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

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Date:   Wed, 15 Dec 2021 18:19:40 +0000
From:   Liam Howlett <liam.howlett@...cle.com>
To:     Vlastimil Babka <vbabka@...e.cz>
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>,
        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

* Vlastimil Babka <vbabka@...e.cz> [211215 11:40]:
> 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?


Yes, you are correct that mt_find() will handle the RCU locking, while
the advanced interface does not use the locking.  However, now that the
maple tree can use an external lock and uses the mmap_lock(), it's not
necessary to use RCU locks in these cases until we have users that are
removed from the mmap_lock().  I will add a comment so that this is not
missed when things are removed from the mmap_lock().

> 
> > 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;
> >  }
> >  
>