docs: move nommu-mmap.txt to admin-guide and rename to ReST

The nommu-mmap.txt file provides description of user visible
behaviuour. So, move it to the admin-guide.

As it is already at the ReST, also rename it.

Suggested-by: Mike Rapoport <rppt@linux.ibm.com>
Suggested-by: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/3a63d1833b513700755c85bf3bda0a6c4ab56986.1592918949.git.mchehab+huawei@kernel.org
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

1 files changed, 0 insertions, 283 deletions

diff --git a/Documentation/nommu-mmap.txt b/Documentation/nommu-mmap.txt
deleted file mode 100644
index 530fed08de2c..000000000000
--- a/Documentation/nommu-mmap.txt
+++ /dev/null
@@ -1,283 +0,0 @@
-=============================
-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 NOMMU_MAP_DIRECT capability and
-           appropriate mapping protection capabilities. Ramfs, romfs, cramfs
-           and mtd might all permit this.
-
-	 - If the backing device can't or won't permit direct sharing,
-           but does have the NOMMU_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 NOMMU_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 NOMMU_MAP_COPY is not specified, and a
-copy mapped otherwise.
-
-.. important::
-
-	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``.