[RFC PATCH 1/5] lguest: mmap backing file

Rusty Russell
2008-03-20T06:05:44Z

From: Paul TBBle Hampson <Paul.Hampson-vM6MUUi4OUAAvxtiuMwx3w@public.gmane.org>

This creates a file in $HOME/.lguest/ to directly back the RAM and DMA memory
mappings created by map_zeroed_pages.

Signed-off-by: Rusty Russell <rusty-8n+1lVoiYb80n/F98K4Iww@public.gmane.org>
---
Documentation/lguest/lguest.c | 59 ++++++++++++++++++++++++++++--------------
1 file changed, 40 insertions(+), 19 deletions(-)

diff --git a/Documentation/lguest/lguest.c b/Documentation/lguest/lguest.c
--- a/Documentation/lguest/lguest.c
+++ b/Documentation/lguest/lguest.c
@@ -236,19 +236,51 @@ static int open_or_die(const char *name,
return fd;
}

-/* map_zeroed_pages() takes a number of pages. */
+/* unlink_memfile() removes the backing file for the Guest's memory, if we exit
+ * cleanly. */
+static char memfile_path[PATH_MAX];
+
+static void unlink_memfile(void)
+{
+ unlink(memfile_path);
+}
+
+/* map_zeroed_pages() takes a number of pages, and creates a mapping file where
+ * this Guest's memory lives. */
static void *map_zeroed_pages(unsigned int num)
{
- int fd = open_or_die("/dev/zero", O_RDONLY);
+ int fd;
void *addr;

- /* We use a private mapping (ie. if we write to the page, it will be
- * copied). */
+ /* We create a .lguest directory in the user's home, to put the memory
+ * files into. */
+ snprintf(memfile_path, PATH_MAX, "%s/.lguest", getenv("HOME") ?: "");
+ if (mkdir(memfile_path, S_IRWXU) != 0 && errno != EEXIST)
+ err(1, "Creating directory %s", memfile_path);
+
+ /* Name the memfiles by the process ID of this launcher. */
+ snprintf(memfile_path, PATH_MAX, "%s/.lguest/%u",
+ getenv("HOME") ?: "", getpid());
+ fd = open(memfile_path, O_RDWR | O_CREAT | O_TRUNC, S_IRWXU);
+ if (fd < 0)
+ err(1, "Creating memory backing file %s", memfile_path);
+
+ /* Make sure we remove it when we're finished. */
+ atexit(unlink_memfile);
+
+ /* Now, we opened it with O_TRUNC, so the file is 0 bytes long. Here
+ * we expand it to the length we need, and it will be filled with
+ * zeroes. */
+ if (ftruncate(fd, num * getpagesize()) != 0)
+ err(1, "Truncating file %s %u pages", memfile_path, num);
+
+ /* We use a shared mapping, so others can share with us. */
addr = mmap(NULL, getpagesize() * num,
- PROT_READ|PROT_WRITE|PROT_EXEC, MAP_PRIVATE, fd, 0);
+ PROT_READ|PROT_WRITE|PROT_EXEC, MAP_SHARED, fd, 0);
if (addr == MAP_FAILED)
err(1, "Mmaping %u pages of /dev/zero", num);

+ verbose("Memory backing file is %s @ %p\n", memfile_path, addr);
return addr;
}

@@ -263,23 +295,12 @@ static void *get_pages(unsigned int num)
return addr;
}

-/* This routine is used to load the kernel or initrd. It tries mmap, but if
- * that fails (Plan 9's kernel file isn't nicely aligned on page boundaries),
- * it falls back to reading the memory in. */
+/* This routine is used to load the kernel or initrd. We used to mmap, but now
+ * we simply read it in, so it will be present in the shared underlying
+ * file. */
static void map_at(int fd, void *addr, unsigned long offset, unsigned long len)
{
ssize_t r;
-
- /* We map writable even though for some segments are marked read-only.
- * The kernel really wants to be writable: it patches its own
- * instructions.
- *
- * MAP_PRIVATE means that the page won't be copied until a write is
- * done to it. This allows us to share untouched memory between
- * Guests. */
- if (mmap(addr, len, PROT_READ|PROT_WRITE|PROT_EXEC,
- MAP_FIXED|MAP_PRIVATE, fd, offset) != MAP_FAILED)
- return;

/* pread does a seek and a read in one shot: saves a few lines. */
r = pread(fd, addr, len, offset);