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No-MMU memory mapping support
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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 NOMMU_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 NOMMU_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/
(#) 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 NOMMU_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 NOMMU_MAP_COPY is not specified, and a
copy mapped otherwise.
.. important::
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.
