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       subpage_prot - define a subpage protection for an address range


       long subpage_prot(unsigned long addr, unsigned long len,
                         uint32_t *map);


       The  PowerPC-specific  subpage_prot() system call provides the facility
       to control the access permissions on individual 4kB subpages on systems
       configured with a page size of 64kB.

       The  protection  map  is  applied  to  the  memory  pages in the region
       starting at addr and continuing for len bytes.  Both of these arguments
       must be aligned to a 64-kB boundary.

       The  protection  map is specified in the buffer pointed to by map.  The
       map has 2 bits per 4kB subpage; thus each  32-bit  word  specifies  the
       protections  of  16  4kB subpages inside a 64kB page (so, the number of
       32-bit words pointed to by map should equate to  the  number  of  64-kB
       pages  specified  by  len).   Each 2-bit field in the protection map is
       either 0 to allow any access, 1 to prevent writes, or 2 or 3 to prevent
       all accesses.


       On  success,  subpage_prot()  returns  0.   Otherwise, one of the error
       codes specified below is returned.


       EFAULT The buffer referred to by map is not accessible.

       EINVAL The  addr  or  len  arguments  are  incorrect.   Both  of  these
              arguments must be aligned to a multiple of the system page size,
              and they must not refer to a region outside of the address space
              of the process or to a region that consists of huge pages.

       ENOMEM Out of memory.


       This  system  call  is provided on the PowerPC architecture since Linux
       2.6.25.  The system call is provided only if the kernel  is  configured
       with CONFIG_PPC_64K_PAGES.  No library support is provided.


       This system call is Linux-specific.


       Normal  page  protections  (at  the  64-kB  page level) also apply; the
       subpage protection mechanism is an additional constraint, so putting  0
       in  a 2-bit field won't allow writes to a page that is otherwise write-

       This system call is provided to assist writing emulators  that  operate
       using  64-kB  pages on PowerPC systems.  When emulating systems such as
       x86, which uses a smaller page size, the emulator can no longer use the
       memory-management  unit  (MMU)  and normal system calls for controlling
       page protections.  (The emulator could emulate the MMU by checking  and
       possibly  remapping the address for each memory access in software, but
       that is slow.)  The idea is that the  emulator  supplies  an  array  of
       protection  masks  to  apply to a specified range of virtual addresses.
       These masks are applied at the level where hardware page-table  entries
       (PTEs)  are  inserted  into  the hardware page table based on the Linux
       PTEs, so the Linux PTEs are not affected.  Implicit in this is that the
       regions  of  the  address  space that are protected are switched to use
       4-kB hardware pages rather than 64-kB hardware pages (on machines  with
       hardware 64-kB page support).


       mprotect(2), syscall(2);
       the kernel source file Documentation/vm/hugetlbpage.txt.


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