Re: [PATCH v4] staging: kpc2000: Convert put_page() to put_user_page*()

From: John Hubbard <jhubbard@nvidia.com>
To: Bharath Vedartham <linux.bhar@gmail.com>,
	<gregkh@linuxfoundation.org>, <Matt.Sickler@daktronics.com>
Cc: "Ira Weiny" <ira.weiny@intel.com>,
	"Jérôme Glisse" <jglisse@redhat.com>,
	devel@driverdev.osuosl.org, linux-kernel@vger.kernel.org,
	linux-mm@kvack.org
Subject: Re: [PATCH v4] staging: kpc2000: Convert put_page() to put_user_page*()
Date: Fri, 2 Aug 2019 17:13:42 -0700	[thread overview]
Message-ID: <4467d671-d011-0ebc-e2de-48a9745d4fe6@nvidia.com> (raw)
In-Reply-To: <1564058658-3551-1-git-send-email-linux.bhar@gmail.com>

On 7/25/19 5:44 AM, Bharath Vedartham wrote:
> For pages that were retained via get_user_pages*(), release those pages
> via the new put_user_page*() routines, instead of via put_page().
> 
> This is part a tree-wide conversion, as described in commit fc1d8e7cca2d
> ("mm: introduce put_user_page*(), placeholder versions").
> 

Hi Bharath,

If you like, I could re-post your patch here, modified slightly, as part of
the next version of the miscellaneous call site conversion series [1].

As part of that, we should change this to use put_user_pages_dirty_lock() 
(see below).

> Cc: Ira Weiny <ira.weiny@intel.com>
> Cc: John Hubbard <jhubbard@nvidia.com>
> Cc: Jérôme Glisse <jglisse@redhat.com>
> Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> Cc: Matt Sickler <Matt.Sickler@daktronics.com>
> Cc: devel@driverdev.osuosl.org
> Cc: linux-kernel@vger.kernel.org
> Cc: linux-mm@kvack.org
> Reviewed-by: John Hubbard <jhubbard@nvidia.com>
> Signed-off-by: Bharath Vedartham <linux.bhar@gmail.com>
> ---
> Changes since v1
>         - Improved changelog by John's suggestion.
>         - Moved logic to dirty pages below sg_dma_unmap
>          and removed PageReserved check.
> Changes since v2
>         - Added back PageResevered check as
>         suggested by John Hubbard.
> Changes since v3
>         - Changed the changelog as suggested by John.
>         - Added John's Reviewed-By tag.
> Changes since v4
>         - Rebased the patch on the staging tree.
>         - Improved commit log by fixing a line wrap.
> ---
>  drivers/staging/kpc2000/kpc_dma/fileops.c | 17 ++++++-----------
>  1 file changed, 6 insertions(+), 11 deletions(-)
> 
> diff --git a/drivers/staging/kpc2000/kpc_dma/fileops.c b/drivers/staging/kpc2000/kpc_dma/fileops.c
> index 48ca88b..f15e292 100644
> --- a/drivers/staging/kpc2000/kpc_dma/fileops.c
> +++ b/drivers/staging/kpc2000/kpc_dma/fileops.c
> @@ -190,9 +190,7 @@ static int kpc_dma_transfer(struct dev_private_data *priv,
>  	sg_free_table(&acd->sgt);
>   err_dma_map_sg:
>   err_alloc_sg_table:
> -	for (i = 0 ; i < acd->page_count ; i++) {
> -		put_page(acd->user_pages[i]);
> -	}
> +	put_user_pages(acd->user_pages, acd->page_count);
>   err_get_user_pages:
>  	kfree(acd->user_pages);
>   err_alloc_userpages:
> @@ -211,16 +209,13 @@ void  transfer_complete_cb(struct aio_cb_data *acd, size_t xfr_count, u32 flags)
>  	BUG_ON(acd->ldev == NULL);
>  	BUG_ON(acd->ldev->pldev == NULL);
>  
> -	for (i = 0 ; i < acd->page_count ; i++) {
> -		if (!PageReserved(acd->user_pages[i])) {
> -			set_page_dirty(acd->user_pages[i]);
> -		}
> -	}
> -
>  	dma_unmap_sg(&acd->ldev->pldev->dev, acd->sgt.sgl, acd->sgt.nents, acd->ldev->dir);
>  
> -	for (i = 0 ; i < acd->page_count ; i++) {
> -		put_page(acd->user_pages[i]);
> +	for (i = 0; i < acd->page_count; i++) {
> +		if (!PageReserved(acd->user_pages[i]))
> +			put_user_pages_dirty(&acd->user_pages[i], 1);

This would change to:
			put_user_pages_dirty_lock(&acd->user_pages[i], 1, true);

...and we'd add this blurb (this time with CH's name spelled properly) to 
the commit description:

Note that this effectively changes the code's behavior in
qp_release_pages(): it now ultimately calls set_page_dirty_lock(),
instead of set_page_dirty(). This is probably more accurate.

As Christoph Hellwig put it, "set_page_dirty() is only safe if we are
dealing with a file backed page where we have reference on the inode it
hangs off." [1]

[1] https://lore.kernel.org/r/20190723153640.GB720@lst.de

Also, future: I don't know the driver well enough to say, but maybe "true" 
could be replaced by "acd->ldev->dir == DMA_FROM_DEVICE", there, but that
would be a separate patch.

thanks,
-- 
John Hubbard
NVIDIA

> +		else
> +			put_user_page(acd->user_pages[i]);
>  	}
>  
>  	sg_free_table(&acd->sgt);
> 