linux-kernel - Re: [PATCH v4 12/66] kernel/fork: Use maple tree for dup

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Date:   Thu, 16 Dec 2021 12:09:13 +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 12/66] kernel/fork: Use maple tree for dup_mmap()
 during forking

On 12/1/21 15:29, Liam Howlett wrote:
> From: "Liam R. Howlett" <Liam.Howlett@...cle.com>
> 
> The maple tree was already tracking VMAs in this function by an earlier
> commit, but the rbtree iterator was being used to iterate the list.
> Change the iterator to use a maple tree native iterator and switch to
> the maple tree advanced API to avoid multiple walks of the tree during
> insert operations.  Unexport the now-unused vma_store() function.
> 
> We track whether we need to free the VMAs and tree nodes through RCU
> (ie whether there have been multiple threads that can see the mm_struct
> simultaneously; by pthread(), ptrace() or looking at /proc/$pid/maps).
> This setting is sticky because it's too tricky to decide when it's safe
> to exit RCU mode.

I don't immediately see why enabling the RCU tracking in mmget is part of
the dup_mmap() change?

> For performance reasons we bulk allocate the maple tree nodes.  The node
> calculations are done internally to the tree and use the VMA count and
> assume the worst-case node requirements.  The VM_DONT_COPY flag does
> not allow for the most efficient copy method of the tree and so a bulk
> loading algorithm is used.
> 
> Signed-off-by: Liam R. Howlett <Liam.Howlett@...cle.com>
> Signed-off-by: Matthew Wilcox (Oracle) <willy@...radead.org>

Acked-by: Vlastimil Babka <vbabka@...e.cz>

>  static inline bool mmget_not_zero(struct mm_struct *mm)
>  {
> +	/*
> +	 * There is a race below during task tear down that can cause the maple

What does 'below' refer to here?

> +	 * tree to enter rcu mode with only a single user.  If this race
> +	 * happens, the result would be that the maple tree nodes would remain
> +	 * active for an extra RCU read cycle.
> +	 */
> +	if (!mt_in_rcu(&mm->mm_mt))
> +		mm_set_in_rcu(mm);
>  	return atomic_inc_not_zero(&mm->mm_users);
>  }
>  
> diff --git a/kernel/fork.c b/kernel/fork.c
> index cc9bb95c7678..c9f8465d8ae2 100644