linux-kernel - Re: [PATCH v4 08/10] mm/vma: improve and document __is_vma_write

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Message-ID: <47eafa10-6d13-4324-830a-2e7cf4e67f2b@suse.cz>
Date: Mon, 26 Jan 2026 12:30:04 +0100
From: Vlastimil Babka <vbabka@...e.cz>
To: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
 Andrew Morton <akpm@...ux-foundation.org>
Cc: David Hildenbrand <david@...nel.org>,
 "Liam R . Howlett" <Liam.Howlett@...cle.com>, Mike Rapoport
 <rppt@...nel.org>, Suren Baghdasaryan <surenb@...gle.com>,
 Michal Hocko <mhocko@...e.com>, Shakeel Butt <shakeel.butt@...ux.dev>,
 Jann Horn <jannh@...gle.com>, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org, linux-rt-devel@...ts.linux.dev,
 Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...hat.com>,
 Will Deacon <will@...nel.org>, Boqun Feng <boqun.feng@...il.com>,
 Waiman Long <longman@...hat.com>,
 Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
 Clark Williams <clrkwllms@...nel.org>, Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [PATCH v4 08/10] mm/vma: improve and document
 __is_vma_write_locked()

On 1/23/26 21:12, Lorenzo Stoakes wrote:
> We don't actually need to return an output parameter providing mm sequence
> number, rather we can separate that out into another function -
> __vma_raw_mm_seqnum() - and have any callers which need to obtain that
> invoke that instead.
> 
> The access to the raw sequence number requires that we hold the exclusive
> mmap lock such that we know we can't race vma_end_write_all(), so move the
> assert to __vma_raw_mm_seqnum() to make this requirement clear.
> 
> Also while we're here, convert all of the VM_BUG_ON_VMA()'s to
> VM_WARN_ON_ONCE_VMA()'s in line with the convention that we do not invoke
> oopses when we can avoid it.
> 
> Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>

Reviewed-by: Vlastimil Babka <vbabka@...e.cz>

Few nits:

> ---
>  include/linux/mmap_lock.h | 44 ++++++++++++++++++++++-----------------
>  1 file changed, 25 insertions(+), 19 deletions(-)
> 
> diff --git a/include/linux/mmap_lock.h b/include/linux/mmap_lock.h
> index 678f90080fa6..23bde4bd5a85 100644
> --- a/include/linux/mmap_lock.h
> +++ b/include/linux/mmap_lock.h
> @@ -258,17 +258,30 @@ static inline void vma_end_read(struct vm_area_struct *vma)
>  	vma_refcount_put(vma);
>  }
>  
> -/* WARNING! Can only be used if mmap_lock is expected to be write-locked */
> -static inline bool __is_vma_write_locked(struct vm_area_struct *vma, unsigned int *mm_lock_seq)
> +static inline unsigned int __vma_raw_mm_seqnum(struct vm_area_struct *vma)
>  {
> +	const struct mm_struct *mm = vma->vm_mm;
> +
> +	/* We must hold an exclusive write lock for this access to be valid. */
>  	mmap_assert_write_locked(vma->vm_mm);
> +	return mm->mm_lock_seq.sequence;
> +}
>  
> +/*
> + * Determine whether a VMA is write-locked. Must be invoked ONLY if the mmap
> + * write lock is held.
> + *
> + * Returns true if write-locked, otherwise false.
> + *
> + * Note that mm_lock_seq is updated only if the VMA is NOT write-locked.

This line is no longer applicable.

> + */
> +static inline bool __is_vma_write_locked(struct vm_area_struct *vma)
> +{
>  	/*
>  	 * current task is holding mmap_write_lock, both vma->vm_lock_seq and
>  	 * mm->mm_lock_seq can't be concurrently modified.
>  	 */
> -	*mm_lock_seq = vma->vm_mm->mm_lock_seq.sequence;
> -	return (vma->vm_lock_seq == *mm_lock_seq);
> +	return vma->vm_lock_seq == __vma_raw_mm_seqnum(vma);
>  }
>  
>  int __vma_start_write(struct vm_area_struct *vma, unsigned int mm_lock_seq,
> @@ -281,12 +294,10 @@ int __vma_start_write(struct vm_area_struct *vma, unsigned int mm_lock_seq,
>   */
>  static inline void vma_start_write(struct vm_area_struct *vma)
>  {
> -	unsigned int mm_lock_seq;
> -
> -	if (__is_vma_write_locked(vma, &mm_lock_seq))
> +	if (__is_vma_write_locked(vma))
>  		return;
>  
> -	__vma_start_write(vma, mm_lock_seq, TASK_UNINTERRUPTIBLE);
> +	__vma_start_write(vma, __vma_raw_mm_seqnum(vma), TASK_UNINTERRUPTIBLE);

At this point I think __vma_start_write() could just perform
__vma_raw_mm_seqnum() itself and we can remove the param.
It could possibly make the inline code smaller.