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Date:   Tue, 17 Jan 2023 16:47:13 +0100
From:   Michal Hocko <mhocko@suse.com>
To:     Suren Baghdasaryan <surenb@google.com>
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Subject: Re: [PATCH 28/41] mm: introduce lock_vma_under_rcu to be used from
 arch-specific code
Message-ID: <Y8bDAVC/aiL9tCyz@dhcp22.suse.cz>
References: <20230109205336.3665937-1-surenb@google.com>
 <20230109205336.3665937-29-surenb@google.com>
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On Mon 09-01-23 12:53:23, Suren Baghdasaryan wrote:
> Introduce lock_vma_under_rcu function to lookup and lock a VMA during
> page fault handling. When VMA is not found, can't be locked or changes
> after being locked, the function returns NULL. The lookup is performed
> under RCU protection to prevent the found VMA from being destroyed before
> the VMA lock is acquired. VMA lock statistics are updated according to
> the results.
> For now only anonymous VMAs can be searched this way. In other cases the
> function returns NULL.

Could you describe why only anonymous vmas are handled at this stage and
what (roughly) has to be done to support other vmas? lock_vma_under_rcu
doesn't seem to have any anonymous vma specific requirements AFAICS.

Also isn't lock_vma_under_rcu effectively find_read_lock_vma? Not that
the naming is really the most important part but the rcu locking is
internal to the function so why should we spread this implementation
detail to the world...

> Signed-off-by: Suren Baghdasaryan <surenb@google.com>
> ---
>  include/linux/mm.h |  3 +++
>  mm/memory.c        | 51 ++++++++++++++++++++++++++++++++++++++++++++++
>  2 files changed, 54 insertions(+)
> 
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index c464fc8a514c..d0fddf6a1de9 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -687,6 +687,9 @@ static inline void vma_assert_no_reader(struct vm_area_struct *vma)
>  		      vma);
>  }
>  
> +struct vm_area_struct *lock_vma_under_rcu(struct mm_struct *mm,
> +					  unsigned long address);
> +
>  #else /* CONFIG_PER_VMA_LOCK */
>  
>  static inline void vma_init_lock(struct vm_area_struct *vma) {}
> diff --git a/mm/memory.c b/mm/memory.c
> index 9ece18548db1..a658e26d965d 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -5242,6 +5242,57 @@ vm_fault_t handle_mm_fault(struct vm_area_struct *vma, unsigned long address,
>  }
>  EXPORT_SYMBOL_GPL(handle_mm_fault);
>  
> +#ifdef CONFIG_PER_VMA_LOCK
> +/*
> + * Lookup and lock a VMA under RCU protection. Returned VMA is guaranteed to be
> + * stable and not isolated. If the VMA is not found or is being modified the
> + * function returns NULL.
> + */
> +struct vm_area_struct *lock_vma_under_rcu(struct mm_struct *mm,
> +					  unsigned long address)
> +{
> +	MA_STATE(mas, &mm->mm_mt, address, address);
> +	struct vm_area_struct *vma, *validate;
> +
> +	rcu_read_lock();
> +	vma = mas_walk(&mas);
> +retry:
> +	if (!vma)
> +		goto inval;
> +
> +	/* Only anonymous vmas are supported for now */
> +	if (!vma_is_anonymous(vma))
> +		goto inval;
> +
> +	if (!vma_read_trylock(vma))
> +		goto inval;
> +
> +	/* Check since vm_start/vm_end might change before we lock the VMA */
> +	if (unlikely(address < vma->vm_start || address >= vma->vm_end)) {
> +		vma_read_unlock(vma);
> +		goto inval;
> +	}
> +
> +	/* Check if the VMA got isolated after we found it */
> +	mas.index = address;
> +	validate = mas_walk(&mas);
> +	if (validate != vma) {
> +		vma_read_unlock(vma);
> +		count_vm_vma_lock_event(VMA_LOCK_MISS);
> +		/* The area was replaced with another one. */
> +		vma = validate;
> +		goto retry;
> +	}
> +
> +	rcu_read_unlock();
> +	return vma;
> +inval:
> +	rcu_read_unlock();
> +	count_vm_vma_lock_event(VMA_LOCK_ABORT);
> +	return NULL;
> +}
> +#endif /* CONFIG_PER_VMA_LOCK */
> +
>  #ifndef __PAGETABLE_P4D_FOLDED
>  /*
>   * Allocate p4d page table.
> -- 
> 2.39.0

-- 
Michal Hocko
SUSE Labs