Re: [PATCH 02/13] drm/i915: rewrite shmem_pwrite_slow to use copy_from_user

Ben Widawsky
2011-11-21T05:56:32Z

On Sun, 6 Nov 2011 20:13:49 +0100
Daniel Vetter <daniel.vetter@ffwll.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@ffwll.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?