[PATCH] oom_reaper: don't call mmput_async() on uninitialized mm

[PATCH] oom_reaper: don't call mmput_async() on uninitialized mm

[Arnd Bergmann]

The change to the oom_reaper to hold a mutex inside __oom_reap_task()
accidentally started calling mmput_async() on the local
mm before that variable got initialized, as reported by gcc
in linux-next:

mm/oom_kill.c: In function '__oom_reap_task':
mm/oom_kill.c:537:2: error: 'mm' may be used uninitialized in this function [-Werror=maybe-uninitialized]

This rearranges the code slightly back to the state before patch
but leaves the lock in place. The error handling in the function
still looks a bit confusing and could probably be improved
but I could not come up with a solution that made me happy
for now.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Fixes: mmotm ("oom_reaper: close race with exiting task")
---
 mm/oom_kill.c | 12 ++++++++----
 1 file changed, 8 insertions(+), 4 deletions(-)

diff --git a/mm/oom_kill.c b/mm/oom_kill.c
index 1685890d424e..255cb5f48019 100644
--- a/mm/oom_kill.c
+++ b/mm/oom_kill.c
@@ -447,7 +447,7 @@ static bool __oom_reap_task(struct task_struct *tsk)
 	struct task_struct *p;
 	struct zap_details details = {.check_swap_entries = true,
 				      .ignore_dirty = true};
-	bool ret = true;
+	bool ret;
 
 	/*
 	 * We have to make sure to not race with the victim exit path
@@ -472,13 +472,16 @@ static bool __oom_reap_task(struct task_struct *tsk)
 	 * is no mm.
 	 */
 	p = find_lock_task_mm(tsk);
-	if (!p)
-		goto unlock_oom;
+	if (!p) {
+		mutex_unlock(&oom_lock);
+		return true;
+	}
 
 	mm = p->mm;
 	if (!atomic_inc_not_zero(&mm->mm_users)) {
 		task_unlock(p);
-		goto unlock_oom;
+		mutex_unlock(&oom_lock);
+		return true;
 	}
 
 	task_unlock(p);
@@ -527,6 +530,7 @@ static bool __oom_reap_task(struct task_struct *tsk)
 	 * to release its memory.
 	 */
 	set_bit(MMF_OOM_REAPED, &mm->flags);
+	ret = true;
 unlock_oom:
 	mutex_unlock(&oom_lock);
 	/*
-- 
2.7.0

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Re: [PATCH] oom_reaper: don't call mmput_async() on uninitialized mm

[Michal Hocko]

On Fri 27-05-16 10:00:48, Arnd Bergmann wrote:
> The change to the oom_reaper to hold a mutex inside __oom_reap_task()
> accidentally started calling mmput_async() on the local
> mm before that variable got initialized, as reported by gcc
> in linux-next:
> 
> mm/oom_kill.c: In function '__oom_reap_task':
> mm/oom_kill.c:537:2: error: 'mm' may be used uninitialized in this function [-Werror=maybe-uninitialized]
> 
> This rearranges the code slightly back to the state before patch
> but leaves the lock in place. The error handling in the function
> still looks a bit confusing and could probably be improved
> but I could not come up with a solution that made me happy
> for now.
> 
> Signed-off-by: Arnd Bergmann <arnd@arndb.de>
> Fixes: mmotm ("oom_reaper: close race with exiting task")

Thanks for catching that Arnd?

Acked-by: Michal Hocko <mhocko@suse.com>

> ---
>  mm/oom_kill.c | 12 ++++++++----
>  1 file changed, 8 insertions(+), 4 deletions(-)
> 
> diff --git a/mm/oom_kill.c b/mm/oom_kill.c
> index 1685890d424e..255cb5f48019 100644
> --- a/mm/oom_kill.c
> +++ b/mm/oom_kill.c
> @@ -447,7 +447,7 @@ static bool __oom_reap_task(struct task_struct *tsk)
>  	struct task_struct *p;
>  	struct zap_details details = {.check_swap_entries = true,
>  				      .ignore_dirty = true};
> -	bool ret = true;
> +	bool ret;
>  
>  	/*
>  	 * We have to make sure to not race with the victim exit path
> @@ -472,13 +472,16 @@ static bool __oom_reap_task(struct task_struct *tsk)
>  	 * is no mm.
>  	 */
>  	p = find_lock_task_mm(tsk);
> -	if (!p)
> -		goto unlock_oom;
> +	if (!p) {
> +		mutex_unlock(&oom_lock);
> +		return true;
> +	}
>  
>  	mm = p->mm;
>  	if (!atomic_inc_not_zero(&mm->mm_users)) {
>  		task_unlock(p);
> -		goto unlock_oom;
> +		mutex_unlock(&oom_lock);
> +		return true;
>  	}
>  
>  	task_unlock(p);
> @@ -527,6 +530,7 @@ static bool __oom_reap_task(struct task_struct *tsk)
>  	 * to release its memory.
>  	 */
>  	set_bit(MMF_OOM_REAPED, &mm->flags);
> +	ret = true;
>  unlock_oom:
>  	mutex_unlock(&oom_lock);
>  	/*
> -- 
> 2.7.0
> 

-- 
Michal Hocko
SUSE Labs

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Re: [PATCH] oom_reaper: don't call mmput_async() on uninitialized mm

[Andrew Morton]

On Fri, 27 May 2016 10:10:59 +0200 Michal Hocko <mhocko@kernel.org> wrote:

> On Fri 27-05-16 10:00:48, Arnd Bergmann wrote:
> > The change to the oom_reaper to hold a mutex inside __oom_reap_task()
> > accidentally started calling mmput_async() on the local
> > mm before that variable got initialized, as reported by gcc
> > in linux-next:
> > 
> > mm/oom_kill.c: In function '__oom_reap_task':
> > mm/oom_kill.c:537:2: error: 'mm' may be used uninitialized in this function [-Werror=maybe-uninitialized]
> > 
> > This rearranges the code slightly back to the state before patch
> > but leaves the lock in place. The error handling in the function
> > still looks a bit confusing and could probably be improved
> > but I could not come up with a solution that made me happy
> > for now.
> > 
> > Signed-off-by: Arnd Bergmann <arnd@arndb.de>
> > Fixes: mmotm ("oom_reaper: close race with exiting task")
> 
> Thanks for catching that Arnd?
> 
> Acked-by: Michal Hocko <mhocko@suse.com>

I think I preferred my version - all those unwinding return statements
can cause problems..

--- a/mm/oom_kill.c~oom_reaper-close-race-with-exiting-task-fix
+++ a/mm/oom_kill.c
@@ -443,7 +443,7 @@ static bool __oom_reap_task(struct task_
 {
 	struct mmu_gather tlb;
 	struct vm_area_struct *vma;
-	struct mm_struct *mm;
+	struct mm_struct *mm = NULL;
 	struct task_struct *p;
 	struct zap_details details = {.check_swap_entries = true,
 				      .ignore_dirty = true};
@@ -534,7 +534,8 @@ unlock_oom:
 	 * different context because we shouldn't risk we get stuck there and
 	 * put the oom_reaper out of the way.
 	 */
-	mmput_async(mm);
+	if (mm)
+		mmput_async(mm);
 	return ret;
 }
 
_

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Re: [PATCH] oom_reaper: don't call mmput_async() on uninitialized mm

[Michal Hocko]

On Fri 27-05-16 12:55:34, Andrew Morton wrote:
> On Fri, 27 May 2016 10:10:59 +0200 Michal Hocko <mhocko@kernel.org> wrote:
> 
> > On Fri 27-05-16 10:00:48, Arnd Bergmann wrote:
> > > The change to the oom_reaper to hold a mutex inside __oom_reap_task()
> > > accidentally started calling mmput_async() on the local
> > > mm before that variable got initialized, as reported by gcc
> > > in linux-next:
> > > 
> > > mm/oom_kill.c: In function '__oom_reap_task':
> > > mm/oom_kill.c:537:2: error: 'mm' may be used uninitialized in this function [-Werror=maybe-uninitialized]
> > > 
> > > This rearranges the code slightly back to the state before patch
> > > but leaves the lock in place. The error handling in the function
> > > still looks a bit confusing and could probably be improved
> > > but I could not come up with a solution that made me happy
> > > for now.
> > > 
> > > Signed-off-by: Arnd Bergmann <arnd@arndb.de>
> > > Fixes: mmotm ("oom_reaper: close race with exiting task")
> > 
> > Thanks for catching that Arnd?
> > 
> > Acked-by: Michal Hocko <mhocko@suse.com>
> 
> I think I preferred my version - all those unwinding return statements
> can cause problems..

looks good to me as well.

Thanks!

> --- a/mm/oom_kill.c~oom_reaper-close-race-with-exiting-task-fix
> +++ a/mm/oom_kill.c
> @@ -443,7 +443,7 @@ static bool __oom_reap_task(struct task_
>  {
>  	struct mmu_gather tlb;
>  	struct vm_area_struct *vma;
> -	struct mm_struct *mm;
> +	struct mm_struct *mm = NULL;
>  	struct task_struct *p;
>  	struct zap_details details = {.check_swap_entries = true,
>  				      .ignore_dirty = true};
> @@ -534,7 +534,8 @@ unlock_oom:
>  	 * different context because we shouldn't risk we get stuck there and
>  	 * put the oom_reaper out of the way.
>  	 */
> -	mmput_async(mm);
> +	if (mm)
> +		mmput_async(mm);
>  	return ret;
>  }
>  
> _

-- 
Michal Hocko
SUSE Labs

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