Re: [PATCH v4 1/3] mm: protect free_pgtables with mmap_lock write lock in exit_mmap

[Michal Hocko]

On Wed 08-12-21 13:22:09, Suren Baghdasaryan wrote:
> oom-reaper and process_mrelease system call should protect against
> races with exit_mmap which can destroy page tables while they
> walk the VMA tree. oom-reaper protects from that race by setting
> MMF_OOM_VICTIM and by relying on exit_mmap to set MMF_OOM_SKIP
> before taking and releasing mmap_write_lock. process_mrelease has
> to elevate mm->mm_users to prevent such race. Both oom-reaper and
> process_mrelease hold mmap_read_lock when walking the VMA tree.
> The locking rules and mechanisms could be simpler if exit_mmap takes
> mmap_write_lock while executing destructive operations such as
> free_pgtables.
> Change exit_mmap to hold the mmap_write_lock when calling
> free_pgtables and remove_vma. Operations like unmap_vmas and
> unlock_range are not destructive and could run under mmap_read_lock
> but for simplicity we take one mmap_write_lock during almost the entire
> operation.

unlock_range is not safe to be called under read lock. See 27ae357fa82b
("mm, oom: fix concurrent munlock and oom reaper unmap, v3").

> Note also that because oom-reaper checks VM_LOCKED flag,
> unlock_range() should not be allowed to race with it.
> Before this patch, remove_vma used to be called with no locks held,
> however with fput being executed asynchronously and vm_ops->close
> not being allowed to hold mmap_lock (it is called from __split_vma
> with mmap_sem held for write), changing that should be fine.
> In most cases this lock should be uncontended. Previously, Kirill
> reported ~4% regression caused by a similar change [1]. We reran the
> same test and although the individual results are quite noisy, the
> percentiles show lower regression with 1.6% being the worst case [2].
> The change allows oom-reaper and process_mrelease to execute safely
> under mmap_read_lock without worries that exit_mmap might destroy page
> tables from under them.
> 
> [1] https://lore.kernel.org/all/20170725141723.ivukwhddk2voyhuc@xxxxxxxxxxxxxxxxxx/
> [2] https://lore.kernel.org/all/CAJuCfpGC9-c9P40x7oy=jy5SphMcd0o0G_6U1-+JAziGKG6dGA@xxxxxxxxxxxxxx/
> 
> Signed-off-by: Suren Baghdasaryan <surenb@xxxxxxxxxx>

The patch looks good otherwise. Btw. when I was trying to do something
similar in the past Hugh has noted that we can get rid of the same 
lock&&unlock trick in ksm. Maybe you want to have a look at that as well
;)

Acked-by: Michal Hocko <mhocko@xxxxxxxx>

Thanks!

> ---
> changes in v4
> - Separated comments describing vm_operations_struct::close locking
> requirements into a separate patch, per Matthew Wilcox
> 
>  mm/mmap.c | 16 ++++++++--------
>  1 file changed, 8 insertions(+), 8 deletions(-)
> 
> diff --git a/mm/mmap.c b/mm/mmap.c
> index bfb0ea164a90..f4e09d390a07 100644
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -3142,25 +3142,27 @@ void exit_mmap(struct mm_struct *mm)
>  		 * to mmu_notifier_release(mm) ensures mmu notifier callbacks in
>  		 * __oom_reap_task_mm() will not block.
>  		 *
> -		 * This needs to be done before calling munlock_vma_pages_all(),
> +		 * This needs to be done before calling unlock_range(),
>  		 * which clears VM_LOCKED, otherwise the oom reaper cannot
>  		 * reliably test it.
>  		 */
>  		(void)__oom_reap_task_mm(mm);
>  
>  		set_bit(MMF_OOM_SKIP, &mm->flags);
> -		mmap_write_lock(mm);
> -		mmap_write_unlock(mm);
>  	}
>  
> +	mmap_write_lock(mm);
>  	if (mm->locked_vm)
>  		unlock_range(mm->mmap, ULONG_MAX);
>  
>  	arch_exit_mmap(mm);
>  
>  	vma = mm->mmap;
> -	if (!vma)	/* Can happen if dup_mmap() received an OOM */
> +	if (!vma) {
> +		/* Can happen if dup_mmap() received an OOM */
> +		mmap_write_unlock(mm);
>  		return;
> +	}
>  
>  	lru_add_drain();
>  	flush_cache_mm(mm);
> @@ -3171,16 +3173,14 @@ void exit_mmap(struct mm_struct *mm)
>  	free_pgtables(&tlb, vma, FIRST_USER_ADDRESS, USER_PGTABLES_CEILING);
>  	tlb_finish_mmu(&tlb);
>  
> -	/*
> -	 * Walk the list again, actually closing and freeing it,
> -	 * with preemption enabled, without holding any MM locks.
> -	 */
> +	/* Walk the list again, actually closing and freeing it. */
>  	while (vma) {
>  		if (vma->vm_flags & VM_ACCOUNT)
>  			nr_accounted += vma_pages(vma);
>  		vma = remove_vma(vma);
>  		cond_resched();
>  	}
> +	mmap_write_unlock(mm);
>  	vm_unacct_memory(nr_accounted);
>  }
>  
> -- 
> 2.34.1.400.ga245620fadb-goog

-- 
Michal Hocko
SUSE Labs