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: fe115628d567 ("drm/i915: Implement pwrite without struct-mutex")
References: https://bugs.freedesktop.org/show_bug.cgi?id=99107
Signed-off-by: Chris 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: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170307120338.7277-2-chris@chris-wilson.co.uk

diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
index f8a5f5c..b2beb02 100644 (file)
--- a/drivers/gpu/drm/i915/i915_gem.c
+++ b/drivers/gpu/drm/i915/i915_gem.c
@@ -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 @@ err_unlock:
        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 *

diff --git a/drivers/gpu/drm/i915/i915_gem_object.h b/drivers/gpu/drm/i915/i915_gem_object.h
index 1495eeb..e89bfdb 100644 (file)
--- a/drivers/gpu/drm/i915/i915_gem_object.h
+++ b/drivers/gpu/drm/i915/i915_gem_object.h
@@ -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 *);
 };