linux-kernel - Re: [PATCH 2/2] vhost: convert get_user_pages() --> pin_user

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Message-ID: <20200602002212-mutt-send-email-mst@kernel.org>
Date:   Tue, 2 Jun 2020 00:22:25 -0400
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     John Hubbard <jhubbard@...dia.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Jason Wang <jasowang@...hat.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Jérôme Glisse <jglisse@...hat.com>,
        Jan Kara <jack@...e.cz>, Dave Chinner <david@...morbit.com>,
        Souptick Joarder <jrdr.linux@...il.com>,
        Jonathan Corbet <corbet@....net>, linux-doc@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, kvm@...r.kernel.org,
        virtualization@...ts.linux-foundation.org, netdev@...r.kernel.org,
        LKML <linux-kernel@...r.kernel.org>, linux-mm@...ck.org
Subject: Re: [PATCH 2/2] vhost: convert get_user_pages() --> pin_user_pages()

On Fri, May 29, 2020 at 04:43:09PM -0700, John Hubbard wrote:
> This code was using get_user_pages*(), in approximately a "Case 5"
> scenario (accessing the data within a page), using the categorization
> from [1]. That means that it's time to convert the get_user_pages*() +
> put_page() calls to pin_user_pages*() + unpin_user_pages() calls.
> 
> There is some helpful background in [2]: basically, this is a small
> part of fixing a long-standing disconnect between pinning pages, and
> file systems' use of those pages.
> 
> [1] Documentation/core-api/pin_user_pages.rst
> 
> [2] "Explicit pinning of user-space pages":
>     https://lwn.net/Articles/807108/
> 
> Cc: Michael S. Tsirkin <mst@...hat.com>
> Cc: Jason Wang <jasowang@...hat.com>
> Cc: kvm@...r.kernel.org
> Cc: virtualization@...ts.linux-foundation.org
> Cc: netdev@...r.kernel.org
> Signed-off-by: John Hubbard <jhubbard@...dia.com>

Acked-by: Michael S. Tsirkin <mst@...hat.com>

> ---
>  drivers/vhost/vhost.c | 5 ++---
>  1 file changed, 2 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
> index 21a59b598ed8..596132a96cd5 100644
> --- a/drivers/vhost/vhost.c
> +++ b/drivers/vhost/vhost.c
> @@ -1762,15 +1762,14 @@ static int set_bit_to_user(int nr, void __user *addr)
>  	int bit = nr + (log % PAGE_SIZE) * 8;
>  	int r;
>  
> -	r = get_user_pages_fast(log, 1, FOLL_WRITE, &page);
> +	r = pin_user_pages_fast(log, 1, FOLL_WRITE, &page);
>  	if (r < 0)
>  		return r;
>  	BUG_ON(r != 1);
>  	base = kmap_atomic(page);
>  	set_bit(bit, base);
>  	kunmap_atomic(base);
> -	set_page_dirty_lock(page);
> -	put_page(page);
> +	unpin_user_pages_dirty_lock(&page, 1, true);
>  	return 0;
>  }
>  
> -- 
> 2.26.2