LKML Archive on lore.kernel.org
help / color / mirror / Atom feed
From: ebiederm@xmission.com (Eric W. Biederman)
To: Al Viro <viro@ZenIV.linux.org.uk>
Cc: John Ogness <john.ogness@linutronix.de>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	linux-fsdevel <linux-fsdevel@vger.kernel.org>,
	Christoph Hellwig <hch@lst.de>,
	Thomas Gleixner <tglx@linutronix.de>,
	Peter Zijlstra <peterz@infradead.org>,
	Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: dcache: remove trylock loops (was Re: [BUG] lock_parent() breakage when used from shrink_dentry_list())
Date: Mon, 12 Mar 2018 15:23:44 -0500	[thread overview]
Message-ID: <877eqhcab3.fsf@xmission.com> (raw)
In-Reply-To: <20180312191351.GN30522@ZenIV.linux.org.uk> (Al Viro's message of "Mon, 12 Mar 2018 19:13:51 +0000")

Al Viro <viro@ZenIV.linux.org.uk> writes:

> On Tue, Feb 27, 2018 at 06:16:28AM +0100, John Ogness wrote:
>
>> If someone else has grabbed a reference, it shouldn't be added to the
>> lru list. Only decremented.
>> 
>> if (entry->d_lockref.count == 1)
>
> Nah, better handle that in retain_dentry() itself.  See updated
> #work.dcache.
>
> Note: another potentially fun thing in that branch is that I've
> finally decided to bite the bullet and make __d_move() preserve
> ->d_parent of target.
>
> Mainline:
> al@sonny:/tmp$ touch d
> al@sonny:/tmp$ sleep 100 >/tmp/a/b/c &
> [1] 16487
> al@sonny:/tmp$ ls -l /proc/16487/fd
> total 0
> lrwx------ 1 al al 64 Mar 12 11:33 0 -> /dev/pts/13
> l-wx------ 1 al al 64 Mar 12 11:33 1 -> /tmp/a/b/c
> lrwx------ 1 al al 64 Mar 12 11:33 2 -> /dev/pts/13
> al@sonny:/tmp$ mv /tmp/d /tmp/a/b/c 
> al@sonny:/tmp$ ls -l /proc/16487/fd
> total 0
> lrwx------ 1 al al 64 Mar 12 11:33 0 -> /dev/pts/13
> l-wx------ 1 al al 64 Mar 12 11:33 1 -> /tmp/c (deleted)
> lrwx------ 1 al al 64 Mar 12 11:33 2 -> /dev/pts/13
>
> With that branch:
> root@kvm1:/tmp# touch d
> root@kvm1:/tmp# sleep 100 >/tmp/a/b/c &
> [1] 2263
> root@kvm1:/tmp# ls -l /proc/2263/fd
> total 0
> lrwx------ 1 root root 64 Mar 12 11:33 0 -> /dev/pts/0
> l-wx------ 1 root root 64 Mar 12 11:33 1 -> /tmp/a/b/c
> lrwx------ 1 root root 64 Mar 12 11:33 2 -> /dev/pts/0
> root@kvm1:/tmp# mv /tmp/d /tmp/a/b/c
> root@kvm1:/tmp# ls -l /proc/2263/fd
> total 0
> lrwx------ 1 root root 64 Mar 12 11:33 0 -> /dev/pts/0
> l-wx------ 1 root root 64 Mar 12 11:33 1 -> '/tmp/a/b/c (deleted)'
> lrwx------ 1 root root 64 Mar 12 11:33 2 -> /dev/pts/0
>
> It doesn't come quite for free; cross-directory d_move()
> and d_exchange() callers are responsible for having both
> parents pinned (all of them do that, mostly since the usual
> sequence is "look parents up, lock_rename(), *then* look
> children up, then do renaming"; those that are not part of
> rename(2) are also OK) and d_splice_alias() has become potentially
> blocking in one case.  AFAICS, none of the callers is in
> locking environment that would not allow that.  Survives
> the local beating and doesn't seem to cause any performance
> regressions.
>
> What we get out of that is
> 	a) much saner semantics for d_move() et.al.
> 	b) saner behaviour of d_path() (see above)
> 	c) dentry can be IS_ROOT only if it has been
> such all along; that simplifies the hell out of analysis.
>
> FWIW, there's another trylock loop on dentries - one in
> autofs get_next_positive_dentry().  Any plans re dealing
> with that one?
>
> I'd spent the last couple of weeks (when not being too sick
> for any work) going through dcache.c and related code; hopefully
> this time I will get the documentation into postable shape ;-/
>
> There's an unpleasant area around the ->s_root vs. NFS.  There's
> code that makes assumptions about ->s_root that are simply not true
> for NFS.  Is path_connected() correct wrt NFS multiple imports from
> the same server? Ditto for mnt_already_visible() (that one might
> be mitigated at the moment, but probably won't last).  Eric, am
> I missing something subtle in there?

I don't have the entire context in my head.  But I don't think we
have problems today.

NFS before it uses paths from an unconnected root in the rest of
the vfs walks those paths backwards and makes the paths connect.
I don't remember where all of that code that performs those connections
but I do remember the code in fs/fhandle.c shares that code with nfs, to
perform the same operation in open_by_handle_at.

So I don't think the nfs peculiarities are actually relevant to
anything on an ordinary code path.


Of the two code paths you are concert about:

For path path_connected looking at s_root is a heuristic to avoid
calling is_subdir every time we need to do that check.  If the heuristic
fails we still have is_subdir which should remain accurate.  If
is_subdir fails the path is genuinely not connected at that moment
and failing is the correct thing to do.


For mnt_too_revealing the only filesystems under consideration are
proc and sysfs.   So nfs oddities are of no consequence.
mnt_too_revealing probably won't be extended to other filesystems.
Certainly nfs is not a candidate for having setting SB_I_USERNS_VISIBLE.


Al is that sufficient to address your concerns?

Eric

  parent reply	other threads:[~2018-03-12 20:24 UTC|newest]

Thread overview: 46+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-02-22 23:50 [PATCH v2 0/6] fs/dcache: avoid trylock loops John Ogness
2018-02-22 23:50 ` [PATCH v2 1/6] fs/dcache: Remove stale comment from dentry_kill() John Ogness
2018-02-22 23:50 ` [PATCH v2 2/6] fs/dcache: Move dentry_kill() below lock_parent() John Ogness
2018-02-22 23:50 ` [PATCH v2 3/6] fs/dcache: Avoid the try_lock loop in d_delete() John Ogness
2018-02-23  2:08   ` Al Viro
2018-02-22 23:50 ` [PATCH v2 4/6] fs/dcache: Avoid the try_lock loops in dentry_kill() John Ogness
2018-02-23  2:22   ` Al Viro
2018-02-23  3:12     ` Al Viro
2018-02-23  3:16       ` Al Viro
2018-02-23  5:46       ` Al Viro
2018-02-22 23:50 ` [PATCH v2 5/6] fs/dcache: Avoid a try_lock loop in shrink_dentry_list() John Ogness
2018-02-23  3:48   ` Al Viro
2018-02-22 23:50 ` [PATCH v2 6/6] fs/dcache: Avoid remaining " John Ogness
2018-02-23  3:58   ` Al Viro
2018-02-23  4:08     ` Al Viro
2018-02-23 13:57       ` John Ogness
2018-02-23 15:09         ` Al Viro
2018-02-23 17:42           ` Al Viro
2018-02-23 20:13             ` [BUG] lock_parent() breakage when used from shrink_dentry_list() (was Re: [PATCH v2 6/6] fs/dcache: Avoid remaining try_lock loop in shrink_dentry_list()) Al Viro
2018-02-23 21:35               ` Linus Torvalds
2018-02-24  0:22                 ` Al Viro
2018-02-25  7:40                   ` Al Viro
2018-02-27  5:16                     ` dcache: remove trylock loops (was Re: [BUG] lock_parent() breakage when used from shrink_dentry_list()) John Ogness
2018-03-12 19:13                       ` Al Viro
2018-03-12 20:05                         ` Al Viro
2018-03-12 20:33                           ` Al Viro
2018-03-13  1:12                           ` NeilBrown
2018-04-28  0:10                             ` Al Viro
2018-03-12 20:23                         ` Eric W. Biederman [this message]
2018-03-12 20:39                           ` Al Viro
2018-03-12 23:28                             ` Eric W. Biederman
2018-03-12 23:52                               ` Eric W. Biederman
2018-03-13  0:37                                 ` Al Viro
2018-03-13  0:50                                   ` Al Viro
2018-03-13  4:02                                     ` Eric W. Biederman
2018-03-14 23:20                                     ` [PATCH] fs: Teach path_connected to handle nfs filesystems with multiple roots Eric W. Biederman
2018-03-15 22:34                                       ` Al Viro
2018-03-13  0:36                               ` dcache: remove trylock loops (was Re: [BUG] lock_parent() breakage when used from shrink_dentry_list()) Al Viro
2018-03-12 22:14                         ` Thomas Gleixner
2018-03-13 20:46                         ` John Ogness
2018-03-13 21:05                           ` John Ogness
2018-03-13 23:59                             ` Al Viro
2018-03-14  2:58                               ` Matthew Wilcox
2018-03-14  8:18                               ` John Ogness
2018-03-02  9:04                     ` [BUG] lock_parent() breakage when used from shrink_dentry_list() (was Re: [PATCH v2 6/6] fs/dcache: Avoid remaining try_lock loop in shrink_dentry_list()) Sebastian Andrzej Siewior
2018-02-23  0:59 ` [PATCH v2 0/6] fs/dcache: avoid trylock loops Linus Torvalds

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=877eqhcab3.fsf@xmission.com \
    --to=ebiederm@xmission.com \
    --cc=bigeasy@linutronix.de \
    --cc=hch@lst.de \
    --cc=john.ogness@linutronix.de \
    --cc=linux-fsdevel@vger.kernel.org \
    --cc=linux-kernel@vger.kernel.org \
    --cc=peterz@infradead.org \
    --cc=tglx@linutronix.de \
    --cc=torvalds@linux-foundation.org \
    --cc=viro@ZenIV.linux.org.uk \
    --subject='Re: dcache: remove trylock loops (was Re: [BUG] lock_parent() breakage when used from shrink_dentry_list())' \
    /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

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).