linux-kernel - Re: [PATCH 13/41] mm: introduce vma->vm

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Date:   Tue, 17 Jan 2023 16:09:02 +0100
From:   Michal Hocko <mhocko@...e.com>
To:     Suren Baghdasaryan <surenb@...gle.com>
Cc:     akpm@...ux-foundation.org, michel@...pinasse.org,
        jglisse@...gle.com, vbabka@...e.cz, hannes@...xchg.org,
        mgorman@...hsingularity.net, dave@...olabs.net,
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Subject: Re: [PATCH 13/41] mm: introduce vma->vm_flags modifier functions

On Mon 09-01-23 12:53:08, Suren Baghdasaryan wrote:
> To keep vma locking correctness when vm_flags are modified, add modifier
> functions to be used whenever flags are updated.
> 
> Signed-off-by: Suren Baghdasaryan <surenb@...gle.com>
> ---
>  include/linux/mm.h       | 38 ++++++++++++++++++++++++++++++++++++++
>  include/linux/mm_types.h |  8 +++++++-
>  2 files changed, 45 insertions(+), 1 deletion(-)
> 
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index ec2c4c227d51..35cf0a6cbcc2 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -702,6 +702,44 @@ static inline void vma_init(struct vm_area_struct *vma, struct mm_struct *mm)
>  	vma_init_lock(vma);
>  }
>  
> +/* Use when VMA is not part of the VMA tree and needs no locking */
> +static inline
> +void init_vm_flags(struct vm_area_struct *vma, unsigned long flags)
> +{
> +	WRITE_ONCE(vma->vm_flags, flags);
> +}

Why do we need WRITE_ONCE here? Isn't vma invisible during its
initialization?

> +
> +/* Use when VMA is part of the VMA tree and needs appropriate locking */
> +static inline
> +void reset_vm_flags(struct vm_area_struct *vma, unsigned long flags)
> +{
> +	vma_write_lock(vma);
> +	init_vm_flags(vma, flags);
> +}
> +
> +static inline
> +void set_vm_flags(struct vm_area_struct *vma, unsigned long flags)
> +{
> +	vma_write_lock(vma);
> +	vma->vm_flags |= flags;
> +}
> +
> +static inline
> +void clear_vm_flags(struct vm_area_struct *vma, unsigned long flags)
> +{
> +	vma_write_lock(vma);
> +	vma->vm_flags &= ~flags;
> +}
> +
> +static inline
> +void mod_vm_flags(struct vm_area_struct *vma,
> +		  unsigned long set, unsigned long clear)
> +{
> +	vma_write_lock(vma);
> +	vma->vm_flags |= set;
> +	vma->vm_flags &= ~clear;
> +}
> +

This is rather unusual pattern. There is no note about locking involved
in the naming and also why is the locking part of this interface in the
first place? I can see reason for access functions to actually check for
lock asserts.
-- 
Michal Hocko
SUSE Labs