linux-kernel - Re: [RFC PATCH RESEND 17/28] mm/mmap: prevent pagefault handler from racing with mmu

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Date:   Fri, 9 Sep 2022 16:20:35 +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 17/28] mm/mmap: prevent pagefault handler from
 racing with mmu_notifier registration

Le 01/09/2022 à 19:35, Suren Baghdasaryan a écrit :
> Pagefault handlers might need to fire MMU notifications while a new
> notifier is being registered. Modify mm_take_all_locks to mark all VMAs
> as locked and prevent this race with fault handlers that would hold VMA
> locks.
> 
> Signed-off-by: Suren Baghdasaryan <surenb@...gle.com>
> ---
>  mm/mmap.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/mm/mmap.c b/mm/mmap.c
> index b31cc97c2803..1edfcd384f5e 100644
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -3538,6 +3538,7 @@ static void vm_lock_mapping(struct mm_struct *mm, struct address_space *mapping)
>   *     hugetlb mapping);
>   *   - all i_mmap_rwsem locks;
>   *   - all anon_vma->rwseml
> + *   - all vmas marked locked

IIRC, the anon_vma may be locked during the page fault handling, and this
happens after the VMA is read lock. I think the same applies to
i_mmap_rwsem lock.

Thus, the VMA should be marked locked first.

>   *
>   * We can take all locks within these types randomly because the VM code
>   * doesn't nest them and we protected from parallel mm_take_all_locks() by
> @@ -3579,6 +3580,7 @@ int mm_take_all_locks(struct mm_struct *mm)
>  		if (vma->anon_vma)
>  			list_for_each_entry(avc, &vma->anon_vma_chain, same_vma)
>  				vm_lock_anon_vma(mm, avc->anon_vma);
> +		vma_mark_locked(vma);
>  	}
>  
>  	return 0;
> @@ -3636,6 +3638,7 @@ void mm_drop_all_locks(struct mm_struct *mm)
>  	mmap_assert_write_locked(mm);
>  	BUG_ON(!mutex_is_locked(&mm_all_locks_mutex));
>  
> +	vma_mark_unlocked_all(mm);
>  	for (vma = mm->mmap; vma; vma = vma->vm_next) {
>  		if (vma->anon_vma)
>  			list_for_each_entry(avc, &vma->anon_vma_chain, same_vma)