linux-kernel - Re: [PATCH 1/1] mm: introduce vm_flags_reset_once to replace WRITE_ONCE vm

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Date:   Wed, 1 Feb 2023 08:43:43 +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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        ldufour@...ux.ibm.com, paulmck@...nel.org, mingo@...hat.com,
        will@...nel.org, luto@...nel.org, songliubraving@...com,
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        hughd@...gle.com, bigeasy@...utronix.de, kent.overstreet@...ux.dev,
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        linux-mm@...ck.org, linux-arm-kernel@...ts.infradead.org,
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Subject: Re: [PATCH 1/1] mm: introduce vm_flags_reset_once to replace
 WRITE_ONCE vm_flags updates

On Tue 31-01-23 16:01:16, Suren Baghdasaryan wrote:
> Provide vm_flags_reset_once() and replace the vm_flags updates which used
> WRITE_ONCE() to prevent compiler optimizations.
> 
> Fixes: 0cce31a0aa0e ("mm: replace vma->vm_flags direct modifications with modifier calls")
> Reported-by: Hyeonggon Yoo <42.hyeyoo@...il.com>
> Signed-off-by: Suren Baghdasaryan <surenb@...gle.com>

This would have been better folded into the vm_flags modification patch
because it would be more obvious change. Hugh has provided a very nice
comment in mlock_vma_pages_range but the git blame would be more visible
when the conversion is from WRITE_ONCE.

One way or the other
Acked-by: Michal Hocko <mhocko@...e.com>

> ---
> Notes:
> - The patch applies cleanly over mm-unstable
> - The SHA in Fixes: line is from mm-unstable, so is... unstable
> 
>  include/linux/mm.h | 7 +++++++
>  mm/mlock.c         | 4 ++--
>  2 files changed, 9 insertions(+), 2 deletions(-)
> 
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 5bf0ad48faaa..23ce04f6e91e 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -648,6 +648,13 @@ static inline void vm_flags_reset(struct vm_area_struct *vma,
>  	vm_flags_init(vma, flags);
>  }
>  
> +static inline void vm_flags_reset_once(struct vm_area_struct *vma,
> +				       vm_flags_t flags)
> +{
> +	mmap_assert_write_locked(vma->vm_mm);
> +	WRITE_ONCE(ACCESS_PRIVATE(vma, __vm_flags), flags);
> +}
> +
>  static inline void vm_flags_set(struct vm_area_struct *vma,
>  				vm_flags_t flags)
>  {
> diff --git a/mm/mlock.c b/mm/mlock.c
> index ed49459e343e..617469fce96d 100644
> --- a/mm/mlock.c
> +++ b/mm/mlock.c
> @@ -380,7 +380,7 @@ static void mlock_vma_pages_range(struct vm_area_struct *vma,
>  	 */
>  	if (newflags & VM_LOCKED)
>  		newflags |= VM_IO;
> -	vm_flags_reset(vma, newflags);
> +	vm_flags_reset_once(vma, newflags);
>  
>  	lru_add_drain();
>  	walk_page_range(vma->vm_mm, start, end, &mlock_walk_ops, NULL);
> @@ -388,7 +388,7 @@ static void mlock_vma_pages_range(struct vm_area_struct *vma,
>  
>  	if (newflags & VM_IO) {
>  		newflags &= ~VM_IO;
> -		vm_flags_reset(vma, newflags);
> +		vm_flags_reset_once(vma, newflags);
>  	}
>  }
>  
> -- 
> 2.39.1.456.gfc5497dd1b-goog

-- 
Michal Hocko
SUSE Labs