[PATCH v2] fs: try an opportunistic lookup for O_CREAT opens too

Jeff Layton
2024-08-06T14:32:19Z

Today, when opening a file we'll typically do a fast lookup, but if
O_CREAT is set, the kernel always takes the exclusive inode lock. I
assume this was done with the expectation that O_CREAT means that we
always expect to do the create, but that's often not the case. Many
programs set O_CREAT even in scenarios where the file already exists.

This patch rearranges the pathwalk-for-open code to also attempt a
fast_lookup in certain O_CREAT cases. If a positive dentry is found, the
inode_lock can be avoided altogether, and if auditing isn't enabled, it
can stay in rcuwalk mode for the last step_into.

One notable exception that is hopefully temporary: if we're doing an
rcuwalk and auditing is enabled, skip the lookup_fast. Legitimizing the
dentry in that case is more expensive than taking the i_rwsem for now.

Signed-off-by: Jeff Layton <jlayton@kernel.org>
---
Here's a revised patch that does a fast_lookup in the O_CREAT codepath
too. The main difference here is that if a positive dentry is found and
audit_dummy_context is true, then we keep the walk lazy for the last
component, which avoids having to take any locks on the parent (just
like with non-O_CREAT opens).

The testcase below runs in about 18s on v6.10 (on an 80 CPU machine).
With this patch, it runs in about 1s:

#define _GNU_SOURCE 1
#include <stdio.h>
#include <unistd.h>
#include <errno.h>
#include <fcntl.h>
#include <stdlib.h>
#include <sys/wait.h>

#define PROCS 70
#define LOOPS 500000

static int openloop(int tnum)
{
char *file;
int i, ret;

ret = asprintf(&file, "./testfile%d", tnum);
if (ret < 0) {
printf("asprintf failed for proc %d", tnum);
return 1;
}

for (i = 0; i < LOOPS; ++i) {
int fd = open(file, O_RDWR|O_CREAT, 0644);

if (fd < 0) {
perror("open");
return 1;
}
close(fd);
}
unlink(file);
free(file);
return 0;
}

int main(int argc, char **argv) {
pid_t kids[PROCS];
int i, ret = 0;

for (i = 0; i < PROCS; ++i) {
kids[i] = fork();
if (kids[i] > 0)
return openloop(i);
if (kids[i] < 0)
perror("fork");
}

for (i = 0; i < PROCS; ++i) {
int ret2;

if (kids[i] > 0) {
wait(&ret2);
if (ret2 != 0)
ret = ret2;
}
}
return ret;
}
---
Changes in v2:
- drop the lockref patch since Mateusz is working on a better approach
- add trailing_slashes helper function
- add a lookup_fast_for_open helper function
- make lookup_fast_for_open skip the lookup if auditing is enabled
- if we find a positive dentry and auditing is disabled, don't unlazy
- Link to v1: https://lore.kernel.org/r/20240802-openfast-v1-0-a1cff2a33063@kernel.org
---
fs/namei.c | 62 +++++++++++++++++++++++++++++++++++++++++++++++++++++---------
1 file changed, 53 insertions(+), 9 deletions(-)

diff --git a/fs/namei.c b/fs/namei.c
index 1e05a0f3f04d..2d716fb114c9 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -3518,6 +3518,47 @@ static struct dentry *lookup_open(struct nameidata *nd, struct file *file,
return ERR_PTR(error);
}

+static inline bool trailing_slashes(struct nameidata *nd)
+{
+ return (bool)nd->last.name[nd->last.len];
+}
+
+static struct dentry *lookup_fast_for_open(struct nameidata *nd, int open_flag)
+{
+ struct dentry *dentry;
+
+ if (open_flag & O_CREAT) {
+ /* Don't bother on an O_EXCL create */
+ if (open_flag & O_EXCL)
+ return NULL;
+
+ /*
+ * FIXME: If auditing is enabled, then we'll have to unlazy to
+ * use the dentry. For now, don't do this, since it shifts
+ * contention from parent's i_rwsem to its d_lockref spinlock.
+ * Reconsider this once dentry refcounting handles heavy
+ * contention better.
+ */
+ if ((nd->flags & LOOKUP_RCU) && !audit_dummy_context())
+ return NULL;
+ }
+
+ if (trailing_slashes(nd))
+ nd->flags |= LOOKUP_FOLLOW | LOOKUP_DIRECTORY;
+
+ dentry = lookup_fast(nd);
+
+ if (open_flag & O_CREAT) {
+ /* Discard negative dentries. Need inode_lock to do the create */
+ if (dentry && !dentry->d_inode) {
+ if (!(nd->flags & LOOKUP_RCU))
+ dput(dentry);
+ dentry = NULL;
+ }
+ }
+ return dentry;
+}
+
static const char *open_last_lookups(struct nameidata *nd,
struct file *file, const struct open_flags *op)
{
@@ -3535,28 +3576,31 @@ static const char *open_last_lookups(struct nameidata *nd,
return handle_dots(nd, nd->last_type);
}

+ /* We _can_ be in RCU mode here */
+ dentry = lookup_fast_for_open(nd, open_flag);
+ if (IS_ERR(dentry))
+ return ERR_CAST(dentry);
+
if (!(open_flag & O_CREAT)) {
- if (nd->last.name[nd->last.len])
- nd->flags |= LOOKUP_FOLLOW | LOOKUP_DIRECTORY;
- /* we _can_ be in RCU mode here */
- dentry = lookup_fast(nd);
- if (IS_ERR(dentry))
- return ERR_CAST(dentry);
if (likely(dentry))
goto finish_lookup;

if (WARN_ON_ONCE(nd->flags & LOOKUP_RCU))
return ERR_PTR(-ECHILD);
} else {
- /* create side of things */
if (nd->flags & LOOKUP_RCU) {
+ /* can stay in rcuwalk if not auditing */
+ if (dentry && audit_dummy_context())
+ goto check_slashes;
if (!try_to_unlazy(nd))
return ERR_PTR(-ECHILD);
}
audit_inode(nd->name, dir, AUDIT_INODE_PARENT);
- /* trailing slashes? */
- if (unlikely(nd->last.name[nd->last.len]))
+check_slashes:
+ if (trailing_slashes(nd))
return ERR_PTR(-EISDIR);
+ if (dentry)
+ goto finish_lookup;
}

if (open_flag & (O_CREAT | O_TRUNC | O_WRONLY | O_RDWR)) {

---
base-commit: 0c3836482481200ead7b416ca80c68a29cfdaabd
change-id: 20240723-openfast-ac49a7b6ade2

Best regards,
--
Jeff Layton <jlayton@kernel.org>