linux-kernel - Re: [PATCH] userfaultfd: clear flag if remap event not enabled

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Message-Id: <20181210175115.GB6380@rapoport-lnx>
Date:   Mon, 10 Dec 2018 19:51:16 +0200
From:   Mike Rapoport <rppt@...ux.ibm.com>
To:     Peter Xu <peterx@...hat.com>
Cc:     linux-kernel@...r.kernel.org,
        Andrea Arcangeli <aarcange@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Mike Rapoport <rppt@...ux.vnet.ibm.com>,
        "Kirill A . Shutemov" <kirill@...temov.name>,
        Hugh Dickins <hughd@...gle.com>,
        Pavel Emelyanov <xemul@...tuozzo.com>,
        Pravin Shedge <pravin.shedge4linux@...il.com>,
        linux-mm@...ck.org
Subject: Re: [PATCH] userfaultfd: clear flag if remap event not enabled

On Mon, Dec 10, 2018 at 02:51:21PM +0800, Peter Xu wrote:
> When the process being tracked do mremap() without
> UFFD_FEATURE_EVENT_REMAP on the corresponding tracking uffd file
> handle, we should not generate the remap event, and at the same
> time we should clear all the uffd flags on the new VMA.  Without
> this patch, we can still have the VM_UFFD_MISSING|VM_UFFD_WP
> flags on the new VMA even the fault handling process does not
> even know the existance of the VMA.
> 
> CC: Andrea Arcangeli <aarcange@...hat.com>
> CC: Andrew Morton <akpm@...ux-foundation.org>
> CC: Mike Rapoport <rppt@...ux.vnet.ibm.com>
> CC: Kirill A. Shutemov <kirill@...temov.name>
> CC: Hugh Dickins <hughd@...gle.com>
> CC: Pavel Emelyanov <xemul@...tuozzo.com>
> CC: Pravin Shedge <pravin.shedge4linux@...il.com>
> CC: linux-mm@...ck.org
> CC: linux-kernel@...r.kernel.org
> Signed-off-by: Peter Xu <peterx@...hat.com>
> ---
>  fs/userfaultfd.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
> index cd58939dc977..798ae8a438ff 100644
> --- a/fs/userfaultfd.c
> +++ b/fs/userfaultfd.c
> @@ -740,6 +740,9 @@ void mremap_userfaultfd_prep(struct vm_area_struct *vma,
>  		vm_ctx->ctx = ctx;
>  		userfaultfd_ctx_get(ctx);
>  		WRITE_ONCE(ctx->mmap_changing, true);
> +	} else if (ctx) {
> +		vma->vm_userfaultfd_ctx = NULL_VM_UFFD_CTX;
> +		vma->vm_flags &= ~(VM_UFFD_WP | VM_UFFD_MISSING);

My preference would be 

	if (!ctx)
		return;
	
	if (ctx->features & UFFD_FEATURE_EVENT_REMAP) {
		...
	} else {
		...
	}

but I don't feel strongly about it.

I'd appreciate a comment in the code and with it 

Acked-by: Mike Rapoport <rppt@...ux.ibm.com>


>  	}
>  }
> 
> -- 
> 2.17.1
> 

-- 
Sincerely yours,
Mike.