[Pvfs2-developers] ACL/getxattr overhead in ls

[Murali Vilayannur]

Hi Phil,

> We have noticed on RedHat RHEL3 and RHEL4 that when you do a long
> listing of a directory (on any file system, not just pvfs2) that the ls
> triggers a getxattr of "system.posix_acl_access" and
> "system.posix_acl_default" for every single entry that is listed.  This
> happens regardless of whether ACLs are enabled for the file system or not.
>
> Digging in the coreutils/ls source, it turns out that ls (with -l
> options at least) is checking to see if any acl's are set on each entry
> so that it can display them differently, maybe with a "+" character.
>
> On RHEL3 at least (2.4 kernel) these extra getxattrs are causing 2
> serialized getxattr requests that are probably significantly slowing
> down the listing, which is a shame because the 2.4 kernel module doesn't
> even support ACLs so there is never any point in checking.
>
> Does it seem reasonable to add a test in the kernel module so that if a)
> acl's are not enabled (which can be determined by checking the super
> block acl flag), and b) the getxattr name matches "system.posix_acl"
> then we just immediately return an appropriate error code rather than
> servicing the getxattr?

I don't understand how this situation even happens..
There is already such a check in acl.c in the function
pvfs2_xattr_get_acl() to see if we support acl's or not before calling the
underlying getxattr.
Can you find out if the strace of an ls -l causes the getxattr() to
return -EOPNOTSUPP instead of -ENODATA?

Thanks,
Murali

>
> The stock coreutils package doesn't currently have this problem, because
> their acl check is broken for Linux.  RedHat supplies a patch with their
> rpm that adds acl checks that work on linux (using the
> acl_extended_file() function rather than the unsupported acl()
> function).  I imagine there are other distributions that have done
> likewise, and probably it will be "fixed" in the official gnu version
> eventually also.
>
> -Phil
>
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