linux-kernel - Re: [PATCH v4 49/66] bpf: Remove VMA linked list

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Date:   Wed, 19 Jan 2022 18:04:46 +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 49/66] bpf: Remove VMA linked list

On 12/1/21 15:30, Liam Howlett wrote:
> From: "Liam R. Howlett" <Liam.Howlett@...cle.com>
> 
> Use vma_next() and remove reference to the start of the linked list
> 
> Signed-off-by: Liam R. Howlett <Liam.Howlett@...cle.com>
> ---
>  kernel/bpf/task_iter.c | 21 ++++-----------------
>  1 file changed, 4 insertions(+), 17 deletions(-)
> 
> diff --git a/kernel/bpf/task_iter.c b/kernel/bpf/task_iter.c
> index b48750bfba5a..2d964743f1e6 100644
> --- a/kernel/bpf/task_iter.c
> +++ b/kernel/bpf/task_iter.c
> @@ -299,8 +299,8 @@ struct bpf_iter_seq_task_vma_info {
>  };
>  
>  enum bpf_task_vma_iter_find_op {
> -	task_vma_iter_first_vma,   /* use mm->mmap */
> -	task_vma_iter_next_vma,    /* use curr_vma->vm_next */
> +	task_vma_iter_first_vma,   /* use find_vma() with addr 0 */
> +	task_vma_iter_next_vma,    /* use vma_next() with curr_vma */
>  	task_vma_iter_find_vma,    /* use find_vma() to find next vma */
>  };
>  
> @@ -400,24 +400,11 @@ task_vma_seq_get_next(struct bpf_iter_seq_task_vma_info *info)
>  
>  	switch (op) {
>  	case task_vma_iter_first_vma:
> -		curr_vma = curr_task->mm->mmap;
> +		curr_vma = find_vma(curr_task->mm, 0);
>  		break;
>  	case task_vma_iter_next_vma:
> -		curr_vma = curr_vma->vm_next;
> -		break;
>  	case task_vma_iter_find_vma:
> -		/* We dropped mmap_lock so it is necessary to use find_vma
> -		 * to find the next vma. This is similar to the  mechanism
> -		 * in show_smaps_rollup().
> -		 */
> -		curr_vma = find_vma(curr_task->mm, info->prev_vm_end - 1);
> -		/* case 1) and 4.2) above just use curr_vma */
> -
> -		/* check for case 2) or case 4.1) above */
> -		if (curr_vma &&
> -		    curr_vma->vm_start == info->prev_vm_start &&
> -		    curr_vma->vm_end == info->prev_vm_end)
> -			curr_vma = curr_vma->vm_next;
> +		curr_vma = find_vma(curr_task->mm, curr_vma->vm_end);

Are you sure curr_vma is valid here and we can read its vm_end? Because I
have no idea, but lots of doubts.

>  		break;
>  	}
>  	if (!curr_vma) {