On Fri, 2018-10-19 at 22:48 +0200, Miklos Szeredi wrote:
> On Fri, Oct 19, 2018 at 8:14 PM, Trond Myklebust
> <trondmy@xxxxxxxxxxxxxxx> wrote:
> > On Fri, 2018-10-19 at 19:46 +0200, Miklos Szeredi wrote:
> > > How is it then that only STATX_ATIME is cleared and not the other
> > > fields?
> >
> > It isn't just the atime. We can also fail to revalidate the ctime
> > and
> > mtime if they are not being requested by the user.
> >
> > >
> > > Note: junk != stale. The statx definition doesn't talk about the
> > > fields being up-to-date, except for AT_STATX_FORCE_SYNC, so stale
> > > attributes are okay, and do not warrant clearing the result_mask.
> > >
> >
> > I disagree. stale == junk here, because the default of
> > AT_STATX_SYNC_AS_STAT is described by the manpage as "Do whatever
> > stat(2) does." which this is not.
>
> Ah, you are talking about this:
>
> /* Is the user requesting attributes that might need
> revalidation? */
> if (!(request_mask &
> (STATX_MODE|STATX_NLINK|STATX_ATIME|STATX_CTIME|
> STATX_MTIME|STATX_UID|STATX_GID|
> STATX_SIZE|STATX_BLOCKS)))
> goto out_no_update;
>
> Well, if this is triggered for statx(..., STATX_ATIME,
> AT_STATX_SYNC_AS_STAT) and MNT_NOATIME, then yes, result will be
> junk.
> Which means that the code is wrong, it shouldn't do that.
The problem is that vfs_getattr_nosec() populates stat->result_mask
with a default of STATX_BASIC_STATS, which makes no sense unless you
assume that the user will always ask for a superset of
STATX_BASIC_STATS (or you assume that those attributes never need
revalidation, which is obviously braindead).
> Otherwise (if something other than STATX_ATIME or STATX_INO or
> STATX_TYPE is given as well) it *will* do the same thing as what
> stat(2) does, so in that case STATX_ATIME should not be cleared (yet
> it is cleared).
As far as I'm concerned, we can definitely get rid of the
/*
* We may force a getattr if the user cares about atime.
*
* Note that we only have to check the vfsmount flags here:
* - NFS always sets S_NOATIME by so checking it would give a
* bogus result
* - NFS never sets SB_NOATIME or SB_NODIRATIME so there is
* no point in checking those.
*/
if ((path->mnt->mnt_flags & MNT_NOATIME) ||
((path->mnt->mnt_flags & MNT_NODIRATIME) && S_ISDIR(inode->i_mode)))
request_mask &= ~STATX_ATIME;
however the rest needs to stay, or there is no way we can use statx()
to allow optimised retrieval of only those attributes that your
application cares about.
--
Trond Myklebust
Linux NFS client maintainer, Hammerspace
trond.myklebust@xxxxxxxxxxxxxxx
