Summary

One of the core concepts of Unix is that nearly every resource can be represented by a file, and Linux has inherited this point of view. Files are therefore very prominent members of the kernel world, and a considerable effort goes into their representation. This chapter has introduced the virtual filesystem, a glue layer that sits between deeper kernel layers and userland. It provides various abstract data structures to represent files and inodes, and implementations of real filesystems must fill in these structures such that applications can always use the same interface to access and manipulate files irregardless of the underlying filesystem.

I have discussed how filesystems are mounted into the filesystem tree visible for userland applications, and have additionally shown how shared subtrees can be used to create different views of the ''global'' filesystem depending on the namespace. You have also learned that the kernel employs a number of pseudo-filesystems that are not visible to userland, but nevertheless contain some information that is interesting for internal purposes.

Opening files requires a traversal of the file tree, and you have seen how this problem is solved by the VFS layer. Once a file has been opened, it can be written to and read from, and you have also seen how the VFS is involved in these operations.

Finally, you have learned that the kernel provides some generic standard functions that make things easier for real filesystems like Ext3, as discussed in the next chapter, and how the kernel ensures that only appropriately privileged users may access objects located in the filesystem.

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