• David Howells's avatar
    NOMMU: Make VMAs per MM as for MMU-mode linux · 8feae131
    David Howells authored
    
    
    Make VMAs per mm_struct as for MMU-mode linux.  This solves two problems:
    
     (1) In SYSV SHM where nattch for a segment does not reflect the number of
         shmat's (and forks) done.
    
     (2) In mmap() where the VMA's vm_mm is set to point to the parent mm by an
         exec'ing process when VM_EXECUTABLE is specified, regardless of the fact
         that a VMA might be shared and already have its vm_mm assigned to another
         process or a dead process.
    
    A new struct (vm_region) is introduced to track a mapped region and to remember
    the circumstances under which it may be shared and the vm_list_struct structure
    is discarded as it's no longer required.
    
    This patch makes the following additional changes:
    
     (1) Regions are now allocated with alloc_pages() rather than kmalloc() and
         with no recourse to __GFP_COMP, so the pages are not composite.  Instead,
         each page has a reference on it held by the region.  Anything else that is
         interested in such a page will have to get a reference on it to retain it.
         When the pages are released due to unmapping, each page is passed to
         put_page() and will be freed when the page usage count reaches zero.
    
     (2) Excess pages are trimmed after an allocation as the allocation must be
         made as a power-of-2 quantity of pages.
    
     (3) VMAs are added to the parent MM's R/B tree and mmap lists.  As an MM may
         end up with overlapping VMAs within the tree, the VMA struct address is
         appended to the sort key.
    
     (4) Non-anonymous VMAs are now added to the backing inode's prio list.
    
     (5) Holes may be punched in anonymous VMAs with munmap(), releasing parts of
         the backing region.  The VMA and region structs will be split if
         necessary.
    
     (6) sys_shmdt() only releases one attachment to a SYSV IPC shared memory
         segment instead of all the attachments at that addresss.  Multiple
         shmat()'s return the same address under NOMMU-mode instead of different
         virtual addresses as under MMU-mode.
    
     (7) Core dumping for ELF-FDPIC requires fewer exceptions for NOMMU-mode.
    
     (8) /proc/maps is now the global list of mapped regions, and may list bits
         that aren't actually mapped anywhere.
    
     (9) /proc/meminfo gains a line (tagged "MmapCopy") that indicates the amount
         of RAM currently allocated by mmap to hold mappable regions that can't be
         mapped directly.  These are copies of the backing device or file if not
         anonymous.
    
    These changes make NOMMU mode more similar to MMU mode.  The downside is that
    NOMMU mode requires some extra memory to track things over NOMMU without this
    patch (VMAs are no longer shared, and there are now region structs).
    Signed-off-by: default avatarDavid Howells <dhowells@redhat.com>
    Tested-by: default avatarMike Frysinger <vapier.adi@gmail.com>
    Acked-by: default avatarPaul Mundt <lethal@linux-sh.org>
    8feae131