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

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Date:   Tue, 26 Sep 2023 03:30:35 +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:
> The naming becomes quite confusing, with drm_gem_shmem_unpin_locked()
> and drm_gem_shmem_unpin_pages_locked(). By the look of it, it seems to
> do exactly the opposite of drm_gem_shmem_swapin_locked(), except for
> the missing ->evicted = true, which we can move here anyway, given
> drm_gem_shmem_purge_locked() explicitly set it to false anyway. The
> other thing that's missing is the
> drm_gem_shmem_update_pages_state_locked(), but it can also be moved
> there I think, if the the ->madv update happens before the
> drm_gem_shmem_unpin_pages_locked() call in
> drm_gem_shmem_purge_locked().
> 
> So, how about renaming this function drm_gem_shmem_swapout_locked()?

The swapout name would be misleading to me because pages aren't moved to
swap, but allowed to be moved. I'll rename it to
drm_gem_shmem_shrinker_unpin_locked().

>>  {
>>  	struct drm_gem_object *obj = &shmem->base;
>>  	struct drm_device *dev = obj->dev;
>>  
>>  	dma_resv_assert_held(shmem->base.resv);
>>  
>> -	drm_WARN_ON(obj->dev, !drm_gem_shmem_is_purgeable(shmem));
>> +	if (shmem->evicted)
>> +		return;
>>  
>>  	dma_unmap_sgtable(dev->dev, shmem->sgt, DMA_BIDIRECTIONAL, 0);
> Are we sure we'll always have sgt != NULL? IIRC, if the GEM is only
> mmap-ed in userspace, get_sgt() is not necessarily called by the driver
> (needed to map in GPU space), and we have a potential NULL deref here.
> Maybe that changed at some point in the series, and sgt is
> unconditionally populated when get_pages() is called now.

The sgt is always set in this function because it's part of shrinker and
shrinker doesn't touch GEMs without sgt.

>> +	__drm_gem_shmem_release_pages(shmem);
> Make sure you drop the implicit pages_use_count ref the sgt had, this
> way you can still tie the necessity to drop the pages to sgt != NULL in
> drm_gem_shmem_free().

This will require further refcnt re-initialization when pages are
restored if it's dropped to zero. I don't see how this will improve
anything.

-- 
Best regards,
Dmitry