[PATCH mmotm] mm: vmscan: fix shrinker_rwsem in free_shrinker_info()

[PATCH mmotm] mm: vmscan: fix shrinker_rwsem in free_shrinker_info()

[Hugh Dickins]

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

 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)

Re: [PATCH mmotm] mm: vmscan: fix shrinker_rwsem in free_shrinker_info()

[Shakeel Butt]

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)

Re: [PATCH mmotm] mm: vmscan: fix shrinker_rwsem in free_shrinker_info()

[Yang Shi]

On Wed, Mar 31, 2021 at 6:54 AM Shakeel Butt <shakeelb@google.com> wrote:
>
> 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().

Thanks, Hugh and Shakeel. I missed the report.

I think Shakeel is correct, shrinker_rwsem is not required in css_free
path so Shakeel's proposal should be able to fix it. I prepared a
patch:

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 64bf07cc20f2..7348c26d4cac 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -251,7 +251,12 @@ void free_shrinker_info(struct mem_cgroup *memcg)
        for_each_node(nid) {
                pn = memcg->nodeinfo[nid];
-               info = shrinker_info_protected(memcg, nid);
+               /*
+                * Don't use shrinker_info_protected() helper since
+                * free_shrinker_info() could be called by css_free()
+                * without holding shrinker_rwsem.
+                */
+               info = rcu_dereference_protected(pn->shrinker_info, true);
                kvfree(info);
                rcu_assign_pointer(pn->shrinker_info, NULL);
        }
>
> >
> >  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)

Re: [PATCH mmotm] mm: vmscan: fix shrinker_rwsem in free_shrinker_info()

[Hugh Dickins]

On Wed, 31 Mar 2021, Yang Shi wrote:
> On Wed, Mar 31, 2021 at 6:54 AM Shakeel Butt <shakeelb@google.com> wrote:
> > 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().
> 
> Thanks, Hugh and Shakeel. I missed the report.
> 
> I think Shakeel is correct, shrinker_rwsem is not required in css_free
> path so Shakeel's proposal should be able to fix it.

Yes, looking at it again, I am sure that Shakeel is right, and
that my patch was overkill - no need for shrinker_rwsem there.

Whether it's RCU-safe to free the info there, I have not reviewed at
all: but shrinker_rwsem would not help even if there were an issue.

> I prepared a patch:

Unsigned, white-space damaged, so does not apply.

> 
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 64bf07cc20f2..7348c26d4cac 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -251,7 +251,12 @@ void free_shrinker_info(struct mem_cgroup *memcg)
>         for_each_node(nid) {
>                 pn = memcg->nodeinfo[nid];
> -               info = shrinker_info_protected(memcg, nid);
> +               /*
> +                * Don't use shrinker_info_protected() helper since
> +                * free_shrinker_info() could be called by css_free()
> +                * without holding shrinker_rwsem.
> +                */

Just because I mis-inferred from the use of shrinker_info_protected()
that shrinker_rwsem was needed here, is no reason to add that comment:
imagine how unhelpfully bigger the kernel source would be if we added
a comment everywhere I had misunderstood something!

> +               info = rcu_dereference_protected(pn->shrinker_info, true);
>                 kvfree(info);
>                 rcu_assign_pointer(pn->shrinker_info, NULL);
>         }

That does it, but I bikeshedded with myself in the encyclopaedic
rcupdate.h, and decided rcu_replace_pointer(pn->shrinker_info, NULL, true)
would be best.  But now see that patch won't fit so well into your series,
and I can't spend more time writing up a justification for it.

I think Andrew should simply delete my fix patch from his queue,
and edit out the
@@ -232,7 +239,7 @@ void free_shrinker_info(struct mem_cgrou
 
 	for_each_node(nid) {
 		pn = memcg->nodeinfo[nid];
-		info = rcu_dereference_protected(pn->shrinker_info, true);
+		info = shrinker_info_protected(memcg, nid);
 		kvfree(info);
 		rcu_assign_pointer(pn->shrinker_info, NULL);
 	}
hunk from your mm-vmscan-add-shrinker_info_protected-helper.patch
which will then restore free_shrinker_info() to what you propose above.

Thanks,
Hugh

Re: [PATCH mmotm] mm: vmscan: fix shrinker_rwsem in free_shrinker_info()

[Yang Shi]

On Wed, Mar 31, 2021 at 2:13 PM Hugh Dickins <hughd@google.com> wrote:
>
> On Wed, 31 Mar 2021, Yang Shi wrote:
> > On Wed, Mar 31, 2021 at 6:54 AM Shakeel Butt <shakeelb@google.com> wrote:
> > > 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().
> >
> > Thanks, Hugh and Shakeel. I missed the report.
> >
> > I think Shakeel is correct, shrinker_rwsem is not required in css_free
> > path so Shakeel's proposal should be able to fix it.
>
> Yes, looking at it again, I am sure that Shakeel is right, and
> that my patch was overkill - no need for shrinker_rwsem there.
>
> Whether it's RCU-safe to free the info there, I have not reviewed at
> all: but shrinker_rwsem would not help even if there were an issue.
>
> > I prepared a patch:
>
> Unsigned, white-space damaged, so does not apply.
>
> >
> > diff --git a/mm/vmscan.c b/mm/vmscan.c
> > index 64bf07cc20f2..7348c26d4cac 100644
> > --- a/mm/vmscan.c
> > +++ b/mm/vmscan.c
> > @@ -251,7 +251,12 @@ void free_shrinker_info(struct mem_cgroup *memcg)
> >         for_each_node(nid) {
> >                 pn = memcg->nodeinfo[nid];
> > -               info = shrinker_info_protected(memcg, nid);
> > +               /*
> > +                * Don't use shrinker_info_protected() helper since
> > +                * free_shrinker_info() could be called by css_free()
> > +                * without holding shrinker_rwsem.
> > +                */
>
> Just because I mis-inferred from the use of shrinker_info_protected()
> that shrinker_rwsem was needed here, is no reason to add that comment:
> imagine how unhelpfully bigger the kernel source would be if we added
> a comment everywhere I had misunderstood something!

Yes, I agree the comment may incur more confusion. Better remove it.

>
> > +               info = rcu_dereference_protected(pn->shrinker_info, true);
> >                 kvfree(info);
> >                 rcu_assign_pointer(pn->shrinker_info, NULL);
> >         }
>
> That does it, but I bikeshedded with myself in the encyclopaedic
> rcupdate.h, and decided rcu_replace_pointer(pn->shrinker_info, NULL, true)
> would be best.  But now see that patch won't fit so well into your series,
> and I can't spend more time writing up a justification for it.
>
> I think Andrew should simply delete my fix patch from his queue,
> and edit out the
> @@ -232,7 +239,7 @@ void free_shrinker_info(struct mem_cgrou
>
>         for_each_node(nid) {
>                 pn = memcg->nodeinfo[nid];
> -               info = rcu_dereference_protected(pn->shrinker_info, true);
> +               info = shrinker_info_protected(memcg, nid);
>                 kvfree(info);
>                 rcu_assign_pointer(pn->shrinker_info, NULL);
>         }
> hunk from your mm-vmscan-add-shrinker_info_protected-helper.patch
> which will then restore free_shrinker_info() to what you propose above.

Yes. I saw Andrew already had this fix in -mm tree.

>
> Thanks,
> Hugh