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Subject: Re: [PATCH] staging: kpc2000: Convert put_page() to put_user_page*()
To:     Bharath Vedartham <linux.bhar@gmail.com>, <ira.weiny@intel.com>,
        <gregkh@linuxfoundation.org>, <Matt.Sickler@daktronics.com>,
        <jglisse@redhat.com>
CC:     <devel@driverdev.osuosl.org>, <linux-mm@kvack.org>,
        <linux-kernel@vger.kernel.org>
References: <20190715195248.GA22495@bharath12345-Inspiron-5559>
From:   John Hubbard <jhubbard@nvidia.com>
Message-ID: <2604fcd1-4829-d77e-9f7c-d4b731782ff9@nvidia.com>
Date:   Mon, 15 Jul 2019 13:14:13 -0700
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On 7/15/19 12:52 PM, Bharath Vedartham wrote:
> There have been issues with get_user_pages and filesystem writeback.
> The issues are better described in [1].
>=20
> The solution being proposed wants to keep track of gup_pinned pages which=
 will allow to take furthur steps to coordinate between subsystems using gu=
p.
>=20
> put_user_page() simply calls put_page inside for now. But the implementat=
ion will change once all call sites of put_page() are converted.
>=20
> I currently do not have the driver to test. Could I have some suggestions=
 to test this code? The solution is currently implemented in [2] and
> it would be great if we could apply the patch on top of [2] and run some =
tests to check if any regressions occur.

Hi Bharath,

Process point: the above paragraph, and other meta-questions (about the pat=
ch, rather than part of the patch) should be placed either after the "---",=
 or in a cover letter (git-send-email --cover-letter). That way, the patch =
itself is in a commit-able state.

One more below:

>=20
> [1] https://lwn.net/Articles/753027/
> [2] https://github.com/johnhubbard/linux/tree/gup_dma_core
>=20
> Cc: Matt Sickler <Matt.Sickler@daktronics.com>
> Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> Cc: J=C3=A9r=C3=B4me Glisse <jglisse@redhat.com>
> Cc: Ira Weiny <ira.weiny@intel.com>
> Cc: John Hubbard <jhubbard@nvidia.com>
> Cc: linux-mm@kvack.org
> Cc: devel@driverdev.osuosl.org
>=20
> Signed-off-by: Bharath Vedartham <linux.bhar@gmail.com>
> ---
>  drivers/staging/kpc2000/kpc_dma/fileops.c | 8 ++------
>  1 file changed, 2 insertions(+), 6 deletions(-)
>=20
> diff --git a/drivers/staging/kpc2000/kpc_dma/fileops.c b/drivers/staging/=
kpc2000/kpc_dma/fileops.c
> index 6166587..82c70e6 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:
> @@ -229,9 +227,7 @@ void  transfer_complete_cb(struct aio_cb_data *acd, s=
ize_t xfr_count, u32 flags)
>  =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]);
> -	}
> +	put_user_pages(acd->user_pages, acd->page_count);
>  =09
>  	sg_free_table(&acd->sgt);
>  =09
>=20

Because this is a common pattern, and because the code here doesn't likely =
need to set page dirty before the dma_unmap_sg call, I think the following =
would be better (it's untested), instead of the above diff hunk:

diff --git a/drivers/staging/kpc2000/kpc_dma/fileops.c b/drivers/staging/kp=
c2000/kpc_dma/fileops.c
index 48ca88bc6b0b..d486f9866449 100644
--- a/drivers/staging/kpc2000/kpc_dma/fileops.c
+++ b/drivers/staging/kpc2000/kpc_dma/fileops.c
@@ -211,16 +211,13 @@ void  transfer_complete_cb(struct aio_cb_data *acd, s=
ize_t xfr_count, u32 flags)
        BUG_ON(acd->ldev =3D=3D NULL);
        BUG_ON(acd->ldev->pldev =3D=3D NULL);
=20
-       for (i =3D 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);
=20
        for (i =3D 0 ; i < acd->page_count ; i++) {
-               put_page(acd->user_pages[i]);
+               if (!PageReserved(acd->user_pages[i])) {
+                       put_user_pages_dirty(&acd->user_pages[i], 1);
+               else
+                       put_user_page(acd->user_pages[i]);
        }
=20
        sg_free_table(&acd->sgt);

Assuming that you make those two changes, you can add:

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


thanks,
--=20
John Hubbard
NVIDIA