[v3,1/2] sysfs: Invoke iomem_get_mapping() from the sysfs open callback

Defer invocation of the iomem_get_mapping() to the sysfs open callback
so that it can be executed as needed when the binary sysfs object has
been accessed.

To do that, convert the "mapping" member of the struct bin_attribute
from a pointer to the struct address_space into a function pointer with
a signature that requires the same return type, and then updates the
sysfs_kf_bin_open() to invoke provided function should the function
pointer be valid.

Also, convert every invocation of iomem_get_mapping() into a function
pointer assignment, therefore allowing for the iomem_get_mapping()
invocation to be deferred to when the sysfs open callback runs.

Thus, this change removes the need for the fs_initcalls to complete
before any other sub-system that uses the iomem_get_mapping() would be
able to invoke it safely without leading to a failure and an Oops
related to an invalid iomem_get_mapping() access.

Suggested-by: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: Krzysztof Wilczyński <kw@linux.com>
Reviewed-by: Dan Williams <dan.j.williams@intel.com>
---
 drivers/pci/pci-sysfs.c | 6 +++---
 fs/sysfs/file.c         | 2 +-
 include/linux/sysfs.h   | 2 +-
 3 files changed, 5 insertions(+), 5 deletions(-)

On Thu, Jul 29, 2021 at 11:32:34PM +0000, Krzysztof Wilczyński wrote:
> Defer invocation of the iomem_get_mapping() to the sysfs open callback
> so that it can be executed as needed when the binary sysfs object has
> been accessed.
> 
> To do that, convert the "mapping" member of the struct bin_attribute
> from a pointer to the struct address_space into a function pointer with
> a signature that requires the same return type, and then updates the
> sysfs_kf_bin_open() to invoke provided function should the function
> pointer be valid.
> 
> Also, convert every invocation of iomem_get_mapping() into a function
> pointer assignment, therefore allowing for the iomem_get_mapping()
> invocation to be deferred to when the sysfs open callback runs.
> 
> Thus, this change removes the need for the fs_initcalls to complete
> before any other sub-system that uses the iomem_get_mapping() would be
> able to invoke it safely without leading to a failure and an Oops
> related to an invalid iomem_get_mapping() access.
> 
> Suggested-by: Dan Williams <dan.j.williams@intel.com>
> Signed-off-by: Dan Williams <dan.j.williams@intel.com>
> Signed-off-by: Krzysztof Wilczyński <kw@linux.com>
> Reviewed-by: Dan Williams <dan.j.williams@intel.com>

Acked-by: Bjorn Helgaas <bhelgaas@google.com>

> ---
>  drivers/pci/pci-sysfs.c | 6 +++---
>  fs/sysfs/file.c         | 2 +-
>  include/linux/sysfs.h   | 2 +-
>  3 files changed, 5 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/pci/pci-sysfs.c b/drivers/pci/pci-sysfs.c
> index 5d63df7c1820..76e5545d0e73 100644
> --- a/drivers/pci/pci-sysfs.c
> +++ b/drivers/pci/pci-sysfs.c
> @@ -965,7 +965,7 @@ void pci_create_legacy_files(struct pci_bus *b)
>  	b->legacy_io->read = pci_read_legacy_io;
>  	b->legacy_io->write = pci_write_legacy_io;
>  	b->legacy_io->mmap = pci_mmap_legacy_io;
> -	b->legacy_io->mapping = iomem_get_mapping();
> +	b->legacy_io->mapping = iomem_get_mapping;
>  	pci_adjust_legacy_attr(b, pci_mmap_io);
>  	error = device_create_bin_file(&b->dev, b->legacy_io);
>  	if (error)
> @@ -978,7 +978,7 @@ void pci_create_legacy_files(struct pci_bus *b)
>  	b->legacy_mem->size = 1024*1024;
>  	b->legacy_mem->attr.mode = 0600;
>  	b->legacy_mem->mmap = pci_mmap_legacy_mem;
> -	b->legacy_io->mapping = iomem_get_mapping();
> +	b->legacy_io->mapping = iomem_get_mapping;
>  	pci_adjust_legacy_attr(b, pci_mmap_mem);
>  	error = device_create_bin_file(&b->dev, b->legacy_mem);
>  	if (error)
> @@ -1195,7 +1195,7 @@ static int pci_create_attr(struct pci_dev *pdev, int num, int write_combine)
>  		}
>  	}
>  	if (res_attr->mmap)
> -		res_attr->mapping = iomem_get_mapping();
> +		res_attr->mapping = iomem_get_mapping;
>  	res_attr->attr.name = res_attr_name;
>  	res_attr->attr.mode = 0600;
>  	res_attr->size = pci_resource_len(pdev, num);
> diff --git a/fs/sysfs/file.c b/fs/sysfs/file.c
> index 9aefa7779b29..a3ee4c32a264 100644
> --- a/fs/sysfs/file.c
> +++ b/fs/sysfs/file.c
> @@ -175,7 +175,7 @@ static int sysfs_kf_bin_open(struct kernfs_open_file *of)
>  	struct bin_attribute *battr = of->kn->priv;
>  
>  	if (battr->mapping)
> -		of->file->f_mapping = battr->mapping;
> +		of->file->f_mapping = battr->mapping();
>  
>  	return 0;
>  }
> diff --git a/include/linux/sysfs.h b/include/linux/sysfs.h
> index a12556a4b93a..d5bcc897583c 100644
> --- a/include/linux/sysfs.h
> +++ b/include/linux/sysfs.h
> @@ -176,7 +176,7 @@ struct bin_attribute {
>  	struct attribute	attr;
>  	size_t			size;
>  	void			*private;
> -	struct address_space	*mapping;
> +	struct address_space *(*mapping)(void);
>  	ssize_t (*read)(struct file *, struct kobject *, struct bin_attribute *,
>  			char *, loff_t, size_t);
>  	ssize_t (*write)(struct file *, struct kobject *, struct bin_attribute *,
> -- 
> 2.32.0
>