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@xxxxxxxxxxxxxxxxxx>
Cc: Matthew Auld <matthew.william.auld@xxxxxxxxx>
Cc: Joonas Lahtinen <joonas.lahtinen@xxxxxxxxxxxxxxx>
Cc: Mika Kuoppala <mika.kuoppala@xxxxxxxxxxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx> # v4.10+
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@xxxxxxxxxxxxxxx>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxx>
---
drivers/gpu/drm/i915/i915_gem.c | 78 ++++++++++++++++++++++++++++++++++
drivers/gpu/drm/i915/i915_gem_object.h | 3 ++
2 files changed, 81 insertions(+)
diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
index 1f92d25ca27d..d00203d538d4 100644
--- 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,
@@ -2576,6 +2582,75 @@ 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) &&
@@ -3992,8 +4067,11 @@ 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 33b0dc4782a9..d26155e0a026 100644
--- 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 *);
};
--
2.11.0
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