Add manual page describing kernel buffer management system (i.e.

    kern/vfs_bio.c).

1 files changed, 123 insertions, 0 deletions

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+.\" Copyright (c) 1998
+.\"	The Regents of the University of California.  All rights reserved.
+.\"
+.\" Redistribution and use in source and binary forms, with or without
+.\" modification, are permitted provided that the following conditions
+.\" are met:
+.\" 1. Redistributions of source code must retain the above copyright
+.\"    notice, this list of conditions and the following disclaimer.
+.\" 2. Redistributions in binary form must reproduce the above copyright
+.\"    notice, this list of conditions and the following disclaimer in the
+.\"    documentation and/or other materials provided with the distribution.
+.\" 3. All advertising materials mentioning features or use of this software
+.\"    must display the following acknowledgement:
+.\"	This product includes software developed by the University of
+.\"	California, Berkeley and its contributors.
+.\" 4. Neither the name of the University nor the names of its contributors
+.\"    may be used to endorse or promote products derived from this software
+.\"    without specific prior written permission.
+.\"
+.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
+.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+.\" ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
+.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
+.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
+.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
+.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
+.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
+.\" SUCH DAMAGE.
+.\"
+.\"	$Id: security.7,v 1.1 1998/12/20 20:12:17 dillon Exp $
+.\"
+.Dd December 22, 1998
+.Dt BUF 9
+.Os
+.Sh NAME
+.Nm BUF/BP
+.Nd Kernel Buffer I/O scheme used in FreeBSD VM system
+.Sh DESCRIPTION
+.Pp
+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 filesystem) devices and device I/O.  This abstraction supports
+block sizes from DEV_BSIZE (usually 512) to upwards of several 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.
+.Pp
+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 filesystems
+such as UFS do have to copy a little 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.  Finally, 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
+refered to using the VM_PAGE_BITS_ALL bitmask (i.e. 0xFF if the hardware page
+size is 4096).
+.Pp
+VM buffers also keep track of a byte-granular dirty range and valid range.
+This feature is normally only used by the NFS subsystem.  I'm 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.
+.Pp
+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 unmaps VM buffers the moment
+they are no longer needed but often keeps the 'struct buf' structure 
+instantiated and even bp->b_pages array instantiated 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 placemarker called bogus_page.  The placemarker forces any kernel
+subsystems referencing the associated struct buf to re-lookup the associated
+page.  I believe the placemarker hack is used to allow sophisticated devices
+such as filesystem devices to remap underlying pages in order to deal with,
+for example, remapping a file fragment into a file block.
+.Pp
+VM buffers are used to track I/O operations within the kernel.  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 filesystem devices 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 2.2.8/3.0.0 release.
+The kernel uses an instantiated VM buffer (i.e. struct buf) to placemark pages
+in this special 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 underlying 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 losing that
+clean/dirty state information.  ( XXX do we have to check other flags in
+regards to this situation ??? ).
+.Pp
+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 underlying pages in the
+buffer cache from being freed.  This can complicate the life of the paging
+system.
+.Pp
+.Sh SEE ALSO
+.Pp
+.Xr <fillmein> 9
+.Sh HISTORY
+The
+.Nm
+manual page was originally written by Matthew Dillon and first appeared 
+in FreeBSD-3.0.1, December 1998.
+