linux-kernel - Re: [PATCH v4 24/33] mm: fall back to mmap_lock if vma->anon

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Date:   Wed, 1 Mar 2023 09:54:21 +0000
From:   Hyeonggon Yoo <42.hyeyoo@...il.com>
To:     Suren Baghdasaryan <surenb@...gle.com>
Cc:     akpm@...ux-foundation.org, michel@...pinasse.org,
        jglisse@...gle.com, mhocko@...e.com, vbabka@...e.cz,
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Subject: Re: [PATCH v4 24/33] mm: fall back to mmap_lock if vma->anon_vma is
 not yet set

On Mon, Feb 27, 2023 at 09:36:23AM -0800, Suren Baghdasaryan wrote:
> When vma->anon_vma is not set, page fault handler will set it by either
> reusing anon_vma of an adjacent VMA if VMAs are compatible or by
> allocating a new one. find_mergeable_anon_vma() walks VMA tree to find
> a compatible adjacent VMA and that requires not only the faulting VMA
> to be stable but also the tree structure and other VMAs inside that tree.
> Therefore locking just the faulting VMA is not enough for this search.
> Fall back to taking mmap_lock when vma->anon_vma is not set. This
> situation happens only on the first page fault and should not affect
> overall performance.
> 
> Signed-off-by: Suren Baghdasaryan <surenb@...gle.com>
> ---
>  mm/memory.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/mm/memory.c b/mm/memory.c
> index bda4c1a991f0..8855846a361b 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -5243,6 +5243,10 @@ struct vm_area_struct *lock_vma_under_rcu(struct mm_struct *mm,
>  	if (!vma_is_anonymous(vma))
>  		goto inval;
>  
> +	/* find_mergeable_anon_vma uses adjacent vmas which are not locked */
> +	if (!vma->anon_vma)
> +		goto inval;
> +
>  	if (!vma_start_read(vma))
>  		goto inval;

Reviewed-by: Hyeonggon Yoo <42.hyeyoo@...il.com>

> -- 
> 2.39.2.722.g9855ee24e9-goog
> 
>