Unverified Commit 048832a3 authored by Taotao Chen's avatar Taotao Chen Committed by Christian Brauner
Browse files

drm/i915: Refactor shmem_pwrite() to use kiocb and write_iter 



Refactors shmem_pwrite() to replace the ->write_begin/end logic
with a write_iter-based implementation using kiocb and iov_iter.

While kernel_write() was considered, it caused about 50% performance
regression. vfs_write() is not exported for kernel use. Therefore,
file->f_op->write_iter() is called directly with a synchronously
initialized kiocb to preserve performance and remove write_begin
usage.

Performance results use gem_pwrite on Intel CPU i7-10700
(average of 10 runs):

- ./gem_pwrite --run-subtest bench -s 16384
  Before: 0.205s, After: 0.214s

- ./gem_pwrite --run-subtest bench -s 524288
  Before: 6.1021s, After: 4.8047s

Part of a series refactoring address_space_operations write_begin and
write_end callbacks to use struct kiocb for passing write context and
flags.

Signed-off-by: default avatarTaotao Chen <chentaotao@didiglobal.com>
Link: https://lore.kernel.org/20250716093559.217344-3-chentaotao@didiglobal.com


Signed-off-by: default avatarChristian Brauner <brauner@kernel.org>
parent e7b840fd

drivers/gpu/drm/i915/gem/i915_gem_shmem.c

+22 −60
Original line number Diff line number Diff line
@@ -6,6 +6,7 @@ 
#include <linux/pagevec.h>

#include <linux/shmem_fs.h>

#include <linux/swap.h>

#include <linux/uio.h>



#include <drm/drm_cache.h>
@@ -400,12 +401,12 @@ static int 
shmem_pwrite(struct drm_i915_gem_object *obj,

	     const struct drm_i915_gem_pwrite *arg)

{

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

	const struct address_space_operations *aops = mapping->a_ops;

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

	u64 remain;

	loff_t pos;

	unsigned int pg;

	struct file *file = obj->base.filp;

	struct kiocb kiocb;

	struct iov_iter iter;

	ssize_t written;

	u64 size = arg->size;



	/* Caller already validated user args */

	GEM_BUG_ON(!access_ok(user_data, arg->size));
@@ -428,63 +429,24 @@ shmem_pwrite(struct drm_i915_gem_object *obj, 
	if (obj->mm.madv != I915_MADV_WILLNEED)

		return -EFAULT;



	/*

	 * 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;

	pos = arg->offset;

	pg = offset_in_page(pos);



	do {

		unsigned int len, unwritten;

		struct folio *folio;

		void *data, *vaddr;

		int err;

		char __maybe_unused c;

	if (size > MAX_RW_COUNT)

		return -EFBIG;



		len = PAGE_SIZE - pg;

		if (len > remain)

			len = remain;

	if (!file->f_op->write_iter)

		return -EINVAL;



		/* Prefault the user page to reduce potential recursion */

		err = __get_user(c, user_data);

		if (err)

			return err;

	init_sync_kiocb(&kiocb, file);

	kiocb.ki_pos = arg->offset;

	iov_iter_ubuf(&iter, ITER_SOURCE, (void __user *)user_data, size);



		err = __get_user(c, user_data + len - 1);

		if (err)

			return err;

	written = file->f_op->write_iter(&kiocb, &iter);

	BUG_ON(written == -EIOCBQUEUED);



		err = aops->write_begin(obj->base.filp, mapping, pos, len,

					&folio, &data);

		if (err < 0)

			return err;



		vaddr = kmap_local_folio(folio, offset_in_folio(folio, pos));

		pagefault_disable();

		unwritten = __copy_from_user_inatomic(vaddr, user_data, len);

		pagefault_enable();

		kunmap_local(vaddr);



		err = aops->write_end(obj->base.filp, mapping, pos, len,

				      len - unwritten, folio, data);

		if (err < 0)

			return err;



		/* We don't handle -EFAULT, leave it to the caller to check */

		if (unwritten)

			return -ENODEV;

	if (written != size)

		return -EIO;



		remain -= len;

		user_data += len;

		pos += len;

		pg = 0;

	} while (remain);

	if (written < 0)

		return written;



	return 0;

}