79 lines
3.8 KiB
Plaintext
79 lines
3.8 KiB
Plaintext
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fs/mmap README File
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===================
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NuttX operates in a flat open address space and is focused on MCUs that do
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support Memory Management Units (MMUs). Therefore, NuttX generally does not
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require mmap() functionality and the MCUs generally cannot support true
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memory-mapped files.
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However, memory mapping of files is the mechanism used by NXFLAT, the NuttX
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tiny binary format, to get files into memory in order to execute them.
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mmap() support is therefore required to support NXFLAT. There are two
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conditions where mmap() can be supported:
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1. mmap can be used to support eXecute In Place (XIP) on random access media
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under the following very restrictive conditions:
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a. The filesystem supports the FIOC_MMAP ioctl command. Any file
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system that maps files contiguously on the media should support
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this ioctl. (vs. file system that scatter files over the media
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in non-contiguous sectors). As of this writing, ROMFS is the
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only file system that meets this requirement.
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b. The underlying block driver supports the BIOC_XIPBASE ioctl
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command that maps the underlying media to a randomly accessible
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address. At present, only the RAM/ROM disk driver does this.
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Some limitations of this approach are as follows:
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a. Since no real mapping occurs, all of the file contents are "mapped"
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into memory.
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b. All mapped files are read-only.
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c. There are no access privileges.
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2. If CONFIG_FS_RAMMAP is defined in the configuration, then mmap() will
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support simulation of memory mapped files by copying files whole
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into RAM. These copied files have some of the properties of
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standard memory mapped files. There are many, many exceptions
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exceptions, however. Some of these include:
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a. The goal is to have a single region of memory that represents a single
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file and can be shared by many threads. That is, given a filename a
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thread should be able to open the file, get a file descriptor, and
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call mmap() to get a memory region. Different file descriptors opened
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with the same file path should get the same memory region when mapped.
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The limitation in the current design is that there is insufficient
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knowledge to know that these different file descriptors correspond to
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the same file. So, for the time being, a new memory region is created
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each time that rammap() is called. Not very useful!
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b. The entire mapped portion of the file must be present in memory.
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Since it is assumed the the MCU does not have an MMU, on-demanding
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paging in of file blocks cannot be supported. Since the while mapped
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portion of the file must be present in memory, there are limitations
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in the size of files that may be memory mapped (especially on MCUs
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with no significant RAM resources).
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c. All mapped files are read-only. You can write to the in-memory image,
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but the file contents will not change.
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d. There are no access privileges.
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e. Since there are no processes in NuttX, all mmap() and munmap()
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operations have immediate, global effects. Under Linux, for example,
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munmap() would eliminate only the mapping with a process; the mappings
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to the same file in other processes would not be effected.
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f. Like true mapped file, the region will persist after closing the file
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descriptor. However, at present, these ram copied file regions are
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*not* automatically "unmapped" (i.e., freed) when a thread is terminated.
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This is primarily because it is not possible to know how many users
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of the mapped region there are and, therefore, when would be the
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appropriate time to free the region (other than when munmap is called).
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NOTE: Note, if the design limitation of a) were solved, then it would be
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easy to solve exception d) as well.
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