Re: Union mount and lockdep design issues

[Michal Suchanek]

On 11 July 2011 15:50, Ian Kent <ikent@xxxxxxxxxx> wrote:
> On Mon, 2011-07-11 at 15:36 +0200, Michal Suchanek wrote:
>> On 11 July 2011 14:00, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
>> > On Mon, 2011-07-11 at 12:01 +0100, David Howells wrote:
>> >> Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
>> >>
>>
>> >> > Also, why would you want to have a class per sb-instance? From last
>> >> > talking to David, he said there could only ever be 2 filesystems
>> >> > involved in this, the top and bottom, and it is determined on (union)
>> >> > mount time which is which.
>> >>
>> >> There can be more than 2 - one upperfs (the actual union) and many lowerfs -
>> >> though I think only one lowerfs is accessed at a time.
>> >
>> > Right, however I understood from our earlier discussion that the vfs
>> > would only ever try to lock 2 filesystems at a time, the top and one
>> > lower.
>>
>> This is true from local point of view. However, it is technically
>> possible to use overlayfs as the upper layer of another overlayfs
>> which allows layering multiple readonly "branches" into a single
>> overlay. Since the vfs will lock the "union" and one (or possibly
>> both) of its branches and one of the branches may be itself an union
>> you can get arbitrary depth (which is currently limited by a constant
>> in the code to cut recursion depth and stack usage).
>
> Off topic but can you elaborate on that?
>
> Are you saying the "unioned stack" can consist of more than two file
> systems and can have more than two layers and possibly a mix of multiple
> read-only and read-write file systems?
>

This is how requirements are described in documentation:

> The lower filesystem can be any filesystem supported by Linux and does
> not need to be writable.  The lower filesystem can even be another
> overlayfs.  The upper filesystem will normally be writable and if it
> is it must support the creation of trusted.* extended attributes, and
> must provide valid d_type in readdir responses, at least for symbolic
> links - so NFS is not suitable.

In no place it says that the lower filesystem is required to be
readonly, only that it should not be modified.


This is what the documentation gives as example:

> mount -t overlayfs overlayfs -olowerdir=/lower,upperdir=/upper /overlay

This is how it can be expanded:

mount -t overlayfs overlayfs -olowerdir=/lower2,upperdir=/upper /tmpoverlay
mount -t overlayfs overlayfs -olowerdir=/lower1,upperdir=/tmpoverlay /overlay

lower1 and lower2 are readonly branches you want to union; tmpoverlay
is writable so it fulfills the requirement for upper layer of overlay.
Note that the order is backwards, lower1 appears under lower2 which is
mounted earlier if I understand the layering correctly.

Depending on the way overlayfs handles creation of trusted.* extended
attributes on itself this overlay may work or break in interesting
ways.

Thanks

Michal
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