Re: [PATCH 1/2] drm/ttm: rework ttm_tt page limit v2

[Daniel Vetter <daniel@ffwll.ch>]

On Wed, Dec 16, 2020 at 03:04:26PM +0100, Christian König wrote:
> TTM implements a rather extensive accounting of allocated memory.
> 
> There are two reasons for this:
> 1. It tries to block userspace allocating a huge number of very small
>    BOs without accounting for the kmalloced memory.
> 
> 2. Make sure we don't over allocate and run into an OOM situation
>    during swapout while trying to handle the memory shortage.
> 
> This is only partially a good idea. First of all it is perfectly
> valid for an application to use all of system memory, limiting it to
> 50% is not really acceptable.
> 
> What we need to take care of is that the application is held
> accountable for the memory it allocated. This is what control
> mechanisms like memcg and the normal Linux page accounting already do.
> 
> Making sure that we don't run into an OOM situation while trying to
> cope with a memory shortage is still a good idea, but this is also
> not very well implemented since it means another opportunity of
> recursion from the driver back into TTM.
> 
> So start to rework all of this by implementing a shrinker callback which
> allows for TT object to be swapped out if necessary.
> 
> v2: Switch from limit to shrinker callback.
> 
> Signed-off-by: Christian König <christian.koenig@amd.com>
> ---
>  drivers/gpu/drm/ttm/ttm_bo.c        |  4 +-
>  drivers/gpu/drm/ttm/ttm_memory.c    |  7 ++-
>  drivers/gpu/drm/ttm/ttm_tt.c        | 82 +++++++++++++++++++++++++++--
>  drivers/gpu/drm/vmwgfx/vmwgfx_drv.c |  2 +-
>  include/drm/ttm/ttm_bo_api.h        |  2 +-
>  include/drm/ttm/ttm_tt.h            |  6 ++-
>  6 files changed, 91 insertions(+), 12 deletions(-)
> 
> diff --git a/drivers/gpu/drm/ttm/ttm_bo.c b/drivers/gpu/drm/ttm/ttm_bo.c
> index 31e8b3da5563..f1f3fd085465 100644
> --- a/drivers/gpu/drm/ttm/ttm_bo.c
> +++ b/drivers/gpu/drm/ttm/ttm_bo.c
> @@ -1412,7 +1412,7 @@ EXPORT_SYMBOL(ttm_bo_wait);
>   * A buffer object shrink method that tries to swap out the first
>   * buffer object on the bo_global::swap_lru list.
>   */
> -int ttm_bo_swapout(struct ttm_operation_ctx *ctx)
> +int ttm_bo_swapout(struct ttm_operation_ctx *ctx, gfp_t gfp_flags)
>  {
>  	struct ttm_bo_global *glob = &ttm_bo_glob;
>  	struct ttm_buffer_object *bo;
> @@ -1495,7 +1495,7 @@ int ttm_bo_swapout(struct ttm_operation_ctx *ctx)
>  	if (bo->bdev->driver->swap_notify)
>  		bo->bdev->driver->swap_notify(bo);
>  
> -	ret = ttm_tt_swapout(bo->bdev, bo->ttm);
> +	ret = ttm_tt_swapout(bo->bdev, bo->ttm, gfp_flags);
>  out:
>  
>  	/**
> diff --git a/drivers/gpu/drm/ttm/ttm_memory.c b/drivers/gpu/drm/ttm/ttm_memory.c
> index a3bfbd9cea68..3d2479d0ce2c 100644
> --- a/drivers/gpu/drm/ttm/ttm_memory.c
> +++ b/drivers/gpu/drm/ttm/ttm_memory.c
> @@ -37,6 +37,7 @@
>  #include <linux/slab.h>
>  #include <linux/swap.h>
>  #include <drm/ttm/ttm_pool.h>
> +#include <drm/ttm/ttm_tt.h>
>  
>  #include "ttm_module.h"
>  
> @@ -276,9 +277,9 @@ static void ttm_shrink(struct ttm_mem_global *glob, bool from_wq,
>  
>  	while (ttm_zones_above_swap_target(glob, from_wq, extra)) {
>  		spin_unlock(&glob->lock);
> -		ret = ttm_bo_swapout(ctx);
> +		ret = ttm_bo_swapout(ctx, 0);

General we don't treat gfp_mask as a set of additional flags, but the full
thing. So here we should have GFP_KERNEL.

Also having both the shrinker and the ttm_shrink_work is a bit much, the
shrink work should get deleted completely I think.

>  		spin_lock(&glob->lock);
> -		if (unlikely(ret != 0))
> +		if (unlikely(ret < 0))
>  			break;
>  	}
>  
> @@ -453,6 +454,7 @@ int ttm_mem_global_init(struct ttm_mem_global *glob)
>  			zone->name, (unsigned long long)zone->max_mem >> 10);
>  	}
>  	ttm_pool_mgr_init(glob->zone_kernel->max_mem/(2*PAGE_SIZE));
> +	ttm_tt_mgr_init();
>  	return 0;
>  out_no_zone:
>  	ttm_mem_global_release(glob);
> @@ -466,6 +468,7 @@ void ttm_mem_global_release(struct ttm_mem_global *glob)
>  
>  	/* let the page allocator first stop the shrink work. */
>  	ttm_pool_mgr_fini();
> +	ttm_tt_mgr_fini();
>  
>  	flush_workqueue(glob->swap_queue);
>  	destroy_workqueue(glob->swap_queue);
> diff --git a/drivers/gpu/drm/ttm/ttm_tt.c b/drivers/gpu/drm/ttm/ttm_tt.c
> index 7f75a13163f0..d454c428c56a 100644
> --- a/drivers/gpu/drm/ttm/ttm_tt.c
> +++ b/drivers/gpu/drm/ttm/ttm_tt.c
> @@ -38,6 +38,8 @@
>  #include <drm/drm_cache.h>
>  #include <drm/ttm/ttm_bo_driver.h>
>  
> +static struct shrinker mm_shrinker;
> +
>  /*
>   * Allocates a ttm structure for the given BO.
>   */
> @@ -223,13 +225,23 @@ int ttm_tt_swapin(struct ttm_tt *ttm)
>  	return ret;
>  }
>  
> -int ttm_tt_swapout(struct ttm_bo_device *bdev, struct ttm_tt *ttm)
> +/**
> + * ttm_tt_swapout - swap out tt object
> + *
> + * @bdev: TTM device structure.
> + * @ttm: The struct ttm_tt.
> + * @gfp_flags: Flags to use for memory allocation.
> + *
> + * Swapout a TT object to a shmem_file, return number of pages swapped out or
> + * negative error code.
> + */
> +int ttm_tt_swapout(struct ttm_bo_device *bdev, struct ttm_tt *ttm,
> +		   gfp_t gfp_flags)
>  {
>  	struct address_space *swap_space;
>  	struct file *swap_storage;
>  	struct page *from_page;
>  	struct page *to_page;
> -	gfp_t gfp_mask;
>  	int i, ret;
>  
>  	swap_storage = shmem_file_setup("ttm swap",
> @@ -241,14 +253,14 @@ int ttm_tt_swapout(struct ttm_bo_device *bdev, struct ttm_tt *ttm)
>  	}
>  
>  	swap_space = swap_storage->f_mapping;
> -	gfp_mask = mapping_gfp_mask(swap_space);
> +	gfp_flags |= mapping_gfp_mask(swap_space);

I don't think this combines flags correctly. mapping_gfp_mask is most
likely GFP_KERNEL or something like that, so __GFP_FS (the thing you want
to check for to avoid recursion) is always set.

I think we need an & here, and maybe screaming if the gfp flags for the
swapout space are funny.

>  
>  	for (i = 0; i < ttm->num_pages; ++i) {
>  		from_page = ttm->pages[i];
>  		if (unlikely(from_page == NULL))
>  			continue;
>  
> -		to_page = shmem_read_mapping_page_gfp(swap_space, i, gfp_mask);
> +		to_page = shmem_read_mapping_page_gfp(swap_space, i, gfp_flags);
>  		if (IS_ERR(to_page)) {
>  			ret = PTR_ERR(to_page);
>  			goto out_err;
> @@ -263,7 +275,7 @@ int ttm_tt_swapout(struct ttm_bo_device *bdev, struct ttm_tt *ttm)
>  	ttm->swap_storage = swap_storage;
>  	ttm->page_flags |= TTM_PAGE_FLAG_SWAPPED;
>  
> -	return 0;
> +	return ttm->num_pages;
>  
>  out_err:
>  	fput(swap_storage);
> @@ -341,3 +353,63 @@ void ttm_tt_unpopulate(struct ttm_bo_device *bdev,
>  		ttm_pool_free(&bdev->pool, ttm);
>  	ttm->page_flags &= ~TTM_PAGE_FLAG_PRIV_POPULATED;
>  }
> +
> +/* As long as pages are available make sure to release at least one */
> +static unsigned long ttm_tt_shrinker_scan(struct shrinker *shrink,
> +					  struct shrink_control *sc)
> +{
> +	struct ttm_operation_ctx ctx = {
> +		.no_wait_gpu = true

Iirc there's an eventual shrinker limit where it gets desperate. I think
once we hit that, we should allow gpu waits. But it's not passed to
shrinkers for reasons, so maybe we should have a second round that tries
to more actively shrink objects if we fell substantially short of what
reclaim expected us to do?

Also don't we have a trylock_only flag here to make sure drivers don't do
something stupid?

> +	};
> +	int ret;
> +
> +	if (sc->gfp_mask & GFP_NOFS)
> +		return 0;
> +
> +	ret = ttm_bo_swapout(&ctx, GFP_NOFS);
> +	return ret < 0 ? SHRINK_EMPTY : ret;
> +}
> +
> +/* Return the number of pages available or SHRINK_EMPTY if we have none */
> +static unsigned long ttm_tt_shrinker_count(struct shrinker *shrink,
> +					   struct shrink_control *sc)
> +{
> +	struct ttm_buffer_object *bo;
> +	unsigned long num_pages = 0;
> +	unsigned int i;
> +
> +	if (sc->gfp_mask & GFP_NOFS)
> +		return 0;

The count function should always count, and I'm not seeing a reason why
you couldn't do that here ... Also my understanding is that GFP_NOFS never
goes into shrinkers (the NOFS comes from shrinkers originally only being
used for filesystem objects), so this is double redundant.

My understanding is that gfp_mask is just to convey the right zones and
stuff, so that your shrinker can try to shrink objects in the right zones.
Hence I think the check in the _scan() function should also be removed.

Also the non __ prefixed flags are the combinations callers are supposed
to look at. Memory reclaim code needs to look at the __GFP flags, see e.g. 
gfpflags_allow_blocking() or fs_reclaim_acquire().

> +
> +	/* TODO: Is that to much overhead? */
> +	spin_lock(&ttm_bo_glob.lru_lock);
> +	for (i = 0; i < TTM_MAX_BO_PRIORITY; ++i)
> +		list_for_each_entry(bo, &ttm_bo_glob.swap_lru[i], swap)
> +			num_pages += bo->ttm->num_pages;
> +	spin_unlock(&ttm_bo_glob.lru_lock);
> +
> +	return num_pages ? num_pages : SHRINK_EMPTY;
> +}
> +
> +/**
> + * ttm_tt_mgr_init - register with the MM shrinker
> + *
> + * Register with the MM shrinker for swapping out BOs.
> + */
> +int ttm_tt_mgr_init(void)
> +{
> +	mm_shrinker.count_objects = ttm_tt_shrinker_count;
> +	mm_shrinker.scan_objects = ttm_tt_shrinker_scan;
> +	mm_shrinker.seeks = 1;
> +	return register_shrinker(&mm_shrinker);
> +}
> +
> +/**
> + * ttm_tt_mgr_fini - unregister our MM shrinker
> + *
> + * Unregisters the MM shrinker.
> + */
> +void ttm_tt_mgr_fini(void)
> +{
> +	unregister_shrinker(&mm_shrinker);
> +}
> diff --git a/drivers/gpu/drm/vmwgfx/vmwgfx_drv.c b/drivers/gpu/drm/vmwgfx/vmwgfx_drv.c
> index 0008be02d31c..e141fa9f616d 100644
> --- a/drivers/gpu/drm/vmwgfx/vmwgfx_drv.c
> +++ b/drivers/gpu/drm/vmwgfx/vmwgfx_drv.c
> @@ -1389,7 +1389,7 @@ static int vmw_pm_freeze(struct device *kdev)
>  	vmw_execbuf_release_pinned_bo(dev_priv);
>  	vmw_resource_evict_all(dev_priv);
>  	vmw_release_device_early(dev_priv);
> -	while (ttm_bo_swapout(&ctx) == 0);
> +	while (ttm_bo_swapout(&ctx, 0) > 0);

I think again GFP_KERNEL.

>  	if (dev_priv->enable_fb)
>  		vmw_fifo_resource_dec(dev_priv);
>  	if (atomic_read(&dev_priv->num_fifo_resources) != 0) {
> diff --git a/include/drm/ttm/ttm_bo_api.h b/include/drm/ttm/ttm_bo_api.h
> index e17be324d95f..7c7539d76e1d 100644
> --- a/include/drm/ttm/ttm_bo_api.h
> +++ b/include/drm/ttm/ttm_bo_api.h
> @@ -569,7 +569,7 @@ ssize_t ttm_bo_io(struct ttm_bo_device *bdev, struct file *filp,
>  		  const char __user *wbuf, char __user *rbuf,
>  		  size_t count, loff_t *f_pos, bool write);
>  
> -int ttm_bo_swapout(struct ttm_operation_ctx *ctx);
> +int ttm_bo_swapout(struct ttm_operation_ctx *ctx, gfp_t gfp_flags);
>  
>  /**
>   * ttm_bo_uses_embedded_gem_object - check if the given bo uses the
> diff --git a/include/drm/ttm/ttm_tt.h b/include/drm/ttm/ttm_tt.h
> index 6c8eb9a4de81..4bc3a08644fc 100644
> --- a/include/drm/ttm/ttm_tt.h
> +++ b/include/drm/ttm/ttm_tt.h
> @@ -135,7 +135,8 @@ void ttm_tt_destroy_common(struct ttm_bo_device *bdev, struct ttm_tt *ttm);
>   * Swap in a previously swap out ttm_tt.
>   */
>  int ttm_tt_swapin(struct ttm_tt *ttm);
> -int ttm_tt_swapout(struct ttm_bo_device *bdev, struct ttm_tt *ttm);
> +int ttm_tt_swapout(struct ttm_bo_device *bdev, struct ttm_tt *ttm,
> +		   gfp_t gfp_flags);
>  
>  /**
>   * ttm_tt_populate - allocate pages for a ttm
> @@ -155,6 +156,9 @@ int ttm_tt_populate(struct ttm_bo_device *bdev, struct ttm_tt *ttm, struct ttm_o
>   */
>  void ttm_tt_unpopulate(struct ttm_bo_device *bdev, struct ttm_tt *ttm);
>  
> +int ttm_tt_mgr_init(void);
> +void ttm_tt_mgr_fini(void);
> +
>  #if IS_ENABLED(CONFIG_AGP)
>  #include <linux/agp_backend.h>

For testing I strongly recommend a debugfs file to trigger this shrinker
completely, from the right lockdep context (i.e. using
fs_reclaim_acquire/release()). Much easier to test that way. See
i915_drop_caches_set() in i915_debugfs.c.

That way you can fully test it all without hitting anything remotely
resembling actual OOM, which tends to kill all kinds of things.

Aside from the detail work I think this is going in the right direction.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch
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