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

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Date:   Mon, 2 Oct 2023 22:28:13 +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 9/26/23 10:35, Boris Brezillon wrote:
>> 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().
> If you go this way, I would argue that drm_gem_shmem_swapin_locked() is
> just as incorrect as drm_gem_shmem_swapout_locked(), in that
> drm_gem_get_pages() might just return pages that were flagged
> reclaimable but never reclaimed/swapped-out. I do think that having
> some symmetry in the naming makes more sense than being 100% accurate.

That function is internal to drm-shmem and is used for both eviction and
purging. Having "swap-out" invoked by the purging also doesn't sound good.

Given that the function in question mainly "unmaps" the pages, what
about drm_gem_shmem_shkinker_unmap_pages_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.
> Okay, that's questionable. Why would we not want to reclaim BOs that
> are only mapped in userspace (sgt == NULL && pages_use_count > 0 &&
> pages_pin_count == 0)? I agree that creating such a BO would be
> pointless (why create a buffer through DRM if it's not passed to the
> GPU), but that's still something the API allows...

This is a pre-existing behaviour. There is no driver that uses pages
without sgt, hence there is nobody to test such code paths.

Maybe will worth to explicitly prohibit usage of get_pages() without
having sgt for clarity. But this should be separate to this patchset, IMO.

-- 
Best regards,
Dmitry