vnode-backed VM objects, such as file-backed objects, generally need to maintain their
own clean/dirty info independent from the VM system's idea of clean/dirty. For example,
when the VM system decides to synchronize a physical page to its backing store, the VM
system needs to mark the page clean before the page is actually written to its backing
store. Additionally, filesystems need to be able to map portions of a file or file
metadata into KVM in order to operate on it.
The entities used to manage this are known as filesystem buffers, struct buf's, or bp's. When a filesystem
needs to operate on a portion of a VM object, it typically maps part of the object into a
struct buf and the maps the pages in the struct buf into KVM. In the same manner, disk
I/O is typically issued by mapping portions of objects into buffer structures and then
issuing the I/O on the buffer structures. The underlying vm_page_t's are typically busied
for the duration of the I/O. Filesystem buffers also have their own notion of being busy,
which is useful to filesystem driver code which would rather operate on filesystem
buffers instead of hard VM pages.
FreeBSD reserves a limited amount of KVM to hold mappings from struct bufs, but it
should be made clear that this KVM is used solely to hold mappings and does not limit the
ability to cache data. Physical data caching is strictly a function of vm_page_t's, not filesystem buffers. However, since filesystem
buffers are used to placehold I/O, they do inherently limit the amount of concurrent I/O
possible. However, as there are usually a few thousand filesystem buffers available, this
is not usually a problem.
