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Subject: Re: [PATCH v3] staging: kpc2000: Convert put_page to put_user_page*()
To:     Bharath Vedartham <linux.bhar@gmail.com>, <ira.weiny@intel.com>,
        <jglisse@redhat.com>, <gregkh@linuxfoundation.org>,
        <Matt.Sickler@daktronics.com>
CC:     <linux-mm@kvack.org>, <linux-kernel@vger.kernel.org>,
        <devel@driverdev.osuosl.org>
References: <20190719200235.GA16122@bharath12345-Inspiron-5559>
From:   John Hubbard <jhubbard@nvidia.com>
Message-ID: <8bce5bb2-d9a5-13f1-7d96-27c41057c519@nvidia.com>
Date:   Fri, 19 Jul 2019 14:28:39 -0700
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On 7/19/19 1:02 PM, Bharath Vedartham wrote:
> There have been issues with coordination of various subsystems using
> get_user_pages. These issues are better described in [1].
>=20
> An implementation of tracking get_user_pages is currently underway
> The implementation requires the use put_user_page*() variants to release
> a reference rather than put_page(). The commit that introduced
> put_user_pages, Commit fc1d8e7cca2daa18d2fe56b94874848adf89d7f5 ("mm: int=
roduce
> put_user_page*(), placeholder version").
>=20
> The implementation currently simply calls put_page() within
> put_user_page(). But in the future, it is to change to add a mechanism
> to keep track of get_user_pages. Once a tracking mechanism is
> implemented, we can make attempts to work on improving on coordination
> between various subsystems using get_user_pages.
>=20
> [1] https://lwn.net/Articles/753027/

Optional: I've been fussing about how to keep the change log reasonable,
and finally came up with the following recommended template for these=20
conversion patches. This would replace the text you have above, because the=
=20
put_user_page placeholder commit has all the documentation (and then some)=
=20
that we need:


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").


For the change itself, you will need to rebase it onto the latest=20
linux.git, as it doesn't quite apply there.=20

Testing is good if we can get it, but as far as I can tell this is
correct, so you can also add:

    Reviewed-by: John Hubbard <jhubbard@nvidia.com>

thanks,
--=20
John Hubbard
NVIDIA

>=20
> Cc: Ira Weiny <ira.weiny@intel.com>
> Cc: John Hubbard <jhubbard@nvidia.com>
> Cc: J=C3=A9r=C3=B4me Glisse <jglisse@redhat.com>
> Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> Cc: Matt Sickler <Matt.Sickler@daktronics.com>
> Cc: devel@driverdev.osuosl.org=20
> Cc: linux-kernel@vger.kernel.org
> Cc: linux-mm@kvack.org
> 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.
> =09
> The PageReserved check needs a closer look and is not worth messing
> around with for now.
>=20
> Matt, Could you give any suggestions for testing this patch?
>    =20
> If in-case, you are willing to pick this up to test. Could you
> apply this patch to this tree
> https://github.com/johnhubbard/linux/tree/gup_dma_core
> and test it with your devices?
>=20
> ---
>  drivers/staging/kpc2000/kpc_dma/fileops.c | 17 ++++++-----------
>  1 file changed, 6 insertions(+), 11 deletions(-)
>=20
> diff --git a/drivers/staging/kpc2000/kpc_dma/fileops.c b/drivers/staging/=
kpc2000/kpc_dma/fileops.c
> index 6166587..75ad263 100644
> --- a/drivers/staging/kpc2000/kpc_dma/fileops.c
> +++ b/drivers/staging/kpc2000/kpc_dma/fileops.c
> @@ -198,9 +198,7 @@ int  kpc_dma_transfer(struct dev_private_data *priv, =
struct kiocb *kcb, unsigned
>  	sg_free_table(&acd->sgt);
>   err_dma_map_sg:
>   err_alloc_sg_table:
> -	for (i =3D 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:
> @@ -221,16 +219,13 @@ void  transfer_complete_cb(struct aio_cb_data *acd,=
 size_t xfr_count, u32 flags)
>  =09
>  	dev_dbg(&acd->ldev->pldev->dev, "transfer_complete_cb(acd =3D [%p])\n",=
 acd);
>  =09
> -	for (i =3D 0 ; i < acd->page_count ; i++){
> -		if (!PageReserved(acd->user_pages[i])){
> -			set_page_dirty(acd->user_pages[i]);
> -		}
> -	}
> -=09
>  	dma_unmap_sg(&acd->ldev->pldev->dev, acd->sgt.sgl, acd->sgt.nents, acd-=
>ldev->dir);
>  =09
> -	for (i =3D 0 ; i < acd->page_count ; i++){
> -		put_page(acd->user_pages[i]);
> +	for (i =3D 0; i < acd->page_count; i++) {
> +		if (!PageReserved(acd->user_pages[i]))
> +			put_user_pages_dirty(&acd->user_pages[i], 1);
> +		else
> +			put_user_page(acd->user_pages[i]);
>  	}
>  =09
>  	sg_free_table(&acd->sgt);
>=20