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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 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'.