linux-kernel - Re: [RFC PATCH RESEND 15/28] mm/mmap: mark adjacent VMAs as locked if they can grow into unmapped area

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Date:   Fri, 9 Sep 2022 15:43:28 +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 15/28] mm/mmap: mark adjacent VMAs as locked if
 they can grow into unmapped area

Le 01/09/2022 à 19:35, Suren Baghdasaryan a écrit :
> While unmapping VMAs, adjacent VMAs might be able to grow into the area
> being unmapped. In such cases mark adjacent VMAs as locked to prevent
> this growth.
> 
> Signed-off-by: Suren Baghdasaryan <surenb@...gle.com>
> ---
>  mm/mmap.c | 8 ++++++--
>  1 file changed, 6 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/mmap.c b/mm/mmap.c
> index b0d78bdc0de0..b31cc97c2803 100644
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -2680,10 +2680,14 @@ detach_vmas_to_be_unmapped(struct mm_struct *mm, struct vm_area_struct *vma,
>  	 * VM_GROWSUP VMA. Such VMAs can change their size under
>  	 * down_read(mmap_lock) and collide with the VMA we are about to unmap.
>  	 */
> -	if (vma && (vma->vm_flags & VM_GROWSDOWN))
> +	if (vma && (vma->vm_flags & VM_GROWSDOWN)) {
> +		vma_mark_locked(vma);
>  		return false;
> -	if (prev && (prev->vm_flags & VM_GROWSUP))
> +	}
> +	if (prev && (prev->vm_flags & VM_GROWSUP)) {
> +		vma_mark_locked(prev);
>  		return false;
> +	}
>  	return true;
>  }
>

That looks right to be.

But, in addition to that, like the previous patch, all the VMAs to be
detached from the tree in the loop above, should be marked locked just
before calling vm_rb_erase().