linux-kernel - Re: [GIT PULL] Ceph fixes for 5.1-rc7

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Date:   Sun, 28 Apr 2019 11:47:58 -0400
From:   Jeff Layton <jlayton@...nel.org>
To:     Al Viro <viro@...iv.linux.org.uk>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Ilya Dryomov <idryomov@...il.com>, ceph-devel@...r.kernel.org,
        Linux List Kernel Mailing <linux-kernel@...r.kernel.org>,
        linux-cifs <linux-cifs@...r.kernel.org>
Subject: Re: [GIT PULL] Ceph fixes for 5.1-rc7

On Sun, 2019-04-28 at 15:48 +0100, Al Viro wrote:
> On Sun, Apr 28, 2019 at 09:27:20AM -0400, Jeff Layton wrote:
> 
> > I don't see a problem doing what you suggest. An offset + fixed length
> > buffer would be fine there.
> > 
> > Is there a real benefit to using __getname though? It sucks when we have
> > to reallocate but I doubt that it happens with any frequency. Most of
> > these paths will end up being much shorter than PATH_MAX and that slims
> > down the memory footprint a bit.
> 
> AFAICS, they are all short-lived; don't forget that slabs have cache,
> so in that situation allocations are cheap.
> 

Fair enough. Al also pointed out on IRC that the __getname/__putname
caches are likely to be hot, so using that may be less costly cpu-wise.

> > Also, FWIW -- this code was originally copied from cifs'
> > build_path_from_dentry(). Should we aim to put something in common
> > infrastructure that both can call?
> > 
> > There are some significant logic differences in the two functions though
> > so we might need some sort of callback function or something to know
> > when to stop walking.
> 
> Not if you want it fast...  Indirect calls are not cheap; the cost of
> those callbacks would be considerable.  Besides, you want more than
> "where do I stop", right?  It's also "what output do I use for this
> dentry", both for you and for cifs (there it's "which separator to use",
> in ceph it's "these we want represented as //")...
> 
> Can it be called on detached subtree, during e.g. open_by_handle()?
> There it can get really fishy; you end up with base being at the
> random point on the way towards root.  How does that work, and if
> it *does* work, why do we need the whole path in the first place?
> 

This I'm not sure of. commit 79b33c8874334e (ceph: snapshot nfs re-
export) explains this a bit, but I'm not sure it really covers this
case.

Zheng/Sage, feel free to correct me here:

My understanding is that for snapshots you need the base inode number,
snapid, and the full path from there to the dentry for a ceph MDS call.

There is a filehandle type for a snapshotted inode:

struct ceph_nfs_snapfh {
        u64 ino;
        u64 snapid;
        u64 parent_ino;
        u32 hash;
} __attribute__ ((packed));

So I guess it is possible. You could do name_to_handle_at for an inode
deep down in a snapshotted tree, and then try to open_by_handle_at after
the dcache gets cleaned out for some other reason.

What I'm not clear on is why we need to build paths at all for
snapshots. Why is a parent inode number (inside the snapshot) + a snapid
+ dentry name not sufficient?

> BTW, for cifs there's no need to play with ->d_lock as we go.  For
> ceph, the only need comes from looking at d_inode(), and I wonder if
> it would be better to duplicate that information ("is that a
> snapdir/nosnap") into dentry iself - would certainly be cheaper.
> OTOH, we are getting short on spare bits in ->d_flags...

We could stick that in ceph_dentry_info (->d_fsdata). We have a flags
field in there already.
-- 
Jeff Layton <jlayton@...nel.org>