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fs: tidy up step_into() & friends before inlining

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Symlink handling is already marked as unlikely and pushing out some of
it into pick_link() reduces register spillage on entry to step_into()
with gcc 14.2.

The compiler needed additional convincing that handle_mounts() is
unlikely to fail.

At the same time neither clang nor gcc could be convinced to tail-call
into pick_link().

While pick_link() takes an address of stack-based object as an argument
(which definitely prevents the optimization), splitting it into separate
<dentry, mount> tuple did not help. The issue persists even when
compiled without stack protector. As such nothing was done about this
for the time being to not grow the diff.

Signed-off-by: Mateusz Guzik <mjguzik@gmail.com>
Link: https://patch.msgid.link/20251120003803.2979978-1-mjguzik@gmail.com
Signed-off-by: Christian Brauner <brauner@kernel.org>
This commit is contained in:
Mateusz Guzik
2025-11-20 01:38:02 +01:00
committed by Christian Brauner
parent 1ed45a4ddc
commit 9d2a6211a7
1 changed files with 18 additions and 13 deletions
Download Patch File Download Diff File Expand all files Collapse all files
fs/namei.c
+18 -13
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@@ -1672,13 +1672,15 @@ static inline int handle_mounts(struct nameidata *nd, struct dentry *dentry,
path->dentry = dentry;
if (nd->flags & LOOKUP_RCU) {
unsigned int seq = nd->next_seq;
if (likely(!d_managed(dentry)))
return 0;
if (likely(__follow_mount_rcu(nd, path)))
return 0;
// *path and nd->next_seq might've been clobbered
path->mnt = nd->path.mnt;
path->dentry = dentry;
nd->next_seq = seq;
if (!try_to_unlazy_next(nd, dentry))
if (unlikely(!try_to_unlazy_next(nd, dentry)))
return -ECHILD;
}
ret = traverse_mounts(path, &jumped, &nd->total_link_count, nd->flags);
@@ -1941,13 +1943,23 @@ static int reserve_stack(struct nameidata *nd, struct path *link)
enum {WALK_TRAILING = 1, WALK_MORE = 2, WALK_NOFOLLOW = 4};
static const char *pick_link(struct nameidata *nd, struct path *link,
static noinline const char *pick_link(struct nameidata *nd, struct path *link,
struct inode *inode, int flags)
{
struct saved *last;
const char *res;
int error = reserve_stack(nd, link);
int error;
if (nd->flags & LOOKUP_RCU) {
/* make sure that d_is_symlink from step_into() matches the inode */
if (read_seqcount_retry(&link->dentry->d_seq, nd->next_seq))
return ERR_PTR(-ECHILD);
} else {
if (link->mnt == nd->path.mnt)
mntget(link->mnt);
}
error = reserve_stack(nd, link);
if (unlikely(error)) {
if (!(nd->flags & LOOKUP_RCU))
path_put(link);
@@ -2026,9 +2038,10 @@ static const char *step_into(struct nameidata *nd, int flags,
{
struct path path;
struct inode *inode;
int err = handle_mounts(nd, dentry, &path);
int err;
if (err < 0)
err = handle_mounts(nd, dentry, &path);
if (unlikely(err < 0))
return ERR_PTR(err);
inode = path.dentry->d_inode;
if (likely(!d_is_symlink(path.dentry)) ||
@@ -2050,14 +2063,6 @@ static const char *step_into(struct nameidata *nd, int flags,
nd->seq = nd->next_seq;
return NULL;
}
if (nd->flags & LOOKUP_RCU) {
/* make sure that d_is_symlink above matches inode */
if (read_seqcount_retry(&path.dentry->d_seq, nd->next_seq))
return ERR_PTR(-ECHILD);
} else {
if (path.mnt == nd->path.mnt)
mntget(path.mnt);
}
return pick_link(nd, &path, inode, flags);
}