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When cramfs in physical memory is used then we have the opportunity to map files directly from ROM, directly into user space, saving on RAM usage. This gives us Execute-In-Place (XIP) support. For a file to be mmap()-able, the map area has to correspond to a range of uncompressed and contiguous blocks, and in the MMU case it also has to be page aligned. A version of mkcramfs with appropriate support is necessary to create such a filesystem image. In the MMU case it may happen for a vma structure to extend beyond the actual file size. This is notably the case in binfmt_elf.c:elf_map(). Or the file's last block is shared with other files and cannot be mapped as is. Rather than refusing to mmap it, we do a "mixed" map and let the regular fault handler populate the unmapped area with RAM-backed pages. In practice the unmapped area is seldom accessed so page faults might never occur before this area is discarded. In the non-MMU case it is the get_unmapped_area method that is responsible for providing the address where the actual data can be found. No mapping is necessary of course. Signed-off-by: Nicolas Pitre <nico@linaro.org> Tested-by: Chris Brandt <chris.brandt@renesas.com> Signed-off-by: Al Viro <viro@zeniv.linux.org.uk> |
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Linux kernel ============ This file was moved to Documentation/admin-guide/README.rst Please notice that there are several guides for kernel developers and users. These guides can be rendered in a number of formats, like HTML and PDF. In order to build the documentation, use ``make htmldocs`` or ``make pdfdocs``. There are various text files in the Documentation/ subdirectory, several of them using the Restructured Text markup notation. See Documentation/00-INDEX for a list of what is contained in each file. Please read the Documentation/process/changes.rst file, as it contains the requirements for building and running the kernel, and information about the problems which may result by upgrading your kernel.