On 6/18/24 04:17, Clément Léger wrote:
Since commit 03e471c41d8b ("qemu_init: increase NOFILE soft limit on
POSIX"), the maximum number of file descriptors that can be opened are
raised to nofile.rlim_max. On recent debian distro, this yield a maximum
of 1073741816 file descriptors. Now, when forking to start
qemu-bridge-helper, this actually calls close() on the full possible file
descriptor range (more precisely [3 - sysconf(_SC_OPEN_MAX)]) which
takes a considerable amount of time. In order to reduce that time,
factorize existing code to close all open files descriptors in a new
qemu_close_all_open_fd() function. This function uses various methods
to close all the open file descriptors ranging from the most efficient
one to the least one. It also accepts an ordered array of file
descriptors that should not be closed since this is required by the
callers that calls it after forking.
Signed-off-by: Clément Léger <cle...@rivosinc.com>
----
v2:
- Factorize async_teardown.c close_fds implementation as well as tap.c ones
- Apply checkpatch
- v1:
https://lore.kernel.org/qemu-devel/20240617162520.4045016-1-cle...@rivosinc.com/
---
include/qemu/osdep.h | 8 +++
net/tap.c | 31 ++++++-----
system/async-teardown.c | 37 +------------
util/osdep.c | 115 ++++++++++++++++++++++++++++++++++++++++
4 files changed, 141 insertions(+), 50 deletions(-)
diff --git a/include/qemu/osdep.h b/include/qemu/osdep.h
index f61edcfdc2..9369a97d3d 100644
--- a/include/qemu/osdep.h
+++ b/include/qemu/osdep.h
@@ -755,6 +755,14 @@ static inline void qemu_reset_optind(void)
int qemu_fdatasync(int fd);
+/**
+ * Close all open file descriptors except the ones supplied in the @skip array
+ *
+ * @skip: ordered array of distinct file descriptors that should not be closed
+ * @nskip: number of entries in the @skip array.
+ */
+void qemu_close_all_open_fd(const int *skip, unsigned int nskip);
+
/**
* Sync changes made to the memory mapped file back to the backing
* storage. For POSIX compliant systems this will fallback
diff --git a/net/tap.c b/net/tap.c
index 51f7aec39d..6fc3939078 100644
--- a/net/tap.c
+++ b/net/tap.c
@@ -385,6 +385,21 @@ static TAPState *net_tap_fd_init(NetClientState *peer,
return s;
}
+static void close_all_fds_after_fork(int excluded_fd)
+{
+ const int skip_fd[] = {0, 1, 2, 3, excluded_fd};
3 should not be included here...
+ unsigned int nskip = ARRAY_SIZE(skip_fd);
+
+ /*
+ * skip_fd must be an ordered array of distinct fds, exclude
+ * excluded_fd if already included in the [0 - 3] range
+ */
+ if (excluded_fd <= 3) {
or here -- stdin is 0, stdout is 1, stderr is 2.
Perhaps we need reminding of this and use the STD*_FILENO names instead of raw integer
constants.
@@ -400,13 +415,7 @@ static void launch_script(const char *setup_script, const
char *ifname,
return;
}
if (pid == 0) {
- int open_max = sysconf(_SC_OPEN_MAX), i;
-
- for (i = 3; i < open_max; i++) {
- if (i != fd) {
- close(i);
- }
- }
Note that the original *does* close 3.
+#ifdef CONFIG_LINUX
+static bool qemu_close_all_open_fd_proc(const int *skip, unsigned int nskip)
+{
+ struct dirent *de;
+ int fd, dfd;
+ bool close_fd;
+ DIR *dir;
+ int i;
+
+ dir = opendir("/proc/self/fd");
+ if (!dir) {
+ /* If /proc is not mounted, there is nothing that can be done. */
+ return false;
+ }
+ /* Avoid closing the directory. */
+ dfd = dirfd(dir);
+
+ for (de = readdir(dir); de; de = readdir(dir)) {
+ fd = atoi(de->d_name);
+ close_fd = true;
+ if (fd == dfd) {
+ close_fd = false;
+ } else {
+ for (i = 0; i < nskip; i++) {
The skip list is sorted, so you should remember the point of the last search and begin
from there, and you should not search past fd < skip[i].
+#else
+static bool qemu_close_all_open_fd_proc(const int *skip, unsigned int nskip)
+{
+ return false;
+}
+#endif
I'm not fond of duplicating the function declaration.
I think it's better to move the ifdef inside:
static bool foo(...)
{
#ifdef XYZ
impl
#else
stub
#endif
}
+
+#ifdef CONFIG_CLOSE_RANGE
+static bool qemu_close_all_open_fd_close_range(const int *skip,
+ unsigned int nskip)
+{
+ int max_fd = sysconf(_SC_OPEN_MAX) - 1;
+ int first = 0, last = max_fd;
+ int cur_skip = 0, ret;
+
+ do {
+ if (nskip) {
+ while (first == skip[cur_skip]) {
+ cur_skip++;
+ first++;
+ }
This fails to check cur_skip < nskip in the loop.
Mixing signed cur_skip with unsigned nskip is bad.
There seems to be no good reason for the separate "if (nskip)" check.
A proper check for cur_skip < nskip will work just fine with nskip == 0.
+ /* Fallback */
+ for (i = 0; i < open_max; i++) {
+ if (i == skip[cur_skip]) {
+ cur_skip++;
+ continue;
+ }
+ close(i);
+ }
Missing nskip test.
r~
