"ls -l": Avoid unnecessary getxattr() overhead

[Sven Breuner]

Hi,

when strace'ing "ls -l" on Linux, I see that it runs three syscalls for 
every file in the directory, e.g.:

$ strace ls -l
...
lstat("test", {st_mode=S_IFREG|0644, st_size=0, ...}) = 0
lgetxattr("test", "security.selinux", 0x62d130, 255) = -1 ENODATA (No 
data available)
getxattr("test", "system.posix_acl_access", 0x0, 0) = -1 EOPNOTSUPP 
(Operation not supported)

lstat("test2", {st_mode=S_IFREG|0644, st_size=0, ...}) = 0
lgetxattr("test2", "security.selinux", 0x62b350, 255) = -1 ENODATA (No 
data available)
getxattr("test2", "system.posix_acl_access", 0x0, 0) = -1 EOPNOTSUPP 
(Operation not supported)

lstat("test3", {st_mode=S_IFREG|0644, st_size=0, ...}) = 0
lgetxattr("test3", "security.selinux", 0x62d150, 255) = -1 ENODATA (No 
data available)
getxattr("test3", "system.posix_acl_access", 0x0, 0) = -1 EOPNOTSUPP 
(Operation not supported)
...

The first operation is lstat(), which definitely makes sense. The other 
two operations are
1) lgetxattr(..., "security.selinux", ...): Doesn't make sense imho, 
because selinux isn't even enabled on the system on which I ran "ls -l".
2) getxattr(..., "system.posix_acl_access", ...): Doesn't make sense, as 
ACLs are not supported on this file system (=> EOPNOTSUPP).

While these extra calls might not really be a problem for local file 
systems, they are a real pain for remote file systems, such as NFS or 
FhGFS (on which I'm working) and probably also for FUSE-based file systems.
The problem is that these extra syscalls lead to an inode revalidation 
inside the (Linux) kernel and thus result in extra remote calls for 
every file, so it would be really great if they could be avoided after 
detecting that they don't make sense in a particular directory.

I don't want to start a philosophical discussion about large 
directories, but as a matter of fact, we often have users with tens of 
thousands of files inside a single directory and those users need to run 
"ls -l" in these directories frequently to monitor the status of thei 
compute job and things like that.
Currently, they need to wait three times longer than necessary to see 
the result, due to the two extra syscalls for each file.

What I would like to suggest is detecting whether these extra calls make 
sense and disable them if they don't make sense:
1) For lgetxattr(..., "security.selinux", ...), ls could simply check 
whether selinux is enabled and skip the calls if it isn't enabled.
2) For getxattr(..., "system.posix_acl_access", ...), ls could check 
whether the first call returns EOPNOTSUPP and then skip this call for 
the remaining directory entries, because those will just have the same 
return value.

Best regards,
Sven

---

Sven Breuner
Fraunhofer HPC
FhGFS Team
http://www.fhgfs.com