fs: improve codegen in link_path_walk()

Looking at the asm produced by gcc 13.3 for x86-64:
1. may_lookup() usage was not optimized for succeeding, despite the
   routine being inlined and rightfully starting with likely(!err)
2. the compiler assumed the path will have an indefinite amount of
   slashes to skip, after which the result will be an empty name

As such:
1. predict may_lookup() succeeding
2. check for one slash, no explicit predicts. do roll forward with
   skipping more slashes while predicting there is only one
3. predict the path to find was not a mere slash

This also has a side effect of shrinking the file:
add/remove: 1/1 grow/shrink: 0/3 up/down: 934/-1012 (-78)
Function                                     old     new   delta
link_path_walk                                 -     934    +934
path_parentat                                138     112     -26
path_openat                                 4864    4823     -41
path_lookupat                                418     374     -44
link_path_walk.part.constprop                901       -    -901
Total: Before=46639, After=46561, chg -0.17%

Signed-off-by: Mateusz Guzik <mjguzik@gmail.com>
Link: https://lore.kernel.org/20250412110935.2267703-1-mjguzik@gmail.com
Signed-off-by: Christian Brauner <brauner@kernel.org>

diff --git a/fs/namei.c b/fs/namei.c
index ae2643ff14dc795d7847a9d1d377acda86c159cb..b9cc03faa03350500bdbf1fd39338e9b9cc83699 100644 (file)
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -2424,9 +2424,12 @@ static int link_path_walk(const char *name, struct nameidata *nd)
        nd->flags |= LOOKUP_PARENT;
        if (IS_ERR(name))
                return PTR_ERR(name);
-       while (*name=='/')
-               name++;
-       if (!*name) {
+       if (*name == '/') {
+               do {
+                       name++;
+               } while (unlikely(*name == '/'));
+       }
+       if (unlikely(!*name)) {
                nd->dir_mode = 0; // short-circuit the 'hardening' idiocy
                return 0;
        }
@@ -2439,7 +2442,7 @@ static int link_path_walk(const char *name, struct nameidata *nd)
 
                idmap = mnt_idmap(nd->path.mnt);
                err = may_lookup(idmap, nd);
-               if (err)
+               if (unlikely(err))
                        return err;
 
                nd->last.name = name;