linux-kernel - Re: [RFC PATCH RESEND 07/28] kernel/fork: mark VMAs as locked before copying pages during fork

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Message-ID: <a072fd10-ee7e-469f-c203-978cd7da566c@linux.ibm.com>
Date:   Tue, 6 Sep 2022 16:37:31 +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 07/28] kernel/fork: mark VMAs as locked before
 copying pages during fork

Le 01/09/2022 à 19:34, Suren Baghdasaryan a écrit :
> Protect VMAs from concurrent page fault handler while performing
> copy_page_range for VMAs having VM_WIPEONFORK flag set.

I'm wondering why is that necessary.
The copied mm is write locked, and the destination one is not reachable.
If any other readers are using the VMA, this is only for page fault handling.
I should have miss something because I can't see any need to mark the lock
VMA here.

> Signed-off-by: Suren Baghdasaryan <surenb@...gle.com>
> ---
>  kernel/fork.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/fork.c b/kernel/fork.c
> index bfab31ecd11e..1872ad549fed 100644
> --- a/kernel/fork.c
> +++ b/kernel/fork.c
> @@ -709,8 +709,10 @@ static __latent_entropy int dup_mmap(struct mm_struct *mm,
>  		rb_parent = &tmp->vm_rb;
>  
>  		mm->map_count++;
> -		if (!(tmp->vm_flags & VM_WIPEONFORK))
> +		if (!(tmp->vm_flags & VM_WIPEONFORK)) {
> +			vma_mark_locked(mpnt);
>  			retval = copy_page_range(tmp, mpnt);
> +		}
>  
>  		if (tmp->vm_ops && tmp->vm_ops->open)
>  			tmp->vm_ops->open(tmp);