Date:	Sun, 20 Nov 2011 21:56:32 -0800
From:	Ben Widawsky <ben@...dawsk.net>
To:	Daniel Vetter <daniel.vetter@...ll.ch>
Cc:	intel-gfx <intel-gfx@...ts.freedesktop.org>,
	linux-kernel@...r.kernel.org, dri-devel@...ts.freedesktop.org
Subject: Re: [Intel-gfx] [PATCH 02/13] drm/i915: rewrite shmem_pwrite_slow
 to use copy_from_user

On Sun,  6 Nov 2011 20:13:49 +0100
Daniel Vetter <daniel.vetter@...ll.ch> wrote:

> ... instead of get_user_pages, because that fails on non page-backed
> user addresses like e.g. a gtt mapping of a bo.
> 
> To get there essentially copy the vfs read path into pagecache. We
> can't call that right away because we have to take care of bit17
> swizzling. To not deadlock with our own pagefault handler we need
> to completely drop struct_mutex, reducing the atomicty-guarantees
> of our userspace abi. Implications for racing with other gem ioctl:
> 
> - execbuf, pwrite, pread: Due to -EFAULT fallback to slow paths there's
>   already the risk of the pwrite call not being atomic, no degration.
> - read/write access to mmaps: already fully racy, no degration.
> - set_tiling: Calling set_tiling while reading/writing is already
>   pretty much undefined, now it just got a bit worse. set_tiling is
>   only called by libdrm on unused/new bos, so no problem.
> - set_domain: When changing to the gtt domain while copying (without any
>   read/write access, e.g. for synchronization), we might leave unflushed
>   data in the cpu caches. The clflush_object at the end of pwrite_slow
>   takes care of this problem.
> - truncating of purgeable objects: the shmem_read_mapping_page call could
>   reinstate backing storage for truncated objects. The check at the end
>   of pwrite_slow takes care of this.
> 
> v2:
> - add missing intel_gtt_chipset_flush
> - add __ to copy_from_user_swizzled as suggest by Chris Wilson.
> 
> Signed-off-by: Daniel Vetter <daniel.vetter@...ll.ch>
> ---
>  drivers/gpu/drm/i915/i915_gem.c |  126 ++++++++++++++++++++-------------------
>  1 files changed, 64 insertions(+), 62 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
> index 6fa24bc..f9efebb 100644
> --- a/drivers/gpu/drm/i915/i915_gem.c
> +++ b/drivers/gpu/drm/i915/i915_gem.c
> @@ -58,6 +58,7 @@ static void i915_gem_free_object_tail(struct drm_i915_gem_object *obj);
>  
>  static int i915_gem_inactive_shrink(struct shrinker *shrinker,
>  				    struct shrink_control *sc);
> +static void i915_gem_object_truncate(struct drm_i915_gem_object *obj);
>  
>  /* some bookkeeping */
>  static void i915_gem_info_add_obj(struct drm_i915_private *dev_priv,
> @@ -385,6 +386,32 @@ i915_gem_shmem_pread_fast(struct drm_device *dev,
>  	return 0;
>  }
>  
> +static inline int
> +__copy_from_user_swizzled(char __user *gpu_vaddr, int gpu_offset,
> +			  const char *cpu_vaddr,
> +			  int length)
> +{
> +	int ret, cpu_offset = 0;
> +
> +	while (length > 0) {
> +		int cacheline_end = ALIGN(gpu_offset + 1, 64);
> +		int this_length = min(cacheline_end - gpu_offset, length);
> +		int swizzled_gpu_offset = gpu_offset ^ 64;
> +
> +		ret = __copy_from_user(gpu_vaddr + swizzled_gpu_offset,
> +				       cpu_vaddr + cpu_offset,
> +				       this_length);
> +		if (ret)
> +			return ret + length;
> +
> +		cpu_offset += this_length;
> +		gpu_offset += this_length;
> +		length -= this_length;
> +	}
> +
> +	return 0;
> +}
> +

Bikeshed, but I would much prefer a #define for the swizzle
bit/cacheline size.

>  /**
>   * This is the fallback shmem pread path, which allocates temporary storage
>   * in kernel space to copy_to_user into outside of the struct_mutex, so we
> @@ -841,71 +868,36 @@ i915_gem_shmem_pwrite_slow(struct drm_device *dev,
>  			   struct drm_file *file)
>  {
>  	struct address_space *mapping = obj->base.filp->f_path.dentry->d_inode->i_mapping;
> -	struct mm_struct *mm = current->mm;
> -	struct page **user_pages;
>  	ssize_t remain;
> -	loff_t offset, pinned_pages, i;
> -	loff_t first_data_page, last_data_page, num_pages;
> -	int shmem_page_offset;
> -	int data_page_index,  data_page_offset;
> -	int page_length;
> -	int ret;
> -	uint64_t data_ptr = args->data_ptr;
> -	int do_bit17_swizzling;
> +	loff_t offset;
> +	char __user *user_data;
> +	int shmem_page_offset, page_length, ret;
> +	int obj_do_bit17_swizzling, page_do_bit17_swizzling;
>  
> +	user_data = (char __user *) (uintptr_t) args->data_ptr;
>  	remain = args->size;
>  
> -	/* Pin the user pages containing the data.  We can't fault while
> -	 * holding the struct mutex, and all of the pwrite implementations
> -	 * want to hold it while dereferencing the user data.
> -	 */
> -	first_data_page = data_ptr / PAGE_SIZE;
> -	last_data_page = (data_ptr + args->size - 1) / PAGE_SIZE;
> -	num_pages = last_data_page - first_data_page + 1;
> -
> -	user_pages = drm_malloc_ab(num_pages, sizeof(struct page *));
> -	if (user_pages == NULL)
> -		return -ENOMEM;
> -
> -	mutex_unlock(&dev->struct_mutex);
> -	down_read(&mm->mmap_sem);
> -	pinned_pages = get_user_pages(current, mm, (uintptr_t)args->data_ptr,
> -				      num_pages, 0, 0, user_pages, NULL);
> -	up_read(&mm->mmap_sem);
> -	mutex_lock(&dev->struct_mutex);
> -	if (pinned_pages < num_pages) {
> -		ret = -EFAULT;
> -		goto out;
> -	}
> -
> -	ret = i915_gem_object_set_to_cpu_domain(obj, 1);
> -	if (ret)
> -		goto out;
> -
> -	do_bit17_swizzling = i915_gem_object_needs_bit17_swizzle(obj);
> +	obj_do_bit17_swizzling = i915_gem_object_needs_bit17_swizzle(obj);
>  
>  	offset = args->offset;
>  	obj->dirty = 1;
>  
> +	mutex_unlock(&dev->struct_mutex);
> +
>  	while (remain > 0) {
>  		struct page *page;
> +		char *vaddr;
>  
>  		/* Operation in this page
>  		 *
>  		 * shmem_page_offset = offset within page in shmem file
> -		 * data_page_index = page number in get_user_pages return
> -		 * data_page_offset = offset with data_page_index page.
>  		 * page_length = bytes to copy for this page
>  		 */
>  		shmem_page_offset = offset_in_page(offset);
> -		data_page_index = data_ptr / PAGE_SIZE - first_data_page;
> -		data_page_offset = offset_in_page(data_ptr);
>  
>  		page_length = remain;
>  		if ((shmem_page_offset + page_length) > PAGE_SIZE)
>  			page_length = PAGE_SIZE - shmem_page_offset;
> -		if ((data_page_offset + page_length) > PAGE_SIZE)
> -			page_length = PAGE_SIZE - data_page_offset;
>  
>  		page = shmem_read_mapping_page(mapping, offset >> PAGE_SHIFT);
>  		if (IS_ERR(page)) {
> @@ -913,34 +905,44 @@ i915_gem_shmem_pwrite_slow(struct drm_device *dev,
>  			goto out;
>  		}
>  
> -		if (do_bit17_swizzling) {
> -			slow_shmem_bit17_copy(page,
> -					      shmem_page_offset,
> -					      user_pages[data_page_index],
> -					      data_page_offset,
> -					      page_length,
> -					      0);
> -		} else {
> -			slow_shmem_copy(page,
> -					shmem_page_offset,
> -					user_pages[data_page_index],
> -					data_page_offset,
> -					page_length);
> -		}
> +		page_do_bit17_swizzling = (page_to_phys(page) & (1 << 17)) == 0;
> +
> +		vaddr = kmap(page);
> +		if (obj_do_bit17_swizzling && page_do_bit17_swizzling)
> +			ret = __copy_from_user_swizzled(vaddr, shmem_page_offset,
> +							user_data,
> +							page_length);
> +		else
> +			ret = __copy_from_user(vaddr + shmem_page_offset,
> +					       user_data,
> +					       page_length);
> +		kunmap(page);
>  
>  		set_page_dirty(page);
>  		mark_page_accessed(page);
>  		page_cache_release(page);
>  
> +		if (ret) {
> +			ret = -EFAULT;
> +			goto out;
> +		}
> +
>  		remain -= page_length;
> -		data_ptr += page_length;
> +		user_data += page_length;
>  		offset += page_length;
>  	}
>  
>  out:
> -	for (i = 0; i < pinned_pages; i++)
> -		page_cache_release(user_pages[i]);
> -	drm_free_large(user_pages);
> +	mutex_lock(&dev->struct_mutex);
> +	/* Fixup: Kill any reinstated backing storage pages */
> +	if (obj->madv == __I915_MADV_PURGED)
> +		i915_gem_object_truncate(obj);
> +	/* and flush dirty cachelines in case the object isn't in the cpu write
> +	 * domain anymore. */
> +	if (obj->base.write_domain != I915_GEM_DOMAIN_CPU) {
> +		i915_gem_clflush_object(obj);
> +		intel_gtt_chipset_flush();
> +	}
>  
>  	return ret;
>  }

I must be missing something obvious here...
Can you explain how this can possibly be considered safe without holding
struct_mutex?
--
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