linux-kernel - Re: [PATCH] drm/radeon: Replace kmap() with kmap_local

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Message-ID: <CADnq5_PP3VCXQ5rbC0-8Qsi5W7Ew87U_bRknz4=qxbrPxVQ+qA@mail.gmail.com>
Date:   Mon, 17 Oct 2022 12:52:10 -0400
From:   Alex Deucher <alexdeucher@...il.com>
To:     Christian König <christian.koenig@....com>
Cc:     "Fabio M. De Francesco" <fmdefrancesco@...il.com>,
        Alex Deucher <alexander.deucher@....com>,
        "Pan, Xinhui" <Xinhui.Pan@....com>,
        David Airlie <airlied@...il.com>,
        Daniel Vetter <daniel@...ll.ch>,
        Christian Brauner <brauner@...nel.org>,
        Sumit Semwal <sumit.semwal@...aro.org>,
        Jean Delvare <jdelvare@...e.com>,
        Guenter Roeck <linux@...ck-us.net>,
        Kees Cook <keescook@...omium.org>,
        amd-gfx@...ts.freedesktop.org, dri-devel@...ts.freedesktop.org,
        linux-kernel@...r.kernel.org, bpf@...r.kernel.org,
        linux-media@...r.kernel.org, linaro-mm-sig@...ts.linaro.org,
        linux-hwmon@...r.kernel.org, linux-hardening@...r.kernel.org,
        "Venkataramanan, Anirudh" <anirudh.venkataramanan@...el.com>,
        Ira Weiny <ira.weiny@...el.com>
Subject: Re: [PATCH] drm/radeon: Replace kmap() with kmap_local_page()

Applied.  Thanks!

On Fri, Oct 14, 2022 at 3:03 AM Christian König
<christian.koenig@....com> wrote:
>
> Am 13.10.22 um 23:07 schrieb Fabio M. De Francesco:
> > The use of kmap() is being deprecated in favor of kmap_local_page().
> >
> > There are two main problems with kmap(): (1) It comes with an overhead as
> > the mapping space is restricted and protected by a global lock for
> > synchronization and (2) it also requires global TLB invalidation when the
> > kmap’s pool wraps and it might block when the mapping space is fully
> > utilized until a slot becomes available.
> >
> > With kmap_local_page() the mappings are per thread, CPU local, can take
> > page faults, and can be called from any context (including interrupts).
> > It is faster than kmap() in kernels with HIGHMEM enabled. Furthermore,
> > the tasks can be preempted and, when they are scheduled to run again, the
> > kernel virtual addresses are restored and still valid.
> >
> > Therefore, replace kmap() with kmap_local_page() in radeon_ttm_gtt_read().
> >
> > Cc: "Venkataramanan, Anirudh" <anirudh.venkataramanan@...el.com>
> > Suggested-by: Ira Weiny <ira.weiny@...el.com>
> > Signed-off-by: Fabio M. De Francesco <fmdefrancesco@...il.com>
>
> Reviewed-by: Christian König <christian.koenig@....com>
>
> > ---
> >   drivers/gpu/drm/radeon/radeon_ttm.c | 4 ++--
> >   1 file changed, 2 insertions(+), 2 deletions(-)
> >
> > diff --git a/drivers/gpu/drm/radeon/radeon_ttm.c b/drivers/gpu/drm/radeon/radeon_ttm.c
> > index d33fec488713..bdb4c0e0736b 100644
> > --- a/drivers/gpu/drm/radeon/radeon_ttm.c
> > +++ b/drivers/gpu/drm/radeon/radeon_ttm.c
> > @@ -869,11 +869,11 @@ static ssize_t radeon_ttm_gtt_read(struct file *f, char __user *buf,
> >
> >               page = rdev->gart.pages[p];
> >               if (page) {
> > -                     ptr = kmap(page);
> > +                     ptr = kmap_local_page(page);
> >                       ptr += off;
> >
> >                       r = copy_to_user(buf, ptr, cur_size);
> > -                     kunmap(rdev->gart.pages[p]);
> > +                     kunmap_local(ptr);
> >               } else
> >                       r = clear_user(buf, cur_size);
> >
>