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

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Date:   Tue, 26 Sep 2023 09:35:17 +0200
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 v17 13/18] drm/shmem-helper: Add memory shrinker

On Tue, 26 Sep 2023 03:30:35 +0300
Dmitry Osipenko <dmitry.osipenko@...labora.com> 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.

> 
> >>  {
> >>  	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...

> 
> >> +	__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.

Sorry to disagree, but I do think it matters to have a clear ownership
model, and if I look at the code (drm_gem_shmem_get_pages_sgt_locked()),
the sgt clearly owns a reference to the pages it points to.