Re: [PATCH 0/5] mm: Switch gfp_t to unsigned long

[Brendan Jackman <jackmanb@google.com>]

On Thu Mar 19, 2026 at 4:56 PM UTC, Matthew Wilcox wrote:
> On Thu, Mar 19, 2026 at 04:03:23PM +0000, Brendan Jackman wrote:
>> As pointed out by Vlastimil in [0], my proposal for __GFP_UNMAPPED is
>> probably not needed for 32-bit. This offers a way out of the GFP flag
>> scarcity so in preparation for this, flip gfp_t to be 64-bit on 64-bit
>> machines, while leaving it 32-bit on 32-bit machines.
>
> Ugh.  This grows struct xarray:
>
> struct xarray {
>         spinlock_t      xa_lock;
> /* private: The rest of the data structure is not to be used directly. */
>         gfp_t           xa_flags;
>         void __rcu *    xa_head;
> };
>
> which grows a lot of key data structures.
>
> It would probably have been good for you to have run pahole before/after
> this change and diff the results.

I will paste the diff at the bottom. I _think_ all the problematic
expansions are downstream of struct xarray, but it's quite likely my
sense for problematic struct expansions is weak.

> Now, you're probably saying "But this makes no damn sense, why on earth
> is xa_flags of type gfp_t?"  And the short answer is "because the radix
> tree has a ridiculous API".  But you'll learn all about it with your
> new patch in this series which converts xa_flags from being gfp_t to a
> plain unsigned int ;-)

Hm... so Can We Just™ turn:

#define radix_tree_root		xarray

... into ... 

struct radix_tree_root {
	struct xarray xarray;
	gfp_t gfp;
};

...? Or do we actually need to find a way to keep the radix_tree_root
from growing too? If so, given that (IIUC?) the radix-tree API is soft
deprecated, I guess we can be pretty simple-minded about it and just
define a fixed set of the GFP flag values people use for this:

enum radix_tree_gfp {
	RADIX_TREE_GFP_KERNEL,
	...
	RADIX_TREE_GFP_KERNEL_NOWARN,
};

const gfp_t radix_tree_gfps[] = {
	[RADIX_TREE_GFP_KERNEL] = GFP_KERNEL,
	...
	[RADIX_TREE_GFP_KERNEL_NOWARN] = GFP_KERNEL | __GFP_NOWARN,
};

Would that make sense?

---

  struct acpi_device_bus_id |   +8
  struct address_space  |  +16
  struct amd_iommu_viommu        |  +16
  struct auxiliary_device   |   +8
  struct bdev_inode     |  +16
  struct blkcg          |   +8
  struct cgroup_subsys  |   +8
  struct chain_allocator |   +8
  struct compact_control |   +8
  struct debugfs_inode_info |  +16
  struct dma_device        |   +8
  struct dmar_domain    |   +8
  struct drm_conn_prop_enum_list |   +8
  struct drm_device      |  +32
  struct drm_file        |  +16
  struct drm_i915_file_private     |  +24
  struct drm_i915_gem_object       |  +24
  struct drm_i915_private          |  +40
  struct drm_master      |  +24
  struct drm_mode_config |  +24
  struct dw_dma         |   +8
  struct ethtool_netdev_state |   +8
  struct ext4_inode_info |  +16
  struct fsnotify_group             |   +8
  struct gendisk        |   +8
  struct hsu_dma        |   +8
  struct hugetlbfs_inode_info |  +16
  struct i915_gem_context          |   +8
  struct i915_gem_object_page_iter |   +8
  struct i915_perf                 |   +8
  struct ida               |   +8
  struct idr            |   +8
  struct ieee80211_if_nan     |   +8
  struct ieee80211_local      |   +8
  struct inet_connection_sock |  +16
  struct inet_sock            |  +16
  struct inode          |  +16
  struct inotify_group_private_data |   +8
  struct __intel_generic_device |  +32
  struct intel_gt                  |  +24
  struct intel_guc                 |  +24
  struct intel_iommu    |   +8
  struct intel_uc                  |  +24
  struct iommufd_viommu          |   +8
  struct iommu_group           |   +8
  struct ipc_ids        |   +8
  struct irq_domain           |   +8
  struct iso_inode_info |  +16
  struct kernfs_root    |   +8
  struct kmem_cache     |   +8
  struct kvm                  |  +32
  struct kvm_arch             |  +24
  struct kvm_mmu_memory_cache |   +8
  struct kvm_vcpu             |  +40
  struct kvm_vcpu_arch        |  +40
  struct loop_device             |   +8
  struct mei_aux_device            |   +8
  struct migration_target_control                               |   +8
  struct mqueue_inode_info |  +16
  struct msdos_inode_info |  +16
  struct msi_dev_domain        |   +8
  struct msi_device_data       |   +8
  struct net_devmem_dmabuf_binding |   +8
  struct netfs_inode    |  +16
  struct netlink_sock           |  +16
  struct nfs_inode         |  +16
  struct nfs_net        |   +8
  struct objpool_head |   +8
  struct oom_control     |   +8
  struct p9_client      |  +16
  struct partial_context             |   +8
  struct phy_link_topology    |   +8
  struct proc_inode     |  +16
  struct protection_domain       |   +8
  struct pts_fs_info     |   +8
  struct raw6_sock      |  +64
  struct regmap               |   +8
  struct rethook      |   +8
  struct rpc_inode      |  +16
  struct sctp_sock            |  +16
  struct serial_ctrl_device |   +8
  struct shmem_inode_info |  +16
  struct shrink_control |   +8
  struct snd_card       |  +16
  struct sock           |  +16
  struct swnode         |   +8
  struct tcf_block      |   +8
  struct tcf_idrinfo          |   +8
  struct tcf_net        |   +8
  struct tg3_dev_id           |   +4
  struct thermal_zone_device |   +8
  struct trace_event_raw_dma_alloc_class |   +8
  struct trace_event_raw_dma_alloc_sgt   |   +8
  struct tracefs_inode  |  +16
  struct ttm_bo_swapout_walk |   +8
  struct v9fs_inode     |  +16
  struct virtio_gpu_device |  +16
  struct virtio_pci_admin_vq                    |   +8
  struct virtio_pci_device                      |   +8
  struct vmap_block_queue    |   +8
  struct worker_pool       |   +8
  struct xarray         |   +8
  struct xhci_stream_info |   +8