FreeBSD/Linux Kernel Cross Reference
sys/Documentation/nommu-mmap.txt

                              =============================
                              NO-MMU MEMORY MAPPING SUPPORT
                              =============================
     
     The kernel has limited support for memory mapping under no-MMU conditions, such
     as are used in uClinux environments. From the userspace point of view, memory
     mapping is made use of in conjunction with the mmap() system call, the shmat()
     call and the execve() system call. From the kernel's point of view, execve()
     mapping is actually performed by the binfmt drivers, which call back into the
    mmap() routines to do the actual work.
    
    Memory mapping behaviour also involves the way fork(), vfork(), clone() and
    ptrace() work. Under uClinux there is no fork(), and clone() must be supplied
    the CLONE_VM flag.
    
    The behaviour is similar between the MMU and no-MMU cases, but not identical;
    and it's also much more restricted in the latter case:
    
     (*) Anonymous mapping, MAP_PRIVATE
    
            In the MMU case: VM regions backed by arbitrary pages; copy-on-write
            across fork.
    
            In the no-MMU case: VM regions backed by arbitrary contiguous runs of
            pages.
    
     (*) Anonymous mapping, MAP_SHARED
    
            These behave very much like private mappings, except that they're
            shared across fork() or clone() without CLONE_VM in the MMU case. Since
            the no-MMU case doesn't support these, behaviour is identical to
            MAP_PRIVATE there.
    
     (*) File, MAP_PRIVATE, PROT_READ / PROT_EXEC, !PROT_WRITE
    
            In the MMU case: VM regions backed by pages read from file; changes to
            the underlying file are reflected in the mapping; copied across fork.
    
            In the no-MMU case:
    
             - If one exists, the kernel will re-use an existing mapping to the
               same segment of the same file if that has compatible permissions,
               even if this was created by another process.
    
             - If possible, the file mapping will be directly on the backing device
               if the backing device has the BDI_CAP_MAP_DIRECT capability and
               appropriate mapping protection capabilities. Ramfs, romfs, cramfs
               and mtd might all permit this.
    
             - If the backing device device can't or won't permit direct sharing,
               but does have the BDI_CAP_MAP_COPY capability, then a copy of the
               appropriate bit of the file will be read into a contiguous bit of
               memory and any extraneous space beyond the EOF will be cleared
    
             - Writes to the file do not affect the mapping; writes to the mapping
               are visible in other processes (no MMU protection), but should not
               happen.
    
     (*) File, MAP_PRIVATE, PROT_READ / PROT_EXEC, PROT_WRITE
    
            In the MMU case: like the non-PROT_WRITE case, except that the pages in
            question get copied before the write actually happens. From that point
            on writes to the file underneath that page no longer get reflected into
            the mapping's backing pages. The page is then backed by swap instead.
    
            In the no-MMU case: works much like the non-PROT_WRITE case, except
            that a copy is always taken and never shared.
    
     (*) Regular file / blockdev, MAP_SHARED, PROT_READ / PROT_EXEC / PROT_WRITE
    
            In the MMU case: VM regions backed by pages read from file; changes to
            pages written back to file; writes to file reflected into pages backing
            mapping; shared across fork.
    
            In the no-MMU case: not supported.
    
     (*) Memory backed regular file, MAP_SHARED, PROT_READ / PROT_EXEC / PROT_WRITE
    
            In the MMU case: As for ordinary regular files.
    
            In the no-MMU case: The filesystem providing the memory-backed file
            (such as ramfs or tmpfs) may choose to honour an open, truncate, mmap
            sequence by providing a contiguous sequence of pages to map. In that
            case, a shared-writable memory mapping will be possible. It will work
            as for the MMU case. If the filesystem does not provide any such
            support, then the mapping request will be denied.
    
     (*) Memory backed blockdev, MAP_SHARED, PROT_READ / PROT_EXEC / PROT_WRITE
    
            In the MMU case: As for ordinary regular files.
    
            In the no-MMU case: As for memory backed regular files, but the
            blockdev must be able to provide a contiguous run of pages without
            truncate being called. The ramdisk driver could do this if it allocated
            all its memory as a contiguous array upfront.
    
     (*) Memory backed chardev, MAP_SHARED, PROT_READ / PROT_EXEC / PROT_WRITE
    
            In the MMU case: As for ordinary regular files.
   
           In the no-MMU case: The character device driver may choose to honour
           the mmap() by providing direct access to the underlying device if it
           provides memory or quasi-memory that can be accessed directly. Examples
           of such are frame buffers and flash devices. If the driver does not
           provide any such support, then the mapping request will be denied.
   
   
   ============================
   FURTHER NOTES ON NO-MMU MMAP
   ============================
   
    (*) A request for a private mapping of a file may return a buffer that is not
        page-aligned.  This is because XIP may take place, and the data may not be
        paged aligned in the backing store.
   
    (*) A request for an anonymous mapping will always be page aligned.  If
        possible the size of the request should be a power of two otherwise some
        of the space may be wasted as the kernel must allocate a power-of-2
        granule but will only discard the excess if appropriately configured as
        this has an effect on fragmentation.
   
    (*) The memory allocated by a request for an anonymous mapping will normally
        be cleared by the kernel before being returned in accordance with the
        Linux man pages (ver 2.22 or later).
   
        In the MMU case this can be achieved with reasonable performance as
        regions are backed by virtual pages, with the contents only being mapped
        to cleared physical pages when a write happens on that specific page
        (prior to which, the pages are effectively mapped to the global zero page
        from which reads can take place).  This spreads out the time it takes to
        initialize the contents of a page - depending on the write-usage of the
        mapping.
   
        In the no-MMU case, however, anonymous mappings are backed by physical
        pages, and the entire map is cleared at allocation time.  This can cause
        significant delays during a userspace malloc() as the C library does an
        anonymous mapping and the kernel then does a memset for the entire map.
   
        However, for memory that isn't required to be precleared - such as that
        returned by malloc() - mmap() can take a MAP_UNINITIALIZED flag to
        indicate to the kernel that it shouldn't bother clearing the memory before
        returning it.  Note that CONFIG_MMAP_ALLOW_UNINITIALIZED must be enabled
        to permit this, otherwise the flag will be ignored.
   
        uClibc uses this to speed up malloc(), and the ELF-FDPIC binfmt uses this
        to allocate the brk and stack region.
   
    (*) A list of all the private copy and anonymous mappings on the system is
        visible through /proc/maps in no-MMU mode.
   
    (*) A list of all the mappings in use by a process is visible through
        /proc/<pid>/maps in no-MMU mode.
   
    (*) Supplying MAP_FIXED or a requesting a particular mapping address will
        result in an error.
   
    (*) Files mapped privately usually have to have a read method provided by the
        driver or filesystem so that the contents can be read into the memory
        allocated if mmap() chooses not to map the backing device directly. An
        error will result if they don't. This is most likely to be encountered
        with character device files, pipes, fifos and sockets.
   
   
   ==========================
   INTERPROCESS SHARED MEMORY
   ==========================
   
   Both SYSV IPC SHM shared memory and POSIX shared memory is supported in NOMMU
   mode.  The former through the usual mechanism, the latter through files created
   on ramfs or tmpfs mounts.
   
   
   =======
   FUTEXES
   =======
   
   Futexes are supported in NOMMU mode if the arch supports them.  An error will
   be given if an address passed to the futex system call lies outside the
   mappings made by a process or if the mapping in which the address lies does not
   support futexes (such as an I/O chardev mapping).
   
   
   =============
   NO-MMU MREMAP
   =============
   
   The mremap() function is partially supported.  It may change the size of a
   mapping, and may move it[*] if MREMAP_MAYMOVE is specified and if the new size
   of the mapping exceeds the size of the slab object currently occupied by the
   memory to which the mapping refers, or if a smaller slab object could be used.
   
   MREMAP_FIXED is not supported, though it is ignored if there's no change of
   address and the object does not need to be moved.
   
   Shared mappings may not be moved.  Shareable mappings may not be moved either,
   even if they are not currently shared.
   
   The mremap() function must be given an exact match for base address and size of
   a previously mapped object.  It may not be used to create holes in existing
   mappings, move parts of existing mappings or resize parts of mappings.  It must
   act on a complete mapping.
   
   [*] Not currently supported.
   
   
   ============================================
   PROVIDING SHAREABLE CHARACTER DEVICE SUPPORT
   ============================================
   
   To provide shareable character device support, a driver must provide a
   file->f_op->get_unmapped_area() operation. The mmap() routines will call this
   to get a proposed address for the mapping. This may return an error if it
   doesn't wish to honour the mapping because it's too long, at a weird offset,
   under some unsupported combination of flags or whatever.
   
   The driver should also provide backing device information with capabilities set
   to indicate the permitted types of mapping on such devices. The default is
   assumed to be readable and writable, not executable, and only shareable
   directly (can't be copied).
   
   The file->f_op->mmap() operation will be called to actually inaugurate the
   mapping. It can be rejected at that point. Returning the ENOSYS error will
   cause the mapping to be copied instead if BDI_CAP_MAP_COPY is specified.
   
   The vm_ops->close() routine will be invoked when the last mapping on a chardev
   is removed. An existing mapping will be shared, partially or not, if possible
   without notifying the driver.
   
   It is permitted also for the file->f_op->get_unmapped_area() operation to
   return -ENOSYS. This will be taken to mean that this operation just doesn't
   want to handle it, despite the fact it's got an operation. For instance, it
   might try directing the call to a secondary driver which turns out not to
   implement it. Such is the case for the framebuffer driver which attempts to
   direct the call to the device-specific driver. Under such circumstances, the
   mapping request will be rejected if BDI_CAP_MAP_COPY is not specified, and a
   copy mapped otherwise.
   
   IMPORTANT NOTE:
   
           Some types of device may present a different appearance to anyone
           looking at them in certain modes. Flash chips can be like this; for
           instance if they're in programming or erase mode, you might see the
           status reflected in the mapping, instead of the data.
   
           In such a case, care must be taken lest userspace see a shared or a
           private mapping showing such information when the driver is busy
           controlling the device. Remember especially: private executable
           mappings may still be mapped directly off the device under some
           circumstances!
   
   
   ==============================================
   PROVIDING SHAREABLE MEMORY-BACKED FILE SUPPORT
   ==============================================
   
   Provision of shared mappings on memory backed files is similar to the provision
   of support for shared mapped character devices. The main difference is that the
   filesystem providing the service will probably allocate a contiguous collection
   of pages and permit mappings to be made on that.
   
   It is recommended that a truncate operation applied to such a file that
   increases the file size, if that file is empty, be taken as a request to gather
   enough pages to honour a mapping. This is required to support POSIX shared
   memory.
   
   Memory backed devices are indicated by the mapping's backing device info having
   the memory_backed flag set.
   
   
   ========================================
   PROVIDING SHAREABLE BLOCK DEVICE SUPPORT
   ========================================
   
   Provision of shared mappings on block device files is exactly the same as for
   character devices. If there isn't a real device underneath, then the driver
   should allocate sufficient contiguous memory to honour any supported mapping.
   
   
   =================================
   ADJUSTING PAGE TRIMMING BEHAVIOUR
   =================================
   
   NOMMU mmap automatically rounds up to the nearest power-of-2 number of pages
   when performing an allocation.  This can have adverse effects on memory
   fragmentation, and as such, is left configurable.  The default behaviour is to
   aggressively trim allocations and discard any excess pages back in to the page
   allocator.  In order to retain finer-grained control over fragmentation, this
   behaviour can either be disabled completely, or bumped up to a higher page
   watermark where trimming begins.
   
   Page trimming behaviour is configurable via the sysctl `vm.nr_trim_pages'.

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