292 lines
13 KiB
Plaintext
292 lines
13 KiB
Plaintext
=============================
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NO-MMU MEMORY MAPPING SUPPORT
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=============================
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The kernel has limited support for memory mapping under no-MMU conditions, such
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as are used in uClinux environments. From the userspace point of view, memory
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mapping is made use of in conjunction with the mmap() system call, the shmat()
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call and the execve() system call. From the kernel's point of view, execve()
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mapping is actually performed by the binfmt drivers, which call back into the
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mmap() routines to do the actual work.
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Memory mapping behaviour also involves the way fork(), vfork(), clone() and
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ptrace() work. Under uClinux there is no fork(), and clone() must be supplied
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the CLONE_VM flag.
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The behaviour is similar between the MMU and no-MMU cases, but not identical;
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and it's also much more restricted in the latter case:
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(*) Anonymous mapping, MAP_PRIVATE
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In the MMU case: VM regions backed by arbitrary pages; copy-on-write
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across fork.
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In the no-MMU case: VM regions backed by arbitrary contiguous runs of
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pages.
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(*) Anonymous mapping, MAP_SHARED
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These behave very much like private mappings, except that they're
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shared across fork() or clone() without CLONE_VM in the MMU case. Since
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the no-MMU case doesn't support these, behaviour is identical to
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MAP_PRIVATE there.
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(*) File, MAP_PRIVATE, PROT_READ / PROT_EXEC, !PROT_WRITE
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In the MMU case: VM regions backed by pages read from file; changes to
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the underlying file are reflected in the mapping; copied across fork.
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In the no-MMU case:
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- If one exists, the kernel will re-use an existing mapping to the
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same segment of the same file if that has compatible permissions,
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even if this was created by another process.
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- If possible, the file mapping will be directly on the backing device
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if the backing device has the BDI_CAP_MAP_DIRECT capability and
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appropriate mapping protection capabilities. Ramfs, romfs, cramfs
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and mtd might all permit this.
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- If the backing device device can't or won't permit direct sharing,
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but does have the BDI_CAP_MAP_COPY capability, then a copy of the
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appropriate bit of the file will be read into a contiguous bit of
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memory and any extraneous space beyond the EOF will be cleared
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- Writes to the file do not affect the mapping; writes to the mapping
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are visible in other processes (no MMU protection), but should not
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happen.
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(*) File, MAP_PRIVATE, PROT_READ / PROT_EXEC, PROT_WRITE
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In the MMU case: like the non-PROT_WRITE case, except that the pages in
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question get copied before the write actually happens. From that point
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on writes to the file underneath that page no longer get reflected into
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the mapping's backing pages. The page is then backed by swap instead.
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In the no-MMU case: works much like the non-PROT_WRITE case, except
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that a copy is always taken and never shared.
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(*) Regular file / blockdev, MAP_SHARED, PROT_READ / PROT_EXEC / PROT_WRITE
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In the MMU case: VM regions backed by pages read from file; changes to
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pages written back to file; writes to file reflected into pages backing
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mapping; shared across fork.
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In the no-MMU case: not supported.
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(*) Memory backed regular file, MAP_SHARED, PROT_READ / PROT_EXEC / PROT_WRITE
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In the MMU case: As for ordinary regular files.
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In the no-MMU case: The filesystem providing the memory-backed file
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(such as ramfs or tmpfs) may choose to honour an open, truncate, mmap
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sequence by providing a contiguous sequence of pages to map. In that
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case, a shared-writable memory mapping will be possible. It will work
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as for the MMU case. If the filesystem does not provide any such
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support, then the mapping request will be denied.
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(*) Memory backed blockdev, MAP_SHARED, PROT_READ / PROT_EXEC / PROT_WRITE
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In the MMU case: As for ordinary regular files.
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In the no-MMU case: As for memory backed regular files, but the
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blockdev must be able to provide a contiguous run of pages without
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truncate being called. The ramdisk driver could do this if it allocated
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all its memory as a contiguous array upfront.
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(*) Memory backed chardev, MAP_SHARED, PROT_READ / PROT_EXEC / PROT_WRITE
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In the MMU case: As for ordinary regular files.
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In the no-MMU case: The character device driver may choose to honour
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the mmap() by providing direct access to the underlying device if it
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provides memory or quasi-memory that can be accessed directly. Examples
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of such are frame buffers and flash devices. If the driver does not
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provide any such support, then the mapping request will be denied.
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============================
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FURTHER NOTES ON NO-MMU MMAP
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============================
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(*) A request for a private mapping of a file may return a buffer that is not
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page-aligned. This is because XIP may take place, and the data may not be
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paged aligned in the backing store.
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(*) A request for an anonymous mapping will always be page aligned. If
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possible the size of the request should be a power of two otherwise some
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of the space may be wasted as the kernel must allocate a power-of-2
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granule but will only discard the excess if appropriately configured as
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this has an effect on fragmentation.
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(*) The memory allocated by a request for an anonymous mapping will normally
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be cleared by the kernel before being returned in accordance with the
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Linux man pages (ver 2.22 or later).
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In the MMU case this can be achieved with reasonable performance as
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regions are backed by virtual pages, with the contents only being mapped
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to cleared physical pages when a write happens on that specific page
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(prior to which, the pages are effectively mapped to the global zero page
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from which reads can take place). This spreads out the time it takes to
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initialize the contents of a page - depending on the write-usage of the
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mapping.
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In the no-MMU case, however, anonymous mappings are backed by physical
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pages, and the entire map is cleared at allocation time. This can cause
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significant delays during a userspace malloc() as the C library does an
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anonymous mapping and the kernel then does a memset for the entire map.
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However, for memory that isn't required to be precleared - such as that
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returned by malloc() - mmap() can take a MAP_UNINITIALIZED flag to
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indicate to the kernel that it shouldn't bother clearing the memory before
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returning it. Note that CONFIG_MMAP_ALLOW_UNINITIALIZED must be enabled
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to permit this, otherwise the flag will be ignored.
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uClibc uses this to speed up malloc(), and the ELF-FDPIC binfmt uses this
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to allocate the brk and stack region.
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(*) A list of all the private copy and anonymous mappings on the system is
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visible through /proc/maps in no-MMU mode.
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(*) A list of all the mappings in use by a process is visible through
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/proc/<pid>/maps in no-MMU mode.
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(*) Supplying MAP_FIXED or a requesting a particular mapping address will
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result in an error.
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(*) Files mapped privately usually have to have a read method provided by the
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driver or filesystem so that the contents can be read into the memory
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allocated if mmap() chooses not to map the backing device directly. An
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error will result if they don't. This is most likely to be encountered
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with character device files, pipes, fifos and sockets.
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==========================
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INTERPROCESS SHARED MEMORY
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==========================
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Both SYSV IPC SHM shared memory and POSIX shared memory is supported in NOMMU
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mode. The former through the usual mechanism, the latter through files created
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on ramfs or tmpfs mounts.
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=======
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FUTEXES
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=======
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Futexes are supported in NOMMU mode if the arch supports them. An error will
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be given if an address passed to the futex system call lies outside the
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mappings made by a process or if the mapping in which the address lies does not
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support futexes (such as an I/O chardev mapping).
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=============
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NO-MMU MREMAP
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=============
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The mremap() function is partially supported. It may change the size of a
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mapping, and may move it[*] if MREMAP_MAYMOVE is specified and if the new size
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of the mapping exceeds the size of the slab object currently occupied by the
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memory to which the mapping refers, or if a smaller slab object could be used.
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MREMAP_FIXED is not supported, though it is ignored if there's no change of
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address and the object does not need to be moved.
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Shared mappings may not be moved. Shareable mappings may not be moved either,
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even if they are not currently shared.
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The mremap() function must be given an exact match for base address and size of
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a previously mapped object. It may not be used to create holes in existing
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mappings, move parts of existing mappings or resize parts of mappings. It must
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act on a complete mapping.
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[*] Not currently supported.
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============================================
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PROVIDING SHAREABLE CHARACTER DEVICE SUPPORT
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============================================
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To provide shareable character device support, a driver must provide a
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file->f_op->get_unmapped_area() operation. The mmap() routines will call this
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to get a proposed address for the mapping. This may return an error if it
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doesn't wish to honour the mapping because it's too long, at a weird offset,
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under some unsupported combination of flags or whatever.
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The driver should also provide backing device information with capabilities set
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to indicate the permitted types of mapping on such devices. The default is
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assumed to be readable and writable, not executable, and only shareable
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directly (can't be copied).
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The file->f_op->mmap() operation will be called to actually inaugurate the
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mapping. It can be rejected at that point. Returning the ENOSYS error will
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cause the mapping to be copied instead if BDI_CAP_MAP_COPY is specified.
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The vm_ops->close() routine will be invoked when the last mapping on a chardev
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is removed. An existing mapping will be shared, partially or not, if possible
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without notifying the driver.
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It is permitted also for the file->f_op->get_unmapped_area() operation to
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return -ENOSYS. This will be taken to mean that this operation just doesn't
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want to handle it, despite the fact it's got an operation. For instance, it
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might try directing the call to a secondary driver which turns out not to
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implement it. Such is the case for the framebuffer driver which attempts to
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direct the call to the device-specific driver. Under such circumstances, the
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mapping request will be rejected if BDI_CAP_MAP_COPY is not specified, and a
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copy mapped otherwise.
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IMPORTANT NOTE:
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Some types of device may present a different appearance to anyone
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looking at them in certain modes. Flash chips can be like this; for
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instance if they're in programming or erase mode, you might see the
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status reflected in the mapping, instead of the data.
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In such a case, care must be taken lest userspace see a shared or a
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private mapping showing such information when the driver is busy
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controlling the device. Remember especially: private executable
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mappings may still be mapped directly off the device under some
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circumstances!
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==============================================
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PROVIDING SHAREABLE MEMORY-BACKED FILE SUPPORT
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==============================================
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Provision of shared mappings on memory backed files is similar to the provision
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of support for shared mapped character devices. The main difference is that the
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filesystem providing the service will probably allocate a contiguous collection
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of pages and permit mappings to be made on that.
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It is recommended that a truncate operation applied to such a file that
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increases the file size, if that file is empty, be taken as a request to gather
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enough pages to honour a mapping. This is required to support POSIX shared
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memory.
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Memory backed devices are indicated by the mapping's backing device info having
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the memory_backed flag set.
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========================================
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PROVIDING SHAREABLE BLOCK DEVICE SUPPORT
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========================================
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Provision of shared mappings on block device files is exactly the same as for
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character devices. If there isn't a real device underneath, then the driver
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should allocate sufficient contiguous memory to honour any supported mapping.
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=================================
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ADJUSTING PAGE TRIMMING BEHAVIOUR
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=================================
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NOMMU mmap automatically rounds up to the nearest power-of-2 number of pages
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when performing an allocation. This can have adverse effects on memory
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fragmentation, and as such, is left configurable. The default behaviour is to
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aggressively trim allocations and discard any excess pages back in to the page
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allocator. In order to retain finer-grained control over fragmentation, this
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behaviour can either be disabled completely, or bumped up to a higher page
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watermark where trimming begins.
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Page trimming behaviour is configurable via the sysctl `vm.nr_trim_pages'.
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