linux-kernel - Re: [RFC PATCH 2/6] mm/mmap: Abstract vma clean up from exit

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Message-ID: <bdb27f25-d979-4dce-889e-a2fd17fc06a5@lucifer.local>
Date: Tue, 19 Aug 2025 19:38:43 +0100
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: "Liam R. Howlett" <Liam.Howlett@...cle.com>
Cc: David Hildenbrand <david@...hat.com>, maple-tree@...ts.infradead.org,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Vlastimil Babka <vbabka@...e.cz>, Mike Rapoport <rppt@...nel.org>,
        Suren Baghdasaryan <surenb@...gle.com>, Michal Hocko <mhocko@...e.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Jann Horn <jannh@...gle.com>, Pedro Falcato <pfalcato@...e.de>,
        Charan Teja Kalla <quic_charante@...cinc.com>,
        shikemeng@...weicloud.com, kasong@...cent.com, nphamcs@...il.com,
        bhe@...hat.com, baohua@...nel.org, chrisl@...nel.org,
        Matthew Wilcox <willy@...radead.org>
Subject: Re: [RFC PATCH 2/6] mm/mmap: Abstract vma clean up from exit_mmap()

On Fri, Aug 15, 2025 at 03:10:27PM -0400, Liam R. Howlett wrote:
> Create the new function tear_down_vmas() to remove a range of vmas.
> exit_mmap() will be removing all the vmas.
>
> This is necessary for future patches.
>
> No functional changes intended.
>
> Signed-off-by: Liam R. Howlett <Liam.Howlett@...cle.com>

This function is pure and complete insanity, but this change looks
good. Couple nits below.

Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>

> ---
>  mm/mmap.c | 37 ++++++++++++++++++++++++-------------
>  1 file changed, 24 insertions(+), 13 deletions(-)
>
> diff --git a/mm/mmap.c b/mm/mmap.c
> index c4c315b480af7..0995a48b46d59 100644
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -1250,6 +1250,29 @@ int vm_brk_flags(unsigned long addr, unsigned long request, vm_flags_t vm_flags)
>  }
>  EXPORT_SYMBOL(vm_brk_flags);
>
> +static inline
> +unsigned long tear_down_vmas(struct mm_struct *mm, struct vma_iterator *vmi,
> +		struct vm_area_struct *vma, unsigned long max)
> +{
> +	unsigned long nr_accounted = 0;
> +	int count = 0;
> +
> +	mmap_assert_write_locked(mm);
> +	vma_iter_set(vmi, vma->vm_end);
> +	do {
> +		if (vma->vm_flags & VM_ACCOUNT)
> +			nr_accounted += vma_pages(vma);
> +		vma_mark_detached(vma);
> +		remove_vma(vma);
> +		count++;
> +		cond_resched();
> +		vma = vma_next(vmi);
> +	} while (vma && vma->vm_end <= max);
> +
> +	BUG_ON(count != mm->map_count);

Can we make this a WARN_ON() or WARN_ON_ONCE() while we're at it?

> +	return nr_accounted;
> +}
> +
>  /* Release all mmaps. */
>  void exit_mmap(struct mm_struct *mm)
>  {
> @@ -1257,7 +1280,6 @@ void exit_mmap(struct mm_struct *mm)
>  	struct vm_area_struct *vma;
>  	unsigned long nr_accounted = 0;

No need to initialise this to 0 any more.

>  	VMA_ITERATOR(vmi, mm, 0);
> -	int count = 0;
>
>  	/* mm's last user has gone, and its about to be pulled down */
>  	mmu_notifier_release(mm);
> @@ -1297,18 +1319,7 @@ void exit_mmap(struct mm_struct *mm)
>  	 * enabled, without holding any MM locks besides the unreachable
>  	 * mmap_write_lock.
>  	 */
> -	vma_iter_set(&vmi, vma->vm_end);
> -	do {
> -		if (vma->vm_flags & VM_ACCOUNT)
> -			nr_accounted += vma_pages(vma);
> -		vma_mark_detached(vma);
> -		remove_vma(vma);
> -		count++;
> -		cond_resched();
> -		vma = vma_next(&vmi);
> -	} while (vma && likely(!xa_is_zero(vma)));
> -
> -	BUG_ON(count != mm->map_count);
> +	nr_accounted = tear_down_vmas(mm, &vmi, vma, ULONG_MAX);
>
>  destroy:
>  	__mt_destroy(&mm->mm_mt);
> --
> 2.47.2
>