On Thu, Feb 27, 2020 at 05:55:43PM +0800, Ian Kent wrote:

Not all file systems even produce negative hashed dentries.

The most beneficial use of them is to improve performance of rapid
fire lookups for non-existent names. Longer lived negative hashed
dentries don't give much benefit at all unless they suddenly have
lots of hits and that would cost a single allocation on the first
lookup if the dentry ttl expired and the dentry discarded.

A ttl (say jiffies) set at appropriate times could be a better
choice all round, no sysctl values at all.

The canonical argument in favour of negative dentries is to improve
application startup time as every application searches the library path
for the same libraries.  Only they don't do that any more:

$ strace -e file cat /dev/null
execve("/bin/cat", ["cat", "/dev/null"], 0x7ffd5f7ddda8 /* 44 vars */) = 0
access("/etc/ld.so.preload", R_OK)      = -1 ENOENT (No such file or directory)
openat(AT_FDCWD, "/etc/ld.so.cache", O_RDONLY|O_CLOEXEC) = 3
openat(AT_FDCWD, "/lib/x86_64-linux-gnu/libc.so.6", O_RDONLY|O_CLOEXEC) = 3
openat(AT_FDCWD, "/usr/lib/locale/locale-archive", O_RDONLY|O_CLOEXEC) = 3
openat(AT_FDCWD, "/dev/null", O_RDONLY) = 3

So, are they still useful?  Or should we, say, keep at most 100 around?