@itemize @bullet
@item
Some way of translating in software from virtual page frames to
-physical page frames. Consider using a hash table (@pxref{Hash
-Table}).
+physical page frames. Pintos provides a hash table that you may find
+useful for this purpose (@pxref{Hash Table}).
It is possible to do this translation without adding a new data
structure, by modifying the code in @file{userprog/pagedir.c}. However,
bytes in the final mapped page ``stick out'' beyond the end of the
file. Set these bytes to zero when the page is faulted in from disk,
and discard them when the page is written back to disk.
+A partial page need not be lazily loaded, as in the case of a partial
+page in an executable (@pxref{Lazy Loading}).
If successful, this function returns a ``mapping ID'' that
uniquely identifies the mapping within the process. On failure,
written back to the file, and pages not written must not be. The pages
are then removed from the process's list of virtual pages.
+Closing or removing a file does not unmap any of its mappings. Once
+created, a mapping is valid until @code{munmap} is called or the process
+exits, following the Unix convention. @xref{Removing an Open File}, for
+more information.
+
+If two or more processes map the same file, there is no requirement that
+they see consistent data. Unix handles this by making the two mappings
+share the same physical page, but the @code{mmap} system call also has
+an argument allowing the client to specify whether the page is shared or
+private (i.e.@: copy-on-write).
+
@node Project 3 FAQ
@section FAQ
Yes.
-@item How do I use the hash table provided in @file{lib/kernel/hash.c}?
-@anchor{Hash Table}
-
-First, you need to add a @struct{hash_elem} as a member of the
-object that the hash table will contain. Each @struct{hash_elem} allows
-the object to a member of at most one hash table at a given time. All
-the hash table functions that deal with hash table items actually use
-the address of a @struct{hash_elem}. You can convert a pointer to a
-@struct{hash_elem} member into a pointer to the structure in which
-member is embedded using the @code{hash_entry} macro.
-
-Second, you need to decide on a key type. The key should be something
-that is unique for each object, because a given hash table may not
-contain two objects with equal keys. Then you need to write two
-functions. The first is a @dfn{hash function} that converts a key
-into an integer. Some sample hash functions that you can use or just
-examine are given in @file{lib/kernel/hash.c}. The second needed
-function is a @dfn{comparison function} that compares a pair of objects
-and returns
-true if the first is less than the second. These two functions have
-to be compatible with the prototypes for @code{hash_hash_func} and
-@code{hash_less_func} in @file{lib/kernel/hash.h}.
-
-Here's a quick example. Suppose you want to put @struct{thread}s
-in a hash table. First, add a @struct{hash_elem} to the thread
-structure by adding a line to its definition:
-
-@example
-struct hash_elem h_elem; /* Hash table element. */
-@end example
-
-We'll choose the @code{tid} member in @struct{thread} as the key,
-and write a hash function and a comparison function:
-
-@example
-/* Returns a hash for E. */
-unsigned
-thread_hash (const struct hash_elem *e, void *aux UNUSED)
-@{
- struct thread *t = hash_entry (e, struct thread, h_elem);
- return hash_int (t->tid);
-@}
-
-/* Returns true if A's tid is less than B's tid. */
-bool
-thread_less (const struct hash_elem *a_,
- const struct hash_elem *b_,
- void *aux UNUSED)
-@{
- struct thread *a = hash_entry (a_, struct thread, h_elem);
- struct thread *b = hash_entry (b_, struct thread, h_elem);
- return a->tid < b->tid;
-@}
-@end example
-
-Then we can create a hash table like this:
-
-@example
-struct hash threads;
-
-hash_init (&threads, thread_hash, thread_less, NULL);
-@end example
-
-Finally, if @code{@var{t}} is a pointer to a @struct{thread},
-then we can insert it into the hash table with:
-
-@example
-hash_insert (&threads, &@var{t}->h_elem);
-@end example
-
-The CS109 and CS161 textbooks have chapters on hash tables.
-
-@item Why do the hash table functions have @var{aux} parameters?
-
-In simple cases you won't have any need for the @var{aux} parameters.
-In these cases you can just pass a null pointer to @func{hash_init}
-for @var{aux} and ignore the values passed to the hash function and
-comparison functions. (You'll get a compiler warning if you don't use
-the @var{aux} parameter, but you can turn that off with the
-@code{UNUSED} macro, as shown above, or you can just ignore it.)
-
-@var{aux} is useful when you have some property of the data in the
-hash table that's both constant and needed for hashing or comparisons,
-but which is not stored in the data items themselves. For example, if
-the items in a hash table contain fixed-length strings, but the items
-themselves don't indicate what that fixed length is, you could pass
-the length as an @var{aux} parameter.
-
-@item Can we change the hash table implementation?
-
-You are welcome to modify it. It is not used by any of the code we
-provided, so modifying it won't affect any code but yours. Do
-whatever it takes to make it work the way you want.
-
@item What extra credit is available?
You may implement sharing: when multiple processes are created that use
The @command{mcp} program in @file{src/examples} shows how to copy a
file using memory-mapped I/O.
-
-@item What if two processes map the same file into memory?
-
-There is no requirement in Pintos that the two processes see
-consistent data. Unix handles this by making the two mappings share the
-same physical page, but the @code{mmap} system call also has an
-argument allowing the client to specify whether the page is shared or
-private (i.e.@: copy-on-write).
-
-@item What happens if a user removes a @code{mmap}'d file?
-
-The mapping should remain valid, following the Unix convention.
-@xref{Removing an Open File}, for more information.
-
-@item If a user closes a mapped file, should it be automatically unmapped?
-
-No. Once created the mapping is valid until @code{munmap} is called
-or the process exits.
@end table
projects / pintos-anon / blobdiff