linux-kernel - Re: [PATCH 2/2] xen/privcmd: Convert get_user_pages*() to pin_user

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Date:   Wed, 24 Jun 2020 22:49:02 -0700
From:   John Hubbard <jhubbard@...dia.com>
To:     Souptick Joarder <jrdr.linux@...il.com>,
        <boris.ostrovsky@...cle.com>, <jgross@...e.com>,
        <sstabellini@...nel.org>
CC:     <xen-devel@...ts.xenproject.org>, <linux-kernel@...r.kernel.org>,
        Paul Durrant <xadimgnik@...il.com>
Subject: Re: [PATCH 2/2] xen/privcmd: Convert get_user_pages*() to
 pin_user_pages*()

On 2020-06-24 20:02, Souptick Joarder wrote:
> In 2019, we introduced pin_user_pages*() and now we are converting
> get_user_pages*() to the new API as appropriate. [1] & [2] could
> be referred for more information. This is case 5 as per document [1].
> 
> [1] Documentation/core-api/pin_user_pages.rst
> 
> [2] "Explicit pinning of user-space pages":
>          https://lwn.net/Articles/807108/
> 
> Signed-off-by: Souptick Joarder <jrdr.linux@...il.com>
> Cc: John Hubbard <jhubbard@...dia.com>
> Cc: Boris Ostrovsky <boris.ostrovsky@...cle.com>
> Cc: Paul Durrant <xadimgnik@...il.com>
> ---
> Hi,
> 
> I'm compile tested this, but unable to run-time test, so any testing
> help is much appriciated.
> 
>   drivers/xen/privcmd.c | 10 ++--------
>   1 file changed, 2 insertions(+), 8 deletions(-)
> 
> diff --git a/drivers/xen/privcmd.c b/drivers/xen/privcmd.c
> index 0da417c..eb05254 100644
> --- a/drivers/xen/privcmd.c
> +++ b/drivers/xen/privcmd.c
> @@ -595,7 +595,7 @@ static int lock_pages(
>   		if (requested > nr_pages)
>   			return -ENOSPC;
>   
> -		page_count = get_user_pages_fast(
> +		page_count = pin_user_pages_fast(
>   			(unsigned long) kbufs[i].uptr,
>   			requested, FOLL_WRITE, pages);
>   		if (page_count < 0) {
> @@ -612,13 +612,7 @@ static int lock_pages(
>   
>   static void unlock_pages(struct page *pages[], unsigned int nr_pages)
>   {
> -	unsigned int i;
> -
> -	for (i = 0; i < nr_pages; i++) {
> -		if (!PageDirty(page))
> -			set_page_dirty_lock(page);
> -		put_page(pages[i]);
> -	}
> +	unpin_user_pages_dirty_lock(pages, nr_pages, 1);

"true", not "1", is the correct way to call that function.

Also, this approach changes the behavior slightly, but I think it's
reasonable to just set_page_dirty_lock() on the whole range--hard to
see much benefit in checking PageDirty first.


>   }
>   
>   static long privcmd_ioctl_dm_op(struct file *file, void __user *udata)
> 

thanks,
-- 
John Hubbard
NVIDIA