Date:   Wed, 18 Apr 2018 09:50:51 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     David Rientjes <rientjes@...gle.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Roman Gushchin <guro@...com>, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org
Subject: Re: [patch v2] mm, oom: fix concurrent munlock and oom reaper unmap

On Tue 17-04-18 19:52:41, David Rientjes wrote:
> Since exit_mmap() is done without the protection of mm->mmap_sem, it is
> possible for the oom reaper to concurrently operate on an mm until
> MMF_OOM_SKIP is set.
> 
> This allows munlock_vma_pages_all() to concurrently run while the oom
> reaper is operating on a vma.  Since munlock_vma_pages_range() depends on
> clearing VM_LOCKED from vm_flags before actually doing the munlock to
> determine if any other vmas are locking the same memory, the check for
> VM_LOCKED in the oom reaper is racy.
> 
> This is especially noticeable on architectures such as powerpc where
> clearing a huge pmd requires serialize_against_pte_lookup().  If the pmd
> is zapped by the oom reaper during follow_page_mask() after the check for
> pmd_none() is bypassed, this ends up deferencing a NULL ptl.
> 
> Fix this by reusing MMF_UNSTABLE to specify that an mm should not be
> reaped.  This prevents the concurrent munlock_vma_pages_range() and
> unmap_page_range().  The oom reaper will simply not operate on an mm that
> has the bit set and leave the unmapping to exit_mmap().

This will further complicate the protocol and actually theoretically
restores the oom lockup issues because the oom reaper doesn't set
MMF_OOM_SKIP when racing with exit_mmap so we fully rely that nothing
blocks there... So the resulting code is more fragile and tricky.

Can we try a simpler way and get back to what I was suggesting before
[1] and simply not play tricks with
		down_write(&mm->mmap_sem);
		up_write(&mm->mmap_sem);

and use the write lock in exit_mmap for oom_victims?

Andrea wanted to make this more clever but this is the second fallout
which could have been prevented. The patch would be smaller and the
locking protocol easier

[1] http://lkml.kernel.org/r/20170727065023.GB20970@dhcp22.suse.cz

> Fixes: 212925802454 ("mm: oom: let oom_reap_task and exit_mmap run concurrently")
> Cc: stable@...r.kernel.org [4.14+]
> Signed-off-by: David Rientjes <rientjes@...gle.com>
> ---
>  v2:
>   - oom reaper only sets MMF_OOM_SKIP if MMF_UNSTABLE was never set (either
>     by itself or by exit_mmap(), per Tetsuo
>   - s/kick_all_cpus_sync/serialize_against_pte_lookup/ in changelog as more
>     isolated way of forcing cpus as non-idle on power
> 
>  mm/mmap.c     | 38 ++++++++++++++++++++------------------
>  mm/oom_kill.c | 28 +++++++++++++---------------
>  2 files changed, 33 insertions(+), 33 deletions(-)
> 
> diff --git a/mm/mmap.c b/mm/mmap.c
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -3015,6 +3015,25 @@ void exit_mmap(struct mm_struct *mm)
>  	/* mm's last user has gone, and its about to be pulled down */
>  	mmu_notifier_release(mm);
>  
> +	if (unlikely(mm_is_oom_victim(mm))) {
> +		/*
> +		 * Wait for oom_reap_task() to stop working on this mm.  Because
> +		 * MMF_UNSTABLE is already set before calling down_read(),
> +		 * oom_reap_task() will not run on this mm after up_write().
> +		 * oom_reap_task() also depends on a stable VM_LOCKED flag to
> +		 * indicate it should not unmap during munlock_vma_pages_all().
> +		 *
> +		 * mm_is_oom_victim() cannot be set from under us because
> +		 * victim->mm is already set to NULL under task_lock before
> +		 * calling mmput() and victim->signal->oom_mm is set by the oom
> +		 * killer only if victim->mm is non-NULL while holding
> +		 * task_lock().
> +		 */
> +		set_bit(MMF_UNSTABLE, &mm->flags);
> +		down_write(&mm->mmap_sem);
> +		up_write(&mm->mmap_sem);
> +	}
> +
>  	if (mm->locked_vm) {
>  		vma = mm->mmap;
>  		while (vma) {
> @@ -3036,26 +3055,9 @@ void exit_mmap(struct mm_struct *mm)
>  	/* update_hiwater_rss(mm) here? but nobody should be looking */
>  	/* Use -1 here to ensure all VMAs in the mm are unmapped */
>  	unmap_vmas(&tlb, vma, 0, -1);
> -
> -	if (unlikely(mm_is_oom_victim(mm))) {
> -		/*
> -		 * Wait for oom_reap_task() to stop working on this
> -		 * mm. Because MMF_OOM_SKIP is already set before
> -		 * calling down_read(), oom_reap_task() will not run
> -		 * on this "mm" post up_write().
> -		 *
> -		 * mm_is_oom_victim() cannot be set from under us
> -		 * either because victim->mm is already set to NULL
> -		 * under task_lock before calling mmput and oom_mm is
> -		 * set not NULL by the OOM killer only if victim->mm
> -		 * is found not NULL while holding the task_lock.
> -		 */
> -		set_bit(MMF_OOM_SKIP, &mm->flags);
> -		down_write(&mm->mmap_sem);
> -		up_write(&mm->mmap_sem);
> -	}
>  	free_pgtables(&tlb, vma, FIRST_USER_ADDRESS, USER_PGTABLES_CEILING);
>  	tlb_finish_mmu(&tlb, 0, -1);
> +	set_bit(MMF_OOM_SKIP, &mm->flags);
>  
>  	/*
>  	 * Walk the list again, actually closing and freeing it,
> diff --git a/mm/oom_kill.c b/mm/oom_kill.c
> --- a/mm/oom_kill.c
> +++ b/mm/oom_kill.c
> @@ -521,12 +521,17 @@ static bool __oom_reap_task_mm(struct task_struct *tsk, struct mm_struct *mm)
>  	}
>  
>  	/*
> -	 * MMF_OOM_SKIP is set by exit_mmap when the OOM reaper can't
> -	 * work on the mm anymore. The check for MMF_OOM_SKIP must run
> +	 * Tell all users of get_user/copy_from_user etc... that the content
> +	 * is no longer stable. No barriers really needed because unmapping
> +	 * should imply barriers already and the reader would hit a page fault
> +	 * if it stumbled over reaped memory.
> +	 *
> +	 * MMF_UNSTABLE is also set by exit_mmap when the OOM reaper shouldn't
> +	 * work on the mm anymore. The check for MMF_OOM_UNSTABLE must run
>  	 * under mmap_sem for reading because it serializes against the
>  	 * down_write();up_write() cycle in exit_mmap().
>  	 */
> -	if (test_bit(MMF_OOM_SKIP, &mm->flags)) {
> +	if (test_and_set_bit(MMF_UNSTABLE, &mm->flags)) {
>  		up_read(&mm->mmap_sem);
>  		trace_skip_task_reaping(tsk->pid);
>  		goto unlock_oom;
> @@ -534,14 +539,6 @@ static bool __oom_reap_task_mm(struct task_struct *tsk, struct mm_struct *mm)
>  
>  	trace_start_task_reaping(tsk->pid);
>  
> -	/*
> -	 * Tell all users of get_user/copy_from_user etc... that the content
> -	 * is no longer stable. No barriers really needed because unmapping
> -	 * should imply barriers already and the reader would hit a page fault
> -	 * if it stumbled over a reaped memory.
> -	 */
> -	set_bit(MMF_UNSTABLE, &mm->flags);
> -
>  	for (vma = mm->mmap ; vma; vma = vma->vm_next) {
>  		if (!can_madv_dontneed_vma(vma))
>  			continue;
> @@ -567,6 +564,7 @@ static bool __oom_reap_task_mm(struct task_struct *tsk, struct mm_struct *mm)
>  			tlb_finish_mmu(&tlb, start, end);
>  		}
>  	}
> +	set_bit(MMF_OOM_SKIP, &mm->flags);
>  	pr_info("oom_reaper: reaped process %d (%s), now anon-rss:%lukB, file-rss:%lukB, shmem-rss:%lukB\n",
>  			task_pid_nr(tsk), tsk->comm,
>  			K(get_mm_counter(mm, MM_ANONPAGES)),
> @@ -594,7 +592,6 @@ static void oom_reap_task(struct task_struct *tsk)
>  	    test_bit(MMF_OOM_SKIP, &mm->flags))
>  		goto done;
>  
> -
>  	pr_info("oom_reaper: unable to reap pid:%d (%s)\n",
>  		task_pid_nr(tsk), tsk->comm);
>  	debug_show_all_locks();
> @@ -603,10 +600,11 @@ static void oom_reap_task(struct task_struct *tsk)
>  	tsk->oom_reaper_list = NULL;
>  
>  	/*
> -	 * Hide this mm from OOM killer because it has been either reaped or
> -	 * somebody can't call up_write(mmap_sem).
> +	 * If the oom reaper could not get started on this mm and it has not yet
> +	 * reached exit_mmap(), set MMF_OOM_SKIP to disregard.
>  	 */
> -	set_bit(MMF_OOM_SKIP, &mm->flags);
> +	if (!test_bit(MMF_UNSTABLE, &mm->flags))
> +		set_bit(MMF_OOM_SKIP, &mm->flags);
>  
>  	/* Drop a reference taken by wake_oom_reaper */
>  	put_task_struct(tsk);

-- 
Michal Hocko
SUSE Labs

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