linux-kernel - Re: [RFC PATCH RESEND 09/28] mm/mempolicy: mark VMA as locked when changing protection policy

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Date:   Tue, 6 Sep 2022 16:47:57 +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 09/28] mm/mempolicy: mark VMA as locked when
 changing protection policy

Le 01/09/2022 à 19:34, Suren Baghdasaryan a écrit :
> Protect VMA from concurrent page fault handler while performing VMA
> protection policy changes.
> 
> Signed-off-by: Suren Baghdasaryan <surenb@...gle.com>
> ---
>  mm/mempolicy.c | 6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/mempolicy.c b/mm/mempolicy.c
> index b73d3248d976..6be1e5c75556 100644
> --- a/mm/mempolicy.c
> +++ b/mm/mempolicy.c
> @@ -383,8 +383,10 @@ void mpol_rebind_mm(struct mm_struct *mm, nodemask_t *new)
>  	struct vm_area_struct *vma;
>  
>  	mmap_write_lock(mm);
> -	for (vma = mm->mmap; vma; vma = vma->vm_next)
> +	for (vma = mm->mmap; vma; vma = vma->vm_next) {
> +		vma_mark_locked(vma);
>  		mpol_rebind_policy(vma->vm_policy, new);
> +	}
>  	mmap_write_unlock(mm);
>  }
>  
> @@ -632,6 +634,7 @@ unsigned long change_prot_numa(struct vm_area_struct *vma,
>  	struct mmu_gather tlb;
>  	int nr_updated;
>  
> +	vma_mark_locked(vma);

If I understand that corretly, the VMA itself is not impacted, only the
PMDs/PTEs, and they are protected using the page table locks.

Am I missing something?

>  	tlb_gather_mmu(&tlb, vma->vm_mm);
>  
>  	nr_updated = change_protection(&tlb, vma, addr, end, PAGE_NONE,
> @@ -765,6 +768,7 @@ static int vma_replace_policy(struct vm_area_struct *vma,
>  	if (IS_ERR(new))
>  		return PTR_ERR(new);
>  
> +	vma_mark_locked(vma);
>  	if (vma->vm_ops && vma->vm_ops->set_policy) {
>  		err = vma->vm_ops->set_policy(vma, new);
>  		if (err)