Hello, I'm trying to implement mmap support in tarfs.
The reason I need it is I want to run some binaries directly form a tarfs mount, without unpacking them to disk (ext2fs). ld.so uses mmap to load shared libraries uncoditionally [0]. So I've been digging through libpager, libnetfs, tarfs, and other code, trying to understand how things fit together and hook them up. The patch series I'm attaching implements mmap by lazily creating a pager for each node. The pager reads and writes data the same way tarfs_read_node () and tarfs_write_node () do: namely, by calling into the existing cache implementation. The most challenging part has been "mmap coherence" (perhaps there is an established term for this concept that I'm not aware of?). Basically, the file system has to guarantee that any modificaitons made to a file via io_write () (and other similar APIs) are immediately visible through any mappings of the file, and similarly any changes made through a mapping have to be immediately visible through io_read (). I peeked at how libdiskfs deals with this (since this does work as expected with ext2fs in my testing), and it forwards any reads/writes to pager_memcpy () instead of accessing the underlying data directly. I've tried to do the same, except that I only forward the calls to a pager if there is one. I've written a tiny program to test mmap behavior (I've uploaded its source here [1]), and used it to verify that my implementation behaves somewhat sanely. I'm also able to load binaries and shared libraries from tarfs with this patchset. In fact, I'm able to run code chrooted to tarfs. Please help me out by commenting on the patches. Does the overall approach look fine, or am I missing some crucial details? Have I messed up reference counting? Is there a better way to deal with a held mutex that unlocking it before calling a function that re-locks it? (Making the mutex recursive, perhaps? Or extracting the inner part into its own function?) [0]: https://www.gnu.org/software/hurd/open_issues/libnetfs_io_map.html [1]: https://paste.gg/p/anonymous/7de6f56e7a1c4babbab17bd6ccc2db46 Sergey Bugaev (6): Plumb io_map () through the backend layer Link to libpager and initialize it on startup Implement basic read-only mmap support Implement basic support for writable mappings Attempt to implement mmap coherence Update TODO and BUGS BUGS | 1 - Makefile | 4 +- TODO | 2 - backend.h | 3 ++ main.c | 13 +++++ netfs.c | 36 +++++++++++--- pager.c | 141 ++++++++++++++++++++++++++++++++++++++++++++++++++++++ pager.h | 9 ++++ tarfs.c | 114 ++++++++++++++++++++++++++++++++++++++----- tarfs.h | 2 + 10 files changed, 302 insertions(+), 23 deletions(-) create mode 100644 pager.c create mode 100644 pager.h -- 2.31.1
