fit this in as a fix on top of
> mm-vmscan-add-shrinker_info_protected-helper.patch
> which Oliver reported (and which you notated in mmotm's "series" file).
> This patch just adds the fix to the end of the series, after
> mm-vmscan-shrink-deferred-objects-proportional-to-priority.patch

The patch "mm: vmscan: add shrinker_info_protected() helper" replaces
rcu_dereference_protected(shrinker_info, true) with
rcu_dereference_protected(shrinker_info,
lockdep_is_held(&shrinker_rwsem)).

I think we don't really need shrinker_rwsem in free_shrinker_info()
which is called from css_free(). The bits of the map have already been
'reparented' in css_offline. I think we can remove
lockdep_is_held(&shrinker_rwsem) for free_shrinker_info().

>
> mm/vmscan.c | 10 ++++++----
> 1 file changed, 6 insertions(+), 4 deletions(-)
>
> --- mmotm/mm/vmscan.c 2021-03-28 17:26:54.935553064 -0700
> +++ linux/mm/vmscan.c 2021-03-30 15:55:13.374459559 -0700
> @@ -249,18 +249,20 @@ void free_shrinker_info(struct mem_cgrou
> struct shrinker_info *info;
> int nid;
>
> + down_write(&shrinker_rwsem);
> for_each_node(nid) {
> pn = memcg->nodeinfo[nid];
> info = shrinker_info_protected(memcg, nid);
> kvfree(info);
> rcu_assign_pointer(pn->shrinker_info, NULL);
> }
> + up_write(&shrinker_rwsem);
> }
>
> int alloc_shrinker_info(struct mem_cgroup *memcg)
> {
> struct shrinker_info *info;
> - int nid, size, ret = 0;
> + int nid, size;
> int map_size, defer_size = 0;
>
> down_write(&shrinker_rwsem);
> @@ -270,9 +272,9 @@ int alloc_shrinker_info(struct mem_cgrou
> for_each_node(nid) {
> info = kvzalloc_node(sizeof(*info) + size, GFP_KERNEL, nid);
> if (!info) {
> + up_write(&shrinker_rwsem);
> free_shrinker_info(memcg);
> - ret = -ENOMEM;
> - break;
> + return -ENOMEM;
> }
> info->nr_deferred = (atomic_long_t *)(info + 1);
> info->map = (void *)info->nr_deferred + defer_size;
> @@ -280,7 +282,7 @@ int alloc_shrinker_info(struct mem_cgrou
> }
> up_write(&shrinker_rwsem);
>
> - return ret;
> + return 0;
> }
>
> static inline bool need_expand(int nr_max)

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