Mount writer count API (read-only bind mounts prep)

The most contentious part of the r/o bind mount patches is actually implementing the count tracking. It has NUMA and SMP implications, and is going to need to have a whole discussion on that one patch.

These patches, on the other hand, simply introduce a new
API: mnt_want_write() and mnt_drop_write(). They do not functionally change the kernel, just alter the way in which we check filesystems for our ability to write to them.

These functions should be used in place of IS_RDONLY(inode) as they explicitly spell out when a mount is expected to _stay_ r/w instead of simply checking at a single point in time.

It should take a very small number (like 3) of small patches to actually implement read-only bind mounts on top of this new API.

These apply to current -git (as of July 11th, 2007).

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Why do we need r/o bind mounts?

This feature allows a read-only view into a read-write filesystem. In the process of doing that, it also provides infrastructure for keeping track of the number of writers to any given mount.

This has a number of uses. It allows chroots to have parts of filesystems writable. It will be useful for containers in the future because users may have root inside a container, but should not be allowed to write to somefilesystems. This also replaces patches that vserver has had out of the tree for several years.

It allows security enhancement by making sure that parts of your filesystem read-only (such as when you don't trust your FTP server), when you don't want to have entire new filesystems mounted, or when you want atime selectively updated. I've been using the following script to test that the feature is working as desired. It takes a directory and makes a regular bind and a r/o bind mount of it. It then performs some normal filesystem operations on the three directories, including ones that are expected to fail, like creating a file on the r/o mount.