The mmap and munmap system calls allow UNIX programs
to exert detailed control over their address spaces. They can be used
to share memory among processes, to map files into process address
spaces, and as part of user-level page fault schemes such as the
garbage-collection algorithms discussed in lecture.
In this lab you'll add mmap and munmap
to xv6, focusing on memory-mapped files.
Fetch the xv6 source for the lab and check out the mmap branch:
$ git fetch
$ git checkout mmap
The manual page
(run man 2 mmap) shows this declaration for mmap:
void *mmap(void *addr, size_t length, int prot, int flags,
int fd, off_t offset);
mmap can be called in many ways,
but this lab requires only
a subset of its features relevant to memory-mapping a file.
You can assume that
addr will always be zero, meaning that the
kernel should decide the virtual address at which to map the file.
mmap returns that address, or 0xffffffffffffffff if
length is the number of bytes to map; it might not be
the same as the file's length.
prot indicates whether the memory should be mapped
readable, writeable, and/or executable; you can assume
that prot is PROT_READ or PROT_WRITE
flags will be either MAP_SHARED,
meaning that modifications to the mapped memory should
be written back to the file, or MAP_PRIVATE,
meaning that they should not. You don't have to implement any
other bits in flags.
fd is the open file descriptor of the file to map.
You can assume offset is zero (it's the starting point
in the file at which to map).
It's OK if processes that map the same MAP_SHARED
file do not share physical pages.
munmap(addr, length) should remove mmap mappings in the
indicated address range. If the process has modified the memory and
has it mapped MAP_SHARED, the modifications should first be
written to the file. An munmap call might cover only a
portion of an mmap-ed region, but you can assume that it will either
unmap at the start, or at the end, or the whole region (but not punch
a hole in the middle of a region).
you should implement enough mmap and munmap
functionality to make the
mmaptest test program work. If mmaptest
doesn't use a mmap feature, you don't need to implement
When you're done, you should see this output:
mmaptest: all tests succeeded
test reparent2: OK
test bigdir: OK
ALL TESTS PASSED
Here are some hints:
- Start by adding mmap and munmap system calls
and associated flags (PROT_READ etc), in order to
get user/mmaptest.c to compile. For now, just return
errors from mmap and munmap.
Run mmaptest, which will fail at the first mmap call.
- Fill in the page table lazily, in response to page faults.
That is, mmap should not allocate physical memory or
read the file. Instead, do that in page fault handling code
in (or called by) usertrap, as in the lazy page allocation lab.
The reason to be lazy is to ensure that mmap of
a large file is fast, and that mmap of a file larger
than physical memory is possible.
- Keep track of what mmap has mapped for each process.
Define a structure corresponding to the VMA (virtual
memory area) described in Lecture 15,
recording the address, length, permissions, file, etc.
for a virtual memory range created by mmap. Since the xv6
kernel doesn't have a memory allocator in the kernel, it's OK to
declare a fixed-size array of VMAs and allocate
from that array as needed.
- Implement mmap:
find an unused region in the process's
address space in which to map the file,
and add a VMA to the process's
table of mapped regions.
The VMA should contain a pointer to
a struct file for the file being mapped; mmap should
increase the file's reference count so that the structure doesn't
disappear when the file is closed (hint:
Run mmaptest: the first mmap should
succeed, but the first access to the mmap-ed memory will
cause a page fault and kill mmaptest.
- Add code to cause a page-fault in a mmap-ed region to
allocate a page of physical memory, read 4096 bytes of
the relevant file into
that page, and map it into the user address space.
Read the file with readi,
which takes an offset argument at which to read in the
file (but you will have to lock/unlock the inode passed
to readi). Don't forget to set the permissions correctly
on the page. Run mmaptest; it should get to the
- Implement munmap: find the VMA for the address range and
unmap the specified pages (hint: use uvmunmap).
If munmap removes all pages of a
previous mmap, it should decrement the reference count
of the corresponding struct file. If an unmapped page
has been modified and the file is mapped MAP_SHARED,
write the page back to the file.
Look at filewrite for inspiration.
- Ideally your implementation would only write back
MAP_SHARED pages that the program actually modified.
The dirty bit (D) in the RISC-V PTE indicates whether a
page has been written. However, mmaptest does not check
that non-dirty pages are not written back; thus you can get away
with writing pages back without looking at D bits.
- Modify exit to unmap the process's mapped regions as
if munmap had been called.
Run mmaptest; mmap_test should pass, but
probably not fork_test.
- Modify fork to ensure that the child has the
same mapped regions as the parent.
forget to increment the reference count for a VMA's struct
file. In the page fault handler of the child, it is OK to
allocate a new physical page instead of sharing a page with the
parent. The latter would be cooler, but it would require more
implementation work. Run mmaptest; it should pass
both mmap_test and fork_test.
Run usertests to make sure everything still works.
- If two processes have the same file mmap-ed (as
in fork_test), share their physical pages. You will need
reference counts on physical pages.
- Your solution probably allocates a new physical page for each page
read from the mmap-ed file, even though the data is also in kernel
memory in the buffer cache. Modify your implementation to use
that physical memory, instead of allocating a new page. This requires that
file blocks be the same size as pages (set BSIZE to
4096). You will need to pin mmap-ed blocks into the buffer cache.
You will need worry about reference counts.
- Remove redundancy between your implementation for lazy
allocation and your implementation of mmap-ed files. (Hint:
create a VMA for the lazy allocation area.)
- Modify exec to use a VMA for different sections of
the binary so that you get on-demand-paged executables. This will
make starting programs faster, because exec will not have
to read any data from the file system.
- Implement page-out and page-in: have
the kernel move some parts of processes to disk when
physical memory is low. Then, page in the paged-out memory when
the process references it.