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authorAndré Fabian Silva Delgado <emulatorman@parabola.nu>2015-08-05 17:04:01 -0300
committerAndré Fabian Silva Delgado <emulatorman@parabola.nu>2015-08-05 17:04:01 -0300
commit57f0f512b273f60d52568b8c6b77e17f5636edc0 (patch)
tree5e910f0e82173f4ef4f51111366a3f1299037a7b /Documentation/nommu-mmap.txt
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+ =============================
+ 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 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 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'.