linux-kernel - Re: [PATCH v10 11/69] mm/mmap: use the maple tree in find

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Message-ID: <e25cb58c-464c-8bd4-c7da-9ab80b49f3bb@redhat.com>
Date:   Tue, 21 Jun 2022 23:04:18 +0200
From:   David Hildenbrand <david@...hat.com>
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>,
        "damon @ lists . linux . dev" <damon@...ts.linux.dev>,
        SeongJae Park <sj@...nel.org>
Subject: Re: [PATCH v10 11/69] mm/mmap: use the maple tree in find_vma()
 instead of the rbtree.

On 21.06.22 22:46, Liam Howlett wrote:
> From: "Liam R. Howlett" <Liam.Howlett@...cle.com>
> 
> Using the maple tree interface mt_find() will handle the RCU locking and
> will start searching at the address up to the limit, ULONG_MAX in this
> case.
> 
> Add kernel documentation to this API.
> 
> Link: https://lkml.kernel.org/r/20220504010716.661115-13-Liam.Howlett@oracle.com
> Signed-off-by: Liam R. Howlett <Liam.Howlett@...cle.com>
> Acked-by: Vlastimil Babka <vbabka@...e.cz>
> Cc: Catalin Marinas <catalin.marinas@....com>
> Cc: David Howells <dhowells@...hat.com>
> Cc: "Matthew Wilcox (Oracle)" <willy@...radead.org>
> Cc: SeongJae Park <sj@...nel.org>
> Cc: Will Deacon <will@...nel.org>
> Cc: Davidlohr Bueso <dave@...olabs.net>
> Signed-off-by: Andrew Morton <akpm@...ux-foundation.org>
> ---
>  mm/mmap.c | 28 ++++++++++------------------
>  1 file changed, 10 insertions(+), 18 deletions(-)
> 
> diff --git a/mm/mmap.c b/mm/mmap.c
> index d7e6baa2f40f..fdb61252448f 100644
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -2486,11 +2486,18 @@ get_unmapped_area(struct file *file, unsigned long addr, unsigned long len,
>  
>  EXPORT_SYMBOL(get_unmapped_area);
>  
> -/* Look up the first VMA which satisfies  addr < vm_end,  NULL if none. */
> +/**
> + * find_vma() - Find the VMA for a given address, or the next vma.
> + * @mm: The mm_struct to check
> + * @addr: The address
> + *
> + * Returns: The VMA associated with addr, or the next vma.
> + * May return %NULL in the case of no vma at addr or above.

Nit: inconsistent use of VMA vs. vma.

> + */
>  struct vm_area_struct *find_vma(struct mm_struct *mm, unsigned long addr)
>  {
> -	struct rb_node *rb_node;
>  	struct vm_area_struct *vma;
> +	unsigned long index = addr;
>  
>  	mmap_assert_locked(mm);
>  	/* Check the cache first. */
> @@ -2498,22 +2505,7 @@ struct vm_area_struct *find_vma(struct mm_struct *mm, unsigned long addr)
>  	if (likely(vma))
>  		return vma;
>  
> -	rb_node = mm->mm_rb.rb_node;
> -
> -	while (rb_node) {
> -		struct vm_area_struct *tmp;
> -
> -		tmp = rb_entry(rb_node, struct vm_area_struct, vm_rb);
> -
> -		if (tmp->vm_end > addr) {
> -			vma = tmp;
> -			if (tmp->vm_start <= addr)
> -				break;
> -			rb_node = rb_node->rb_left;
> -		} else
> -			rb_node = rb_node->rb_right;
> -	}
> -
> +	vma = mt_find(&mm->mm_mt, &index, ULONG_MAX);

I guess it would be handy to have a mt_find() variant that simply
consumes an address, because for example here, we don't actually care
about the output semantics? Does anything speak against such a utility
function or is this here really just a corner case?

That would make that code *even easier* to read.

>  	if (vma)
>  		vmacache_update(addr, vma);
>  	return vma;

Reviewed-by: David Hildenbrand <david@...hat.com>

-- 
Thanks,

David / dhildenb