linux-kernel - Re: [RFC PATCH RESEND 10/28] mm/mmap: mark VMAs as locked in vma

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Date:   Tue, 6 Sep 2022 17:35:22 +0200
From:   Laurent Dufour <ldufour@...ux.ibm.com>
To:     Suren Baghdasaryan <surenb@...gle.com>, akpm@...ux-foundation.org
Cc:     michel@...pinasse.org, jglisse@...gle.com, mhocko@...e.com,
        vbabka@...e.cz, hannes@...xchg.org, mgorman@...e.de,
        dave@...olabs.net, willy@...radead.org, liam.howlett@...cle.com,
        peterz@...radead.org, laurent.dufour@...ibm.com,
        paulmck@...nel.org, luto@...nel.org, songliubraving@...com,
        peterx@...hat.com, david@...hat.com, dhowells@...hat.com,
        hughd@...gle.com, bigeasy@...utronix.de, kent.overstreet@...ux.dev,
        rientjes@...gle.com, axelrasmussen@...gle.com, joelaf@...gle.com,
        minchan@...gle.com, kernel-team@...roid.com, linux-mm@...ck.org,
        linux-arm-kernel@...ts.infradead.org,
        linuxppc-dev@...ts.ozlabs.org, x86@...nel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH RESEND 10/28] mm/mmap: mark VMAs as locked in
 vma_adjust

Le 01/09/2022 à 19:34, Suren Baghdasaryan a écrit :
> vma_adjust modifies a VMA and possibly its neighbors. Mark them as locked
> before making the modifications.
> 
> Signed-off-by: Suren Baghdasaryan <surenb@...gle.com>
> ---
>  mm/mmap.c | 11 ++++++++++-
>  1 file changed, 10 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/mmap.c b/mm/mmap.c
> index f89c9b058105..ed58cf0689b2 100644
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -710,6 +710,10 @@ int __vma_adjust(struct vm_area_struct *vma, unsigned long start,
>  	long adjust_next = 0;
>  	int remove_next = 0;
>  
> +	vma_mark_locked(vma);
> +	if (next)
> +		vma_mark_locked(next);
> +

I was wondering if the VMAs insert and expand should be locked too.

For expand, I can't see any valid reason, but for insert, I'm puzzled.
I would think that it is better to lock the VMA to be inserted but I can't
really justify that.

It may be nice to detail why this is not need to lock insert and expand here.

>  	if (next && !insert) {
>  		struct vm_area_struct *exporter = NULL, *importer = NULL;
>  
> @@ -754,8 +758,11 @@ int __vma_adjust(struct vm_area_struct *vma, unsigned long start,
>  			 * If next doesn't have anon_vma, import from vma after
>  			 * next, if the vma overlaps with it.
>  			 */
> -			if (remove_next == 2 && !next->anon_vma)
> +			if (remove_next == 2 && !next->anon_vma) {
>  				exporter = next->vm_next;
> +				if (exporter)
> +					vma_mark_locked(exporter);
> +			}
>  
>  		} else if (end > next->vm_start) {
>  			/*
> @@ -931,6 +938,8 @@ int __vma_adjust(struct vm_area_struct *vma, unsigned long start,
>  			 * "vma->vm_next" gap must be updated.
>  			 */
>  			next = vma->vm_next;
> +			if (next)
> +				vma_mark_locked(next);
>  		} else {
>  			/*
>  			 * For the scope of the comment "next" and