Re: [RFC] shared subtrees

[Ram <linuxram@us.ibm.com>]

On Wed, 2005-02-02 at 11:45, Mike Waychison wrote:
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> Ram wrote:
> > On Tue, 2005-02-01 at 15:21, J. Bruce Fields wrote:
> > 
> >>On Tue, Jan 25, 2005 at 01:07:12PM -0800, Ram wrote:
> >>
> >>>If there exists a private subtree in a larger shared subtree, what
> >>>happens when the larger shared subtree is rbound to some other place? 
> >>>Is a new private subtree created in the new larger shared subtree? or
> >>>will that be pruned out in the new larger subtree?
> >>
> >>"mount --rbind" will always do at least all the mounts that it did
> >>before the introduction of shared subtrees--so certainly it will copy
> >>private subtrees along with shared ones.  (Since subtrees are private by
> >>default, anything else would make --rbind do nothing by default.) My
> >>understanding of Viro's RFC is that the new subtree will have no
> >>connection with the preexisting private subtree (we want private
> >>subtrees to stay private), but that the new copy will end up with
> >>whatever propagation the target of the "mount --rbind" had.  (So the
> >>addition of the copy of the private subtree to the target vfsmount will
> >>be replicated on any vfsmount that the target vfsmount propogates to,
> >>and those copies will propagate among themselves in the same way that
> >>the copies of the target vfsmount propagate to each other.)
> > 
> > 
> > ok. that makes sense. As you said the private subtree shall get copied
> > to the new location, however propogations wont be set in either
> > directions. However I have a rather unusual requirement which forces 
> > multiple rbind of a shared subtree within the same shared subtree.
> > 
> > I did the calculation and found that the tree simply explodes with
> > vfsstructs.  If I mark a subtree within the larger shared tree as
> > private, then the number of vfsstructs grows linearly O(n). However if
> > there was a way of marking a subtree within the larger shared tree as
> > unclonable than the increase in number of vfsstruct is constant.
> > 
> > What I am essentially driving at is, can we add another feature which 
> > allows me to mark a subtree as unclonable?
> > 
> > 
> > Read below to see how the tree explodes:
> > 
> > to run you through an example: 
> > 
> > (In case the tree pictures below gets garbled, it can also be seen at 
> >  http://www.sudhaa.com/~ram/readahead/sharedsubtree/subtree )
> > 
> > step 1:
> >    lets say the root tree has just two directories with one vfsstruct. 
> >                     root
> >                    /    \
> >                   tmp    usr
> >     All I want is to be able to see the entire root tree 
> >    (but not anything under /root/tmp) to be viewable under /root/tmp/m* 
> > 
> > step2:
> >       mount --make-shared /root
> > 
> >       mkdir -p /tmp/m1
> > 
> >       mount --rbind /root /tmp/m1
> > 
> >       the new tree now looks like this:
> > 
> >                     root
> >                    /    \
> >                  tmp    usr
> >                 /
> >                m1
> >               /  \ 
> >              tmp  usr
> >              /
> >             m1
> > 
> >           it has two vfsstructs
> > 
> > step3: 
> >             mkdir -p /tmp/m2
> >             mount --rbind /root /tmp/m2
> 
> At this step, you probably shouldn't be using --rbind, but --bind
> instead to only bind a copy of the root vfsmount, so it now looks like:
> 
> >                       root
> >                      /    \ 
> >                    tmp     usr
> >                   /    \
> >                 m1       m2
> >                / \       /  \
> >              tmp  usr   tmp  usr
> >              / \         / \ 
> >             m1  m2      m1  m2

Well I thought about this. Even Bruce Fields suggested this in a private
thread. But this solution can be racy. You may have to do multiple binds
for all the vfstructs that reside in the subtree under / (but not under
/root/tmp). And doing it atomically without racing with other
simultaneous mounts would be tricky.

RP
> 
> - --
> Mike Waychison
> Sun Microsystems, Inc.
> 1 (650) 352-5299 voice
> 1 (416) 202-8336 voice
> 
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