litmus-rt-ext-res.git - LITMUS^RT with extended reservations for Forbidden Zones paper @ RTAS'20

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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 less than a page in size may not return a page-aligned buffer. This is because the kernel calls kmalloc() to allocate the buffer, not get_free_page(). (*) A list of all the 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


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