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Date:   Tue, 26 Sep 2023 09:43:07 +0200
From:   Boris Brezillon <boris.brezillon@collabora.com>
To:     Dmitry Osipenko <dmitry.osipenko@collabora.com>
Cc:     David Airlie <airlied@gmail.com>,
        Gerd Hoffmann <kraxel@redhat.com>,
        Gurchetan Singh <gurchetansingh@chromium.org>,
        Chia-I Wu <olvaffe@gmail.com>, Daniel Vetter <daniel@ffwll.ch>,
        Maarten Lankhorst <maarten.lankhorst@linux.intel.com>,
        Maxime Ripard <mripard@kernel.org>,
        Thomas Zimmermann <tzimmermann@suse.de>,
        Christian =?UTF-8?B?S8O2bmln?= <christian.koenig@amd.com>,
        Qiang Yu <yuq825@gmail.com>,
        Steven Price <steven.price@arm.com>,
        Emma Anholt <emma@anholt.net>, Melissa Wen <mwen@igalia.com>,
        dri-devel@lists.freedesktop.org, linux-kernel@vger.kernel.org,
        kernel@collabora.com, virtualization@lists.linux-foundation.org
Subject: Re: [PATCH v17 13/18] drm/shmem-helper: Add memory shrinker
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        <20230915104633.0d5c3932@collabora.com>
        <64e8708a-bb73-96ae-90af-f7b51317613b@collabora.com>
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On Tue, 26 Sep 2023 03:37:22 +0300
Dmitry Osipenko <dmitry.osipenko@collabora.com> wrote:

> On 9/15/23 11:46, Boris Brezillon wrote:
> >> -static int drm_gem_shmem_get_pages_locked(struct drm_gem_shmem_object *shmem)
> >> +static int
> >> +drm_gem_shmem_acquire_pages(struct drm_gem_shmem_object *shmem, bool init)
> >>  {
> >>  	struct drm_gem_object *obj = &shmem->base;
> >>  	struct page **pages;
> >>  
> >>  	dma_resv_assert_held(shmem->base.resv);
> >>  
> >> -	if (refcount_inc_not_zero(&shmem->pages_use_count))
> >> +	if (shmem->madv < 0) {
> >> +		drm_WARN_ON(obj->dev, shmem->pages);
> >> +		return -ENOMEM;
> >> +	}
> >> +
> >> +	if (shmem->pages) {
> >> +		drm_WARN_ON(obj->dev, !shmem->evicted);
> >>  		return 0;
> >> +	}
> >> +
> >> +	if (drm_WARN_ON(obj->dev, !(init ^ refcount_read(&shmem->pages_use_count))))
> >> +		return -EINVAL;  
> > OOC, why do we care? Is there any difference between initial and re-pin
> > that make the page allocation impossible? Feels like, if there's a
> > check to do, it should be done in the caller instead, and you can drop
> > the init param here.  
> 
> This is a sanity check that addresses additional refcnt tracking
> complexity imposed by shrinker.
> 
> This function is used by both init and re-pin that is invoked from
> several places in the code. It's not trivial to move that check to the
> callers.

drm_gem_shmem_acquire_pages() is called twice, once with init=false,
once with init=true. If you really care about this check, it can
be moved to the callers so

1/ it's clearer (the XOR operation between init and refcount to check if
refcount is zero on init and non-zero otherwise is convoluted)
2/ it doesn't leak to the function whose purpose it to [re-]acquire
pages