Re: [Linaro-mm-sig] [PATCH v4 04/13] drm/shmem-helper: Provide a vmap function for short-term mappings

On Tue, Jan 12, 2021 at 02:11:24PM +0100, Thomas Zimmermann wrote:

Hi

Am 11.01.21 um 17:50 schrieb Daniel Vetter:

...

On Fri, Jan 08, 2021 at 10:43:31AM +0100, Thomas Zimmermann wrote:

...

Implementations of the vmap/vunmap GEM callbacks may perform pinning of the BO and may acquire the associated reservation object's lock. Callers that only require a mapping of the contained memory can thus interfere with other tasks that require exact pinning, such as scanout. This is less of an issue with private SHMEM buffers, but may happen with imported ones.

Therefore provide the new interfaces drm_gem_shmem_vmap_local() and drm_gem_shmem_vunmap_local(), which only perform the vmap/vunmap operations. Callers have to hold the reservation lock while the mapping persists.

This patch also connects GEM SHMEM helpers to GEM object functions with equivalent functionality.

v4:

• call dma_buf_{vmap,vunmap}_local() where necessary (Daniel)
• move driver changes into separate patches (Daniel)

Signed-off-by: Thomas Zimmermann tzimmermann@suse.de

drivers/gpu/drm/drm_gem_shmem_helper.c | 90 +++++++++++++++++++++++--- include/drm/drm_gem_shmem_helper.h | 2 + 2 files changed, 84 insertions(+), 8 deletions(-)

diff --git a/drivers/gpu/drm/drm_gem_shmem_helper.c b/drivers/gpu/drm/drm_gem_shmem_helper.c index 9825c378dfa6..298832b2b43b 100644 --- a/drivers/gpu/drm/drm_gem_shmem_helper.c +++ b/drivers/gpu/drm/drm_gem_shmem_helper.c @@ -32,6 +32,8 @@ static const struct drm_gem_object_funcs drm_gem_shmem_funcs = { .get_sg_table = drm_gem_shmem_get_sg_table, .vmap = drm_gem_shmem_vmap, .vunmap = drm_gem_shmem_vunmap,

• .vmap_local = drm_gem_shmem_vmap_local,
• .vunmap_local = drm_gem_shmem_vunmap_local, .mmap = drm_gem_shmem_mmap, };

@@ -261,7 +263,8 @@ void drm_gem_shmem_unpin(struct drm_gem_object *obj) } EXPORT_SYMBOL(drm_gem_shmem_unpin); -static int drm_gem_shmem_vmap_locked(struct drm_gem_shmem_object *shmem, struct dma_buf_map *map) +static int drm_gem_shmem_vmap_locked(struct drm_gem_shmem_object *shmem, struct dma_buf_map *map,

• 		     bool local)
  

This is a bit spaghetti and also has the problem that we're not changing shmem->vmap_use_count under different locks, depending upon which path we're taking.

I think the cleanest would be if we pull the if (import_attach) case out of the _locked() version completely, for all cases, and also outside of the shmem->vmap_lock. This means no caching of vmaps in the shmem layer anymore for imported buffers, but this is no longer a problem: We cache them in the exporters instead (I think at least, if not maybe need to fix that where it's expensive).

If we do that, what protects shmem->vaddr from concurrent access near line 281? would it be kept NULL then?

Also, we have some stats in debugfs (see drm_gem_shmem_print_info) which would be incorrect (or misleading at least).

We'd need to disable all that for pass-through vmap of imported objects.

Given all that, would it be possible to remove vmap_lock in favor of taking the resv lock in vmap/vunmap?

All possible (and imo long-term desirable), the trouble is in rolling it out. I've looked at rolling out dma_resv as the one and only lock for shmem helpers before, and gave up. Exynos is the worst (but not the only) offender: - it has it's own per-object lock - that per-object lock is taken most often before calling into various vfuncs, which means for a gradual transition the dma_resv lock would nest within that existing per-object lock (until we've completely replaced it) - but exynos also uses dma_resv already as an outermost lock in its command submission path

iow as soon as you add dma_resv_lock anywhere in shmem helpers, we've angered lockdep with a deadlock.

That means the only path I think is feasible is adding dma_resv lock to all drivers paths first, _outside_ of any existing driver specific per-object locks. Then remove the driver-specific object locks, and only then can we sprinkle dma_resv_assert_locked all over shmem helpers.

Ofc any driver without per-driver locks of their own could directly switch over to dma_resv lock, but until we've converted over all the drivers with their own locking shmem helpers would be stuck where they are right now.

I gave up :-/ But maybe if you only try to tackle vmap it might be feasible, since a lot fewer callers.

Cheers, Daniel

Best regards Thomas

...

Other option would be to unly pull it out for the _vmap_local case, but that's a bit ugly because no longer symmetrical in the various paths.

...

{ struct drm_gem_object *obj = &shmem->base; int ret = 0; @@ -272,7 +275,10 @@ static int drm_gem_shmem_vmap_locked(struct drm_gem_shmem_object *shmem, struct } if (obj->import_attach) {

• ret = dma_buf_vmap(obj->import_attach->dmabuf, map);
  

• if (local)
  

• 	ret = dma_buf_vmap_local(obj->import_attach->dmabuf, map);
  

• else
  

• 	ret = dma_buf_vmap(obj->import_attach->dmabuf, map);
  

  if (!ret) { if (WARN_ON(map->is_iomem)) { ret = -EIO;

@@ -313,7 +319,7 @@ static int drm_gem_shmem_vmap_locked(struct drm_gem_shmem_object *shmem, struct return ret; } -/* +/**

• drm_gem_shmem_vmap - Create a virtual mapping for a shmem GEM object
• @shmem: shmem GEM object
• @map: Returns the kernel virtual address of the SHMEM GEM object's backing

@@ -339,15 +345,53 @@ int drm_gem_shmem_vmap(struct drm_gem_object *obj, struct dma_buf_map *map) ret = mutex_lock_interruptible(&shmem->vmap_lock); if (ret) return ret;

• ret = drm_gem_shmem_vmap_locked(shmem, map);

• ret = drm_gem_shmem_vmap_locked(shmem, map, false); mutex_unlock(&shmem->vmap_lock); return ret; } EXPORT_SYMBOL(drm_gem_shmem_vmap);

+/**

• • drm_gem_shmem_vmap_local - Create a virtual mapping for a shmem GEM object
• • @shmem: shmem GEM object
• • @map: Returns the kernel virtual address of the SHMEM GEM object's backing

• •   store.
    

• • This function makes sure that a contiguous kernel virtual address mapping
• • exists for the buffer backing the shmem GEM object.
• • The function is called with the BO's reservation object locked. Callers must
• • hold the lock until after unmapping the buffer.
• • This function can be used to implement &drm_gem_object_funcs.vmap_local. But
• • it can also be called by drivers directly, in which case it will hide the
• • differences between dma-buf imported and natively allocated objects.

So for the other callbacks I tried to make sure we have different entry points for this, since it's not really the same thing and because of the locking mess we have with dma_resv_lock vs various pre-existing local locking scheme, it's easy to get a mess.

I think the super clean version here would be to also export just the internal stuff for the ->v(un)map_local hooks, but that's maybe a bit too much boilerplate for no real gain. -Daniel

...

• • Acquired mappings should be cleaned up by calling drm_gem_shmem_vunmap_local().
• • Returns:
• • 0 on success or a negative error code on failure.
• */

+int drm_gem_shmem_vmap_local(struct drm_gem_object *obj, struct dma_buf_map *map) +{

• struct drm_gem_shmem_object *shmem = to_drm_gem_shmem_obj(obj);
• int ret;
• dma_resv_assert_held(obj->resv);
• ret = mutex_lock_interruptible(&shmem->vmap_lock);
• if (ret)

• return ret;
  

• ret = drm_gem_shmem_vmap_locked(shmem, map, true);
• mutex_unlock(&shmem->vmap_lock);
• return ret;

+} +EXPORT_SYMBOL(drm_gem_shmem_vmap_local);

• static void drm_gem_shmem_vunmap_locked(struct drm_gem_shmem_object *shmem,

• 			struct dma_buf_map *map)
  

• 			struct dma_buf_map *map, bool local)
  

  { struct drm_gem_object *obj = &shmem->base;

@@ -358,7 +402,10 @@ static void drm_gem_shmem_vunmap_locked(struct drm_gem_shmem_object *shmem, return; if (obj->import_attach)

• dma_buf_vunmap(obj->import_attach->dmabuf, map);
  

• if (local)
  

• 	dma_buf_vunmap_local(obj->import_attach->dmabuf, map);
  

• else
  

• 	dma_buf_vunmap(obj->import_attach->dmabuf, map);
  

  else vunmap(shmem->vaddr);

@@ -366,7 +413,7 @@ static void drm_gem_shmem_vunmap_locked(struct drm_gem_shmem_object *shmem, drm_gem_shmem_put_pages(shmem); } -/* +/**

• drm_gem_shmem_vunmap - Unmap a virtual mapping fo a shmem GEM object
• @shmem: shmem GEM object
• @map: Kernel virtual address where the SHMEM GEM object was mapped

@@ -384,11 +431,38 @@ void drm_gem_shmem_vunmap(struct drm_gem_object *obj, struct dma_buf_map *map) struct drm_gem_shmem_object *shmem = to_drm_gem_shmem_obj(obj); mutex_lock(&shmem->vmap_lock);

• drm_gem_shmem_vunmap_locked(shmem, map);

• drm_gem_shmem_vunmap_locked(shmem, map, false); mutex_unlock(&shmem->vmap_lock); } EXPORT_SYMBOL(drm_gem_shmem_vunmap);

+/**

• • drm_gem_shmem_vunmap_local - Unmap a virtual mapping fo a shmem GEM object
• • @shmem: shmem GEM object
• • @map: Kernel virtual address where the SHMEM GEM object was mapped
• • This function cleans up a kernel virtual address mapping acquired by
• • drm_gem_shmem_vmap_local(). The mapping is only removed when the use count
• • drops to zero.
• • The function is called with the BO's reservation object locked.
• • This function can be used to implement &drm_gem_object_funcs.vmap_local.
• • But it can also be called by drivers directly, in which case it will hide
• • the differences between dma-buf imported and natively allocated objects.
• */

+void drm_gem_shmem_vunmap_local(struct drm_gem_object *obj, struct dma_buf_map *map) +{

• struct drm_gem_shmem_object *shmem = to_drm_gem_shmem_obj(obj);
• dma_resv_assert_held(obj->resv);
• mutex_lock(&shmem->vmap_lock);
• drm_gem_shmem_vunmap_locked(shmem, map, true);
• mutex_unlock(&shmem->vmap_lock);

+} +EXPORT_SYMBOL(drm_gem_shmem_vunmap_local);

• struct drm_gem_shmem_object * drm_gem_shmem_create_with_handle(struct drm_file *file_priv, struct drm_device *dev, size_t size,

diff --git a/include/drm/drm_gem_shmem_helper.h b/include/drm/drm_gem_shmem_helper.h index 434328d8a0d9..3f59bdf749aa 100644 --- a/include/drm/drm_gem_shmem_helper.h +++ b/include/drm/drm_gem_shmem_helper.h @@ -114,7 +114,9 @@ void drm_gem_shmem_put_pages(struct drm_gem_shmem_object *shmem); int drm_gem_shmem_pin(struct drm_gem_object *obj); void drm_gem_shmem_unpin(struct drm_gem_object *obj); int drm_gem_shmem_vmap(struct drm_gem_object *obj, struct dma_buf_map *map); +int drm_gem_shmem_vmap_local(struct drm_gem_object *obj, struct dma_buf_map *map); void drm_gem_shmem_vunmap(struct drm_gem_object *obj, struct dma_buf_map *map); +void drm_gem_shmem_vunmap_local(struct drm_gem_object *obj, struct dma_buf_map *map); int drm_gem_shmem_madvise(struct drm_gem_object *obj, int madv); -- 2.29.2

-- Thomas Zimmermann Graphics Driver Developer SUSE Software Solutions Germany GmbH Maxfeldstr. 5, 90409 Nürnberg, Germany (HRB 36809, AG Nürnberg) Geschäftsführer: Felix Imendörffer