linux-kernel - Re: [PATCH v18 11/26] drm/shmem-helper: Prepare drm_gem_shmem

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Date:   Mon, 20 Nov 2023 12:19:42 +0100
From:   Boris Brezillon <boris.brezillon@...labora.com>
To:     Dmitry Osipenko <dmitry.osipenko@...labora.com>
Cc:     David Airlie <airlied@...il.com>,
        Gerd Hoffmann <kraxel@...hat.com>,
        Gurchetan Singh <gurchetansingh@...omium.org>,
        Chia-I Wu <olvaffe@...il.com>, Daniel Vetter <daniel@...ll.ch>,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Maxime Ripard <mripard@...nel.org>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        Christian König <christian.koenig@....com>,
        Qiang Yu <yuq825@...il.com>,
        Steven Price <steven.price@....com>,
        Emma Anholt <emma@...olt.net>, Melissa Wen <mwen@...lia.com>,
        dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
        kernel@...labora.com, virtualization@...ts.linux-foundation.org
Subject: Re: [PATCH v18 11/26] drm/shmem-helper: Prepare
 drm_gem_shmem_free() to shrinker addition

On Mon, 20 Nov 2023 14:02:29 +0300
Dmitry Osipenko <dmitry.osipenko@...labora.com> wrote:

> On 11/10/23 13:16, Boris Brezillon wrote:
> > On Mon, 30 Oct 2023 02:01:50 +0300
> > Dmitry Osipenko <dmitry.osipenko@...labora.com> wrote:
> >   
> >> Prepare drm_gem_shmem_free() to addition of memory shrinker support
> >> to drm-shmem by adding and using variant of put_pages() that doesn't
> >> touch reservation lock. Reservation shouldn't be touched because lockdep
> >> will trigger a bogus warning about locking contention with fs_reclaim
> >> code paths that can't happen during the time when GEM is freed and
> >> lockdep doesn't know about that.
> >>
> >> Signed-off-by: Dmitry Osipenko <dmitry.osipenko@...labora.com>
> >> ---
> >>  drivers/gpu/drm/drm_gem_shmem_helper.c | 35 +++++++++++++-------------
> >>  1 file changed, 18 insertions(+), 17 deletions(-)
> >>
> >> diff --git a/drivers/gpu/drm/drm_gem_shmem_helper.c b/drivers/gpu/drm/drm_gem_shmem_helper.c
> >> index 08b5a57c59d8..24ff2b99e75b 100644
> >> --- a/drivers/gpu/drm/drm_gem_shmem_helper.c
> >> +++ b/drivers/gpu/drm/drm_gem_shmem_helper.c
> >> @@ -128,6 +128,22 @@ struct drm_gem_shmem_object *drm_gem_shmem_create(struct drm_device *dev, size_t
> >>  }
> >>  EXPORT_SYMBOL_GPL(drm_gem_shmem_create);
> >>  
> >> +static void
> >> +drm_gem_shmem_free_pages(struct drm_gem_shmem_object *shmem)
> >> +{
> >> +	struct drm_gem_object *obj = &shmem->base;
> >> +
> >> +#ifdef CONFIG_X86
> >> +	if (shmem->map_wc)
> >> +		set_pages_array_wb(shmem->pages, obj->size >> PAGE_SHIFT);
> >> +#endif
> >> +
> >> +	drm_gem_put_pages(obj, shmem->pages,
> >> +			  shmem->pages_mark_dirty_on_put,
> >> +			  shmem->pages_mark_accessed_on_put);
> >> +	shmem->pages = NULL;
> >> +}
> >> +
> >>  /**
> >>   * drm_gem_shmem_free - Free resources associated with a shmem GEM object
> >>   * @shmem: shmem GEM object to free
> >> @@ -142,8 +158,6 @@ void drm_gem_shmem_free(struct drm_gem_shmem_object *shmem)
> >>  	if (obj->import_attach) {
> >>  		drm_prime_gem_destroy(obj, shmem->sgt);
> >>  	} else {
> >> -		dma_resv_lock(shmem->base.resv, NULL);
> >> -
> >>  		drm_WARN_ON(obj->dev, refcount_read(&shmem->vmap_use_count));
> >>  
> >>  		if (shmem->sgt) {
> >> @@ -157,8 +171,6 @@ void drm_gem_shmem_free(struct drm_gem_shmem_object *shmem)
> >>    
> > If you drop the dma_resv_lock/unlock(), you should also replace the
> > drm_gem_shmem_put_pages_locked() by a drm_gem_shmem_free_pages() in this
> > commit.  
> 
> drm_gem_shmem_put_pages_locked() is exported by a later patch of this
> series, it's not worthwhile to remove this function

I'm not talking about removing drm_gem_shmem_put_pages_locked(), but
replacing the drm_gem_shmem_put_pages_locked() call you have in
drm_gem_shmem_free() by a drm_gem_shmem_free_pages(), so you don't end
up with a lockdep warning when you stop exactly here in the patch
series, which is important if we want to keep things bisectable.