Linux-2.6.12-rc2

Initial git repository build. I'm not bothering with the full history,
even though we have it. We can create a separate "historical" git
archive of that later if we want to, and in the meantime it's about
3.2GB when imported into git - space that would just make the early
git days unnecessarily complicated, when we don't have a lot of good
infrastructure for it.

Let it rip!

1 files changed, 198 insertions, 0 deletions

diff --git a/Documentation/nommu-mmap.txt b/Documentation/nommu-mmap.txt
new file mode 100644
index 00000000000..b88ebe4d808
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+++ b/Documentation/nommu-mmap.txt
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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.
+
+ (*) 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.
+
+============================================
+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 BDI_CAP_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 BDI_CAP_MAP_COPY is not specified, and a
+copy mapped otherwise.
+
+IMPORTANT NOTE:
+
+        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.