2021-03-30 23:48:31

by Hugh Dickins

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

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/[email protected]
Reported-by: kernel test robot <[email protected]>
Signed-off-by: Hugh Dickins <[email protected]>
Cc: Yang Shi <[email protected]>
---
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)


2021-03-31 13:55:48

by Shakeel Butt

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

On Tue, Mar 30, 2021 at 4:44 PM Hugh Dickins <[email protected]> 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/[email protected]
> Reported-by: kernel test robot <[email protected]>
> Signed-off-by: Hugh Dickins <[email protected]>
> Cc: Yang Shi <[email protected]>
> ---
> 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)

2021-03-31 18:11:02

by Yang Shi

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

On Wed, Mar 31, 2021 at 6:54 AM Shakeel Butt <[email protected]> wrote:
>
> On Tue, Mar 30, 2021 at 4:44 PM Hugh Dickins <[email protected]> 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/[email protected]
> > Reported-by: kernel test robot <[email protected]>
> > Signed-off-by: Hugh Dickins <[email protected]>
> > Cc: Yang Shi <[email protected]>
> > ---
> > 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)

2021-03-31 21:14:24

by Hugh Dickins

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

On Wed, 31 Mar 2021, Yang Shi wrote:
> On Wed, Mar 31, 2021 at 6:54 AM Shakeel Butt <[email protected]> wrote:
> > On Tue, Mar 30, 2021 at 4:44 PM Hugh Dickins <[email protected]> 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/[email protected]
> > > Reported-by: kernel test robot <[email protected]>
> > > Signed-off-by: Hugh Dickins <[email protected]>
> > > Cc: Yang Shi <[email protected]>
> > > ---
> > > 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

2021-03-31 23:21:27

by Yang Shi

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

On Wed, Mar 31, 2021 at 2:13 PM Hugh Dickins <[email protected]> wrote:
>
> On Wed, 31 Mar 2021, Yang Shi wrote:
> > On Wed, Mar 31, 2021 at 6:54 AM Shakeel Butt <[email protected]> wrote:
> > > On Tue, Mar 30, 2021 at 4:44 PM Hugh Dickins <[email protected]> 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/[email protected]
> > > > Reported-by: kernel test robot <[email protected]>
> > > > Signed-off-by: Hugh Dickins <[email protected]>
> > > > Cc: Yang Shi <[email protected]>
> > > > ---
> > > > 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