linux-kernel - Re: [PATCH v17 13/18] drm/shmem-helper: Add memory shrinker

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Message-ID: <64e8708a-bb73-96ae-90af-f7b51317613b@collabora.com>
Date:   Tue, 26 Sep 2023 03:37:22 +0300
From:   Dmitry Osipenko <dmitry.osipenko@...labora.com>
To:     Boris Brezillon <boris.brezillon@...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 v17 13/18] drm/shmem-helper: Add memory shrinker

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.

-- 
Best regards,
Dmitry