SUBPAGE_PROT(2)            Linux Programmer's Manual           SUBPAGE_PROT(2)

       subpage_prot - define a subpage protection for an address range

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

       Note: There is no glibc wrapper for this system call; see NOTES.

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

       The  protection map is applied to the memory pages in the region start-
       ing 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 4 kB subpage; thus each 32-bit  word  specifies  the
       protections  of 16 4 kB subpages inside a 64 kB 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  argu-
              ments 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.

       Glibc  does  not  provide a wrapper for this system call; call it using

       Normal page protections (at the 64-kB page level) also apply; the  sub-
       page  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-pro-

       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 pro-
       tection 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)

       Documentation/vm/hugetlbpage.txt in the Linux kernel source tree

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Linux                             2017-09-15                   SUBPAGE_PROT(2)
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