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authorLinus Torvalds <torvalds@linux-foundation.org>2023-09-03 13:08:03 -0700
committerLinus Torvalds <torvalds@linux-foundation.org>2023-09-07 09:40:30 -0700
commit9013c51c630ac5a046aa2d51d67e966b6185f1cd (patch)
treefd4d3a044b78dd23148fc03a7947e8b039ddbe63
parent7ba2090ca64ea1aa435744884124387db1fac70f (diff)
vfs: mostly undo glibc turning 'fstat()' into 'fstatat(AT_EMPTY_PATH)'
Mateusz reports that glibc turns 'fstat()' calls into 'fstatat()', and that seems to have been going on for quite a long time due to glibc having tried to simplify its stat logic into just one point. This turns out to cause completely unnecessary overhead, where we then go off and allocate the kernel side pathname, and actually look up the empty path. Sure, our path lookup is quite optimized, but it still causes a fair bit of allocation overhead and a couple of completely unnecessary rounds of lockref accesses etc. This is all hopefully getting fixed in user space, and there is a patch floating around for just having glibc use the native fstat() system call. But even with the current situation we can at least improve on things by catching the situation and short-circuiting it. Note that this is still measurably slower than just a plain 'fstat()', since just checking that the filename is actually empty is somewhat expensive due to inevitable user space access overhead from the kernel (ie verifying pointers, and SMAP on x86). But it's still quite a bit faster than actually looking up the path for real. To quote numers from Mateusz: "Sapphire Rapids, will-it-scale, ops/s stock fstat 5088199 patched fstat 7625244 (+49%) real fstat 8540383 (+67% / +12%)" where that 'stock fstat' is the glibc translation of fstat into fstatat() with an empty path, the 'patched fstat' is with this short circuiting of the path lookup, and the 'real fstat' is the actual native fstat() system call with none of this overhead. Link: https://lore.kernel.org/lkml/20230903204858.lv7i3kqvw6eamhgz@f/ Reported-by: Mateusz Guzik <mjguzik@gmail.com> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
-rw-r--r--fs/stat.c17
1 files changed, 17 insertions, 0 deletions
diff --git a/fs/stat.c b/fs/stat.c
index 136711ae72fb..6822ac77aec2 100644
--- a/fs/stat.c
+++ b/fs/stat.c
@@ -311,6 +311,23 @@ int vfs_fstatat(int dfd, const char __user *filename,
int statx_flags = flags | AT_NO_AUTOMOUNT;
struct filename *name;
+ /*
+ * Work around glibc turning fstat() into fstatat(AT_EMPTY_PATH)
+ *
+ * If AT_EMPTY_PATH is set, we expect the common case to be that
+ * empty path, and avoid doing all the extra pathname work.
+ */
+ if (dfd >= 0 && flags == AT_EMPTY_PATH) {
+ char c;
+
+ ret = get_user(c, filename);
+ if (unlikely(ret))
+ return ret;
+
+ if (likely(!c))
+ return vfs_fstat(dfd, stat);
+ }
+
name = getname_flags(filename, getname_statx_lookup_flags(statx_flags), NULL);
ret = vfs_statx(dfd, name, statx_flags, stat, STATX_BASIC_STATS);
putname(name);