Re: [PATCH 1/2] NOMMU: Set BDI capabilities for /dev/mem and/dev/kmem

From: Andrew Morton
Date: Tue Aug 29 2006 - 14:21:06 EST


On Tue, 29 Aug 2006 18:59:49 +0100
David Howells <dhowells@xxxxxxxxxx> wrote:

> From: David Howells <dhowells@xxxxxxxxxx>
>
> Set the backing device info capabilities for /dev/mem and /dev/kmem to permit
> direct sharing under no-MMU conditions.
>
> Also comment the capabilities for /dev/zero.
>
> Signed-Off-By: David Howells <dhowells@xxxxxxxxxx>
> ---
>
> drivers/char/mem.c | 42 ++++++++++++++++++++++++++++++++++++++++++
> 1 files changed, 42 insertions(+), 0 deletions(-)
>
> diff --git a/drivers/char/mem.c b/drivers/char/mem.c
> index 917b204..4c29619 100644
> --- a/drivers/char/mem.c
> +++ b/drivers/char/mem.c
> @@ -238,6 +238,19 @@ #endif
> }
> #endif
>
> +#ifndef CONFIG_MMU
> +static unsigned long get_unmapped_area_mem(struct file *file,
> + unsigned long addr,
> + unsigned long len,
> + unsigned long pgoff,
> + unsigned long flags)
> +{
> + if (!valid_mmap_phys_addr_range(pgoff, len))
> + return (unsigned long) -EINVAL;
> + return pgoff;
> +}

#else
#define get_unmapped_area_mem NULL
#endif

> @@ -782,6 +801,9 @@ static const struct file_operations mem_
> .write = write_mem,
> .mmap = mmap_mem,
> .open = open_mem,
> +#ifndef CONFIG_MMU
> + .get_unmapped_area = get_unmapped_area_mem,
> +#endif
> };

zap ifdefs.

> static const struct file_operations kmem_fops = {
> @@ -790,6 +812,9 @@ static const struct file_operations kmem
> .write = write_kmem,
> .mmap = mmap_kmem,
> .open = open_kmem,
> +#ifndef CONFIG_MMU
> + .get_unmapped_area = get_unmapped_area_mem,
> +#endif
> };

Ditto.

> +/*
> + * capabilities for /dev/zero
> + * - permits private mappings, "copies" are taken of the source of zeros
> + */
> static struct backing_dev_info zero_bdi = {
> .capabilities = BDI_CAP_MAP_COPY,
> };
>
> +/*
> + * capabilities for /dev/mem and /dev/kmem
> + * - permits shared mmap for read, write and/or exec
> + * - does not permit private mmap (add BDI_CAP_MAP_COPY to permit this)
> + */
> +static struct backing_dev_info mem_bdi = {
> + .capabilities = (BDI_CAP_MAP_DIRECT |
> + BDI_CAP_READ_MAP | BDI_CAP_WRITE_MAP |
> + BDI_CAP_EXEC_MAP),
> +};

This changes behaviour, doesn't it? But only for !CONFIG_MMU kernels?

Perhaps some additional commentary around this is needed.


> static const struct file_operations full_fops = {
> .llseek = full_lseek,
> .read = read_full,
> @@ -861,9 +901,11 @@ static int memory_open(struct inode * in
> {
> switch (iminor(inode)) {
> case 1:
> + filp->f_mapping->backing_dev_info = &mem_bdi;
> filp->f_op = &mem_fops;
> break;
> case 2:
> + filp->f_mapping->backing_dev_info = &mem_bdi;
> filp->f_op = &kmem_fops;
> break;
> case 3:

Perhaps one could make mem_bdi==NULL if !CONFIG_MMU, for a minor space
saving.

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