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

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Date:   Sun, 28 Apr 2019 15:48:50 +0100
From:   Al Viro <viro@...iv.linux.org.uk>
To:     Jeff Layton <jlayton@...nel.org>
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, 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.

> 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?

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...