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>

3 files changed, 285 insertions, 1 deletions

diff --git a/Documentation/admin-guide/mm/index.rst b/Documentation/admin-guide/mm/index.rst
index 11db46448354..774dad6d3d29 100644
--- a/Documentation/admin-guide/mm/index.rst
+++ b/Documentation/admin-guide/mm/index.rst
@@ -31,6 +31,7 @@ the Linux memory management.
    idle_page_tracking
    ksm
    memory-hotplug
+   nommu-map
    numa_memory_policy
    numaperf
    pagemap
diff --git a/Documentation/admin-guide/mm/nommu-mmap.rst b/Documentation/admin-guide/mm/nommu-mmap.rst
new file mode 100644
index 000000000000..530fed08de2c
--- /dev/null
+++ b/Documentation/admin-guide/mm/nommu-mmap.rst
@@ -0,0 +1,283 @@
+=============================
+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``.
diff --git a/Documentation/admin-guide/sysctl/vm.rst b/Documentation/admin-guide/sysctl/vm.rst
index d46d5b7013c6..d997cc3c26d0 100644
--- a/Documentation/admin-guide/sysctl/vm.rst
+++ b/Documentation/admin-guide/sysctl/vm.rst
@@ -583,7 +583,7 @@ trimming of allocations is initiated.
 
 The default value is 1.
 
-See Documentation/nommu-mmap.txt for more information.
+See Documentation/admin-guide/mm/nommu-mmap.rst for more information.
 
 
 numa_zonelist_order