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From: NeilBrown <neilb-IBi9RG/b67k@public.gmane.org>
To: "J. Bruce Fields" <bfields-uC3wQj2KruNg9hUCZPvPmw@public.gmane.org>
Cc: Kinglong Mee
	<kinglongmee-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>,
	Al Viro <viro-RmSDqhL/yNMiFSDQTTA3OLVCufUGDwFn@public.gmane.org>,
	"linux-nfs-u79uwXL29TY76Z2rM5mHXA@public.gmane.org"
	<linux-nfs-u79uwXL29TY76Z2rM5mHXA@public.gmane.org>,
	linux-fsdevel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org,
	Trond Myklebust
	<trond.myklebust-7I+n7zu2hftEKMMhf/gKZA@public.gmane.org>
Subject: Re: export table lookup: was [PATCH 10/10 v7] nfsd: Allows user un-mounting filesystem where nfsd exports base on
Date: Fri, 24 Jul 2015 09:46:57 +1000
Message-ID: <20150724094657.0ca793b4@noble> (raw)
In-Reply-To: <20150722150840.GH22718-uC3wQj2KruNg9hUCZPvPmw@public.gmane.org>

On Wed, 22 Jul 2015 11:08:40 -0400 "J. Bruce Fields"
<bfields-uC3wQj2KruNg9hUCZPvPmw@public.gmane.org> wrote:


> I've had this nagging todo to work out if there are other interesting
> consequences of the fact that the cache is internally keyed on one thing
> and appears to mountd to be keyed on another.  (And that there's a
> complicated many<->many relationship between those two things.)  But I
> haven't gotten to it.  Could be all unlikely corner cases, for all I
> know.

Even corner cases are worth resolving - and you got me interested now:-)

I think the distinction between pathnames and mnt+dentry is not quite
the important one.
I think mnt+dentry is the primary object - it is what a filehandle maps
to and what a pathname maps to.

The problem is that some mnt+dentry pairs do not have matching path
names.  If nfsd gets hold of one of these pairs, it shouldn't try
asking mountd about it because there is no way to ask the question, and
even if there was it isn't clear there is any way for mountd to answer.

If think that nfsd should assume that any such mountpoint is not
exported.

So something vaguely like:

diff --git a/fs/dcache.c b/fs/dcache.c
index 5c8ea15e73a5..a0651872ae8e 100644
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -2943,6 +2943,12 @@ restart:
 		if (error)
 			break;
 
+		if (unlikely(d_mountpoint(dentry))) {
+			struct mount *mounted = __lookup_mnt(vfsmnt, dentry);
+			if (mounted)
+				prepend(&bptr, &blen, "//(unreachable)",15);
+		}
+
 		dentry = parent;
 	}
 	if (!(seq & 1))

Would mean that if I

# cd /tmp/a/b/c
# mount --bind /etc /tmp/a
# /bin/pwd

I get 

/tmp//(unreachable)/a/b/c

would could be checked for by nfsd to decide that there is no point asking user-space.
I'm not at all certain that this is a good interface (or that the code isn't racy) - it is just
a proof-of-concept.

We should probably place the (unreachable) at the front rather than in the middle.

Does that seem like a reasonable approach from your understanding of the problem?

Thanks,
NeilBrown









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  parent reply index

Thread overview: 40+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2015-07-11 12:46 [PATCH 00/10 v7] NFSD: Pin to vfsmount for nfsd exports cache Kinglong Mee
2015-07-11 12:49 ` [PATCH 05/10 v7] sunrpc: Store cache_detail in seq_file's private, directly Kinglong Mee
2015-07-11 12:49 ` [PATCH 06/10 v7] sunrpc/nfsd: Remove redundant code by exports seq_operations functions Kinglong Mee
     [not found] ` <55A11010.6050005-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
2015-07-11 12:47   ` [PATCH 01/10 v7] fs_pin: Initialize value for fs_pin explicitly Kinglong Mee
2015-07-11 12:47   ` [PATCH 02/10 v7] fs_pin: Export functions for specific filesystem Kinglong Mee
2015-07-11 12:48   ` [PATCH 03/10 v7] path: New helpers path_get_pin/path_put_unpin for path pin Kinglong Mee
2015-07-11 12:48   ` [PATCH 04/10 v7] fs: New helper legitimize_mntget() for getting a legitimize mnt Kinglong Mee
2015-07-11 12:50   ` [PATCH 07/10 v7] sunrpc: Switch to using list_head instead single list Kinglong Mee
     [not found]     ` <55A11112.8080502-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
2015-07-11 12:54       ` Christoph Hellwig
2015-07-13  1:30     ` NeilBrown
2015-07-13  8:27       ` Kinglong Mee
2015-07-11 12:51   ` [PATCH 09/10 v7] sunrpc: Support get_ref/put_ref for reference change in cache_head Kinglong Mee
2015-07-11 12:52   ` [PATCH 10/10 v7] nfsd: Allows user un-mounting filesystem where nfsd exports base on Kinglong Mee
     [not found]     ` <55A111A8.2040701-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
2015-07-13  3:39       ` NeilBrown
2015-07-13  4:02         ` Al Viro
     [not found]           ` <20150713040258.GM17109-3bDd1+5oDREiFSDQTTA3OLVCufUGDwFn@public.gmane.org>
2015-07-13  5:19             ` NeilBrown
2015-07-13  6:02               ` Al Viro
2015-07-13  4:20         ` NeilBrown
2015-07-13  4:45           ` Al Viro
     [not found]             ` <20150713044553.GN17109-3bDd1+5oDREiFSDQTTA3OLVCufUGDwFn@public.gmane.org>
2015-07-13  5:21               ` NeilBrown
2015-07-13  6:02                 ` NeilBrown
2015-07-13  6:08                   ` Al Viro
     [not found]                     ` <20150713060802.GP17109-3bDd1+5oDREiFSDQTTA3OLVCufUGDwFn@public.gmane.org>
2015-07-13  6:32                       ` NeilBrown
2015-07-13  6:43                         ` Al Viro
2015-07-15  3:49                           ` NeilBrown
2015-07-15  4:57                             ` Al Viro
2015-07-15  6:51                               ` NeilBrown
2015-07-24  2:05             ` NeilBrown
2015-07-27  2:28               ` Kinglong Mee
     [not found]                 ` <55B59764.1020506-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
2015-07-27  2:51                   ` NeilBrown
2015-07-27  3:17                     ` Kinglong Mee
2015-07-15 21:07         ` J. Bruce Fields
     [not found]           ` <20150715210756.GE21669-uC3wQj2KruNg9hUCZPvPmw@public.gmane.org>
2015-07-15 23:40             ` NeilBrown
2015-07-16 20:51               ` J. Bruce Fields
     [not found]                 ` <20150716205148.GC10673-uC3wQj2KruNg9hUCZPvPmw@public.gmane.org>
2015-07-21 21:58                   ` NeilBrown
2015-07-22 15:08                     ` J. Bruce Fields
     [not found]                       ` <20150722150840.GH22718-uC3wQj2KruNg9hUCZPvPmw@public.gmane.org>
2015-07-23 23:46                         ` NeilBrown [this message]
2015-07-24 19:48                           ` export table lookup: was " J. Bruce Fields
2015-07-25  0:40                             ` NeilBrown
2015-07-11 12:51 ` [PATCH 08/10 v7] sunrpc: New helper cache_delete_entry for deleting cache_head directly Kinglong Mee

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