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Commit 7c55e2c5 authored by Chris Wilson's avatar Chris Wilson
Browse files

drm/i915: Use pagecache write to prepopulate shmemfs from pwrite-ioctl 

Before we instantiate/pin the backing store for our use, we
can prepopulate the shmemfs filp efficiently using a write into the
pagecache. We avoid the penalty of instantiating all the pages, important
if the user is just writing to a few and never uses the object on the GPU,
and using a direct write into shmemfs allows it to avoid the cost of
retrieving a page (mostly the clear-before-use, but in theory we could
curtail swapin) before it is overwritten.

This can be extended later to provide additional specialisation for
other backends (other than shmemfs). For now it provides a defense
against very large write-only allocations from exhausting all of system
memory.

v2: Smelling fixes.

Fixes: fe115628 ("drm/i915: Implement pwrite without struct-mutex")
References: https://bugs.freedesktop.org/show_bug.cgi?id=99107


Signed-off-by: default avatarChris Wilson <chris@chris-wilson.co.uk>
Cc: Matthew Auld <matthew.william.auld@gmail.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Cc: <stable@vger.kernel.org> # v4.10+
Reviewed-by: default avatarJoonas Lahtinen <joonas.lahtinen@linux.intel.com>
Reviewed-by: default avatarTvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170307120338.7277-2-chris@chris-wilson.co.uk
parent 4e5462ee

drivers/gpu/drm/i915/i915_gem.c

+78 −0
Original line number Diff line number Diff line
@@ -1452,6 +1452,12 @@ i915_gem_pwrite_ioctl(struct drm_device *dev, void *data, 


	trace_i915_gem_object_pwrite(obj, args->offset, args->size);



	ret = -ENODEV;

	if (obj->ops->pwrite)

		ret = obj->ops->pwrite(obj, args);

	if (ret != -ENODEV)

		goto err;



	ret = i915_gem_object_wait(obj,

				   I915_WAIT_INTERRUPTIBLE |

				   I915_WAIT_ALL,
@@ -2578,6 +2584,75 @@ void *i915_gem_object_pin_map(struct drm_i915_gem_object *obj, 
	goto out_unlock;

}



static int

i915_gem_object_pwrite_gtt(struct drm_i915_gem_object *obj,

			   const struct drm_i915_gem_pwrite *arg)

{

	struct address_space *mapping = obj->base.filp->f_mapping;

	char __user *user_data = u64_to_user_ptr(arg->data_ptr);

	u64 remain, offset;

	unsigned int pg;



	/* Before we instantiate/pin the backing store for our use, we

	 * can prepopulate the shmemfs filp efficiently using a write into

	 * the pagecache. We avoid the penalty of instantiating all the

	 * pages, important if the user is just writing to a few and never

	 * uses the object on the GPU, and using a direct write into shmemfs

	 * allows it to avoid the cost of retrieving a page (either swapin

	 * or clearing-before-use) before it is overwritten.

	 */

	if (READ_ONCE(obj->mm.pages))

		return -ENODEV;



	/* Before the pages are instantiated the object is treated as being

	 * in the CPU domain. The pages will be clflushed as required before

	 * use, and we can freely write into the pages directly. If userspace

	 * races pwrite with any other operation; corruption will ensue -

	 * that is userspace's prerogative!

	 */



	remain = arg->size;

	offset = arg->offset;

	pg = offset_in_page(offset);



	do {

		unsigned int len, unwritten;

		struct page *page;

		void *data, *vaddr;

		int err;



		len = PAGE_SIZE - pg;

		if (len > remain)

			len = remain;



		err = pagecache_write_begin(obj->base.filp, mapping,

					    offset, len, 0,

					    &page, &data);

		if (err < 0)

			return err;



		vaddr = kmap(page);

		unwritten = copy_from_user(vaddr + pg, user_data, len);

		kunmap(page);



		err = pagecache_write_end(obj->base.filp, mapping,

					  offset, len, len - unwritten,

					  page, data);

		if (err < 0)

			return err;



		if (unwritten)

			return -EFAULT;



		remain -= len;

		user_data += len;

		offset += len;

		pg = 0;

	} while (remain);



	return 0;

}



static bool ban_context(const struct i915_gem_context *ctx)

{

	return (i915_gem_context_is_bannable(ctx) &&
@@ -3994,8 +4069,11 @@ void i915_gem_object_init(struct drm_i915_gem_object *obj, 
static const struct drm_i915_gem_object_ops i915_gem_object_ops = {

	.flags = I915_GEM_OBJECT_HAS_STRUCT_PAGE |

		 I915_GEM_OBJECT_IS_SHRINKABLE,



	.get_pages = i915_gem_object_get_pages_gtt,

	.put_pages = i915_gem_object_put_pages_gtt,



	.pwrite = i915_gem_object_pwrite_gtt,

};



struct drm_i915_gem_object *

drivers/gpu/drm/i915/i915_gem_object.h

+3 −0
Original line number Diff line number Diff line
@@ -56,6 +56,9 @@ struct drm_i915_gem_object_ops { 
	struct sg_table *(*get_pages)(struct drm_i915_gem_object *);

	void (*put_pages)(struct drm_i915_gem_object *, struct sg_table *);



	int (*pwrite)(struct drm_i915_gem_object *,

		      const struct drm_i915_gem_pwrite *);



	int (*dmabuf_export)(struct drm_i915_gem_object *);

	void (*release)(struct drm_i915_gem_object *);

};