mirror of
https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git
synced 2024-08-30 04:39:38 +00:00
![Martin Kepplinger](/assets/img/avatar_default.png)
Signed-off-by: Martin Kepplinger <martink@posteo.de> Signed-off-by: Jonathan Corbet <corbet@lwn.net>
283 lines
12 KiB
Text
283 lines
12 KiB
Text
=============================
|
|
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 NOMMU_MAP_DIRECT capability and
|
|
appropriate mapping protection capabilities. Ramfs, romfs, cramfs
|
|
and mtd might all permit this.
|
|
|
|
- If the backing 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/<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 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``.
|