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

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Message-ID: <20220624130455.ygd6sihcaf3okovd@revolver>
Date:   Fri, 24 Jun 2022 13:05:02 +0000
From:   Liam Howlett <liam.howlett@...cle.com>
To:     David Hildenbrand <david@...hat.com>
CC:     "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.

* David Hildenbrand <david@...hat.com> [220621 17:04]:
> 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.

I'll update the comment.

> 
> > + */
> >  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.

That makes sense.  I just don't want to have a whole lot of APIs that
do the same thing.  I'll add it to my list and see how often we see the
pattern though.

> 
> >  	if (vma)
> >  		vmacache_update(addr, vma);
> >  	return vma;
> 
> Reviewed-by: David Hildenbrand <david@...hat.com>
> 
> -- 
> Thanks,
> 
> David / dhildenb
>