From: Jeff Layton <jlayton@kernel.org>
To: Al Viro <viro@zeniv.linux.org.uk>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
Ilya Dryomov <idryomov@gmail.com>,
ceph-devel@vger.kernel.org,
Linux List Kernel Mailing <linux-kernel@vger.kernel.org>,
linux-cifs <linux-cifs@vger.kernel.org>
Subject: Re: [GIT PULL] Ceph fixes for 5.1-rc7
Date: Sun, 28 Apr 2019 11:47:58 -0400 [thread overview]
Message-ID: <b875fdb47e17ab68d18c5e5e5cbd0ec70fec7ce9.camel@kernel.org> (raw)
In-Reply-To: <20190428144850.GA23075@ZenIV.linux.org.uk>
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@kernel.org>
next prev parent reply other threads:[~2019-04-28 15:48 UTC|newest]
Thread overview: 26+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-04-25 17:47 [GIT PULL] Ceph fixes for 5.1-rc7 Ilya Dryomov
2019-04-25 18:02 ` Linus Torvalds
2019-04-25 18:21 ` Al Viro
2019-04-25 18:24 ` Linus Torvalds
2019-04-25 18:31 ` Al Viro
2019-04-25 18:36 ` Jeff Layton
2019-04-25 18:23 ` Jeff Layton
2019-04-25 20:09 ` Al Viro
2019-04-26 16:25 ` Jeff Layton
2019-04-26 16:36 ` Linus Torvalds
2019-04-26 16:43 ` Linus Torvalds
2019-04-26 17:01 ` Al Viro
2019-04-26 17:08 ` Linus Torvalds
2019-04-26 17:11 ` Al Viro
2019-04-26 20:49 ` Jeff Layton
2019-04-26 21:28 ` Al Viro
2019-04-26 16:50 ` Al Viro
2019-04-26 17:30 ` Jeff Layton
2019-04-28 4:38 ` Al Viro
2019-04-28 13:27 ` Jeff Layton
2019-04-28 14:48 ` Al Viro
2019-04-28 15:47 ` Jeff Layton [this message]
2019-04-28 15:52 ` Al Viro
2019-04-28 16:18 ` Jeff Layton
2019-04-28 16:40 ` Al Viro
2019-04-25 18:35 ` pr-tracker-bot
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=b875fdb47e17ab68d18c5e5e5cbd0ec70fec7ce9.camel@kernel.org \
--to=jlayton@kernel.org \
--cc=ceph-devel@vger.kernel.org \
--cc=idryomov@gmail.com \
--cc=linux-cifs@vger.kernel.org \
--cc=linux-kernel@vger.kernel.org \
--cc=torvalds@linux-foundation.org \
--cc=viro@zeniv.linux.org.uk \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).