buf(9)

NAME

buf - kernel buffer I/O scheme used in FreeBSD VM system

DESCRIPTION

The kernel implements a KVM abstraction of the buffer cache

which allows
it to map potentially disparate vm_page's into contiguous

KVM for use by
(mainly file system) devices and device I/O. This abstrac

tion supports
block sizes from DEV_BSIZE (usually 512) to upwards of sev

eral pages or
more. It also supports a relatively primitive byte-granular

valid range
and dirty range currently hardcoded for use by NFS. The

code implementing the VM Buffer abstraction is mostly concentrated in
/usr/src/sys/kern/vfs_bio.c.

One of the most important things to remember when dealing

with buffer
pointers (struct buf) is that the underlying pages are

mapped directly
from the buffer cache. No data copying occurs in the scheme

proper,
though some file systems such as UFS do have to copy a lit

tle when dealing with file fragments. The second most important thing to

remember is
that due to the underlying page mapping, the b_data base

pointer in a buf
is always *page* aligned, not *block* aligned. When you

have a VM buffer
representing some b_offset and b_size, the actual start of

the buffer is
(b_data + (b_offset & PAGE_MASK)) and not just b_data. Fi

nally, the VM
system's core buffer cache supports valid and dirty bits

(m->valid,
m->dirty) for pages in DEV_BSIZE chunks. Thus a platform

with a hardware
page size of 4096 bytes has 8 valid and 8 dirty bits. These

bits are
generally set and cleared in groups based on the device

block size of the
device backing the page. Complete page's worth are often

referred to
using the VM_PAGE_BITS_ALL bitmask (i.e., 0xFF if the hard

ware page size
is 4096).

VM buffers also keep track of a byte-granular dirty range

and valid
range. This feature is normally only used by the NFS sub

system. I am
not sure why it is used at all, actually, since we have

DEV_BSIZE
valid/dirty granularity within the VM buffer. If a buffer

dirty operation creates a 'hole', the dirty range will extend to cover

the hole. If
a buffer validation operation creates a 'hole' the byte

granular valid
range is left alone and will not take into account the new

extension.
Thus the whole byte-granular abstraction is considered a bad

hack and it
would be nice if we could get rid of it completely.

A VM buffer is capable of mapping the underlying VM cache

pages into KVM
in order to allow the kernel to directly manipulate the data

associated
with the (vnode,b_offset,b_size). The kernel typically un

maps VM buffers
the moment they are no longer needed but often keeps the
structure instantiated and even bp->b_pages array instanti

ated despite
having unmapped them from KVM. If a page making up a VM

buffer is about
to undergo I/O, the system typically unmaps it from KVM and

replaces the
page in the b_pages[] array with a place-marker called bo

gus_page. The
place-marker forces any kernel subsystems referencing the

associated
struct buf to re-lookup the associated page. I believe the

place-marker
hack is used to allow sophisticated devices such as file

system devices
to remap underlying pages in order to deal with, for exam

ple, re-mapping
a file fragment into a file block.

VM buffers are used to track I/O operations within the ker

nel. Unfortunately, the I/O implementation is also somewhat of a hack

because the
kernel wants to clear the dirty bit on the underlying pages

the moment it
queues the I/O to the VFS device, not when the physical I/O

is actually
initiated. This can create confusion within file system de

vices that use
delayed-writes because you wind up with pages marked clean

that are actually still dirty. If not treated carefully, these pages

could be thrown
away! Indeed, a number of serious bugs related to this hack

were not
fixed until the 2.2.8/3.0 release. The kernel uses an in

stantiated VM
buffer (i.e., struct buf) to place-mark pages in this spe

cial state. The
buffer is typically flagged B_DELWRI. When a device no

longer needs a
buffer it typically flags it as B_RELBUF. Due to the under

lying pages
being marked clean, the B_DELWRI|B_RELBUF combination must

be interpreted
to mean that the buffer is still actually dirty and must be

written to
its backing store before it can actually be released. In

the case where
B_DELWRI is not set, the underlying dirty pages are still

properly marked
as dirty and the buffer can be completely freed without los

ing that
clean/dirty state information. (XXX do we have to check

other flags in
regards to this situation ???)

The kernel reserves a portion of its KVM space to hold VM

Buffer's data
maps. Even though this is virtual space (since the buffers

are mapped
from the buffer cache), we cannot make it arbitrarily large

because
instantiated VM Buffers (struct buf's) prevent their under

lying pages in
the buffer cache from being freed. This can complicate the

life of the
paging system.

HISTORY

The buf manual page was originally written by Matthew Dillon

and first
appeared in FreeBSD 3.1, December 1998.

BSD December 22, 1998

BSD