linux-kernel - Re: [PATCH v4 10/66] mm/mmap: Use the maple tree for find_vma

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Message-ID: <92ef4cae-ccfb-d952-6b36-71036329e8da@suse.cz>
Date:   Wed, 15 Dec 2021 17:40:05 +0100
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Liam Howlett <liam.howlett@...cle.com>,
        "maple-tree@...ts.infradead.org" <maple-tree@...ts.infradead.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     Song Liu <songliubraving@...com>,
        Davidlohr Bueso <dave@...olabs.net>,
        "Paul E . McKenney" <paulmck@...nel.org>,
        Matthew Wilcox <willy@...radead.org>,
        Laurent Dufour <ldufour@...ux.ibm.com>,
        David Rientjes <rientjes@...gle.com>,
        Axel Rasmussen <axelrasmussen@...gle.com>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Rik van Riel <riel@...riel.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Michel Lespinasse <walken.cr@...il.com>,
        Jerome Glisse <jglisse@...hat.com>,
        Minchan Kim <minchan@...gle.com>,
        Joel Fernandes <joelaf@...gle.com>,
        Rom Lemarchand <romlem@...gle.com>
Subject: Re: [PATCH v4 10/66] mm/mmap: Use the maple tree for find_vma_prev()
 instead of the rbtree

On 12/1/21 15:29, Liam Howlett wrote:
> From: "Liam R. Howlett" <Liam.Howlett@...cle.com>
> 
> Use the maple tree's advanced API and a maple state to walk the tree
> for the entry at the address of the next vma, then use the maple state
> to walk back one entry to find the previous entry.
> 
> Add kernel documentation comments for this API.

Additional note: the previous patch for find_vma() mentioned "Using the
maple tree interface mt_find() will handle the RCU locking" while
find_vma_prev() uses the advanced maple tree API, thus IIUC without RCU
locking, and doesn't add its own. This can easily result in a bug,
especially if the documentation comments don't mention it at all?

> Signed-off-by: Liam R. Howlett <Liam.Howlett@...cle.com>
> Signed-off-by: Matthew Wilcox (Oracle) <willy@...radead.org>
> ---
>  mm/mmap.c | 24 ++++++++++++++----------
>  1 file changed, 14 insertions(+), 10 deletions(-)
> 
> diff --git a/mm/mmap.c b/mm/mmap.c
> index 6a7502f74190..8425ab573770 100644
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -2454,23 +2454,27 @@ struct vm_area_struct *find_vma(struct mm_struct *mm, unsigned long addr)
>  }
>  EXPORT_SYMBOL(find_vma);
>  
> -/*
> - * Same as find_vma, but also return a pointer to the previous VMA in *pprev.
> +/**
> + * find_vma_prev() - Find the VMA for a given address, or the next vma and
> + * set %pprev to the previous VMA, if any.
> + * @mm: The mm_struct to check
> + * @addr: The address
> + * @pprev: The pointer to set to the previous VMA
> + *
> + * Returns: The VMA associated with @addr, or the next vma.
> + * May return %NULL in the case of no vma at addr or above.
>   */
>  struct vm_area_struct *
>  find_vma_prev(struct mm_struct *mm, unsigned long addr,
>  			struct vm_area_struct **pprev)
>  {
>  	struct vm_area_struct *vma;
> +	MA_STATE(mas, &mm->mm_mt, addr, addr);
>  
> -	vma = find_vma(mm, addr);
> -	if (vma) {
> -		*pprev = vma->vm_prev;
> -	} else {
> -		struct rb_node *rb_node = rb_last(&mm->mm_rb);
> -
> -		*pprev = rb_node ? rb_entry(rb_node, struct vm_area_struct, vm_rb) : NULL;
> -	}
> +	vma = mas_walk(&mas);
> +	*pprev = mas_prev(&mas, 0);
> +	if (!vma)
> +		vma = mas_next(&mas, ULONG_MAX);
>  	return vma;
>  }
>