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diff --git a/share/man/man9/buf.9 b/share/man/man9/buf.9 deleted file mode 100644 index 869c90e29476..000000000000 --- a/share/man/man9/buf.9 +++ /dev/null @@ -1,123 +0,0 @@ -.\" 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. - |