1.\" Copyright (c) 1998 2.\" The Regents of the University of California. All rights reserved. 3.\" 4.\" Redistribution and use in source and binary forms, with or without 5.\" modification, are permitted provided that the following conditions 6.\" are met: 7.\" 1. Redistributions of source code must retain the above copyright 8.\" notice, this list of conditions and the following disclaimer. 9.\" 2. Redistributions in binary form must reproduce the above copyright 10.\" notice, this list of conditions and the following disclaimer in the 11.\" documentation and/or other materials provided with the distribution. 12.\" 3. All advertising materials mentioning features or use of this software 13.\" must display the following acknowledgement: 14.\" This product includes software developed by the University of 15.\" California, Berkeley and its contributors. 16.\" 4. Neither the name of the University nor the names of its contributors 17.\" may be used to endorse or promote products derived from this software 18.\" without specific prior written permission. 19.\" 20.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND 21.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 22.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 23.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE 24.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 25.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 26.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 27.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 28.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 29.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 30.\" SUCH DAMAGE. 31.\" 32.\" $FreeBSD$ 33.\" 34.Dd December 22, 1998 35.Dt BUF 9 36.Os 37.Sh NAME 38.Nm BUF 39.Nd Kernel Buffer I/O scheme used in 40.Fx 41VM system 42.Sh DESCRIPTION 43.Pp 44The kernel implements a KVM abstraction of the buffer cache which allows it 45to map potentially disparate vm_page's into contiguous KVM for use by 46(mainly filesystem) devices and device I/O. This abstraction supports 47block sizes from DEV_BSIZE (usually 512) to upwards of several pages or more. 48It also supports a relatively primitive byte-granular valid range and dirty 49range currently hardcoded for use by NFS. The code implementing the 50VM Buffer abstraction is mostly concentrated in 51.Pa /usr/src/sys/kern/vfs_bio.c . 52.Pp 53One of the most important things to remember when dealing with buffer pointers 54(struct buf) is that the underlying pages are mapped directly from the buffer 55cache. No data copying occurs in the scheme proper, though some filesystems 56such as UFS do have to copy a little when dealing with file fragments. The 57second most important thing to remember is that due to the underlying page 58mapping, the b_data base pointer in a buf is always *page* aligned, not 59*block* aligned. When you have a VM buffer representing some b_offset and 60b_size, the actual start of the buffer is (b_data + (b_offset & PAGE_MASK)) 61and not just b_data. Finally, the VM system's core buffer cache supports 62valid and dirty bits (m->valid, m->dirty) for pages in DEV_BSIZE chunks. Thus 63a platform with a hardware page size of 4096 bytes has 8 valid and 8 dirty 64bits. These bits are generally set and cleared in groups based on the device 65block size of the device backing the page. Complete page's worth are often 66referred to using the VM_PAGE_BITS_ALL bitmask (i.e. 0xFF if the hardware page 67size is 4096). 68.Pp 69VM buffers also keep track of a byte-granular dirty range and valid range. 70This feature is normally only used by the NFS subsystem. I'm not sure why it 71is used at all, actually, since we have DEV_BSIZE valid/dirty granularity 72within the VM buffer. If a buffer dirty operation creates a 'hole', 73the dirty range will extend to cover the hole. If a buffer validation 74operation creates a 'hole' the byte-granular valid range is left alone and 75will not take into account the new extension. Thus the whole byte-granular 76abstraction is considered a bad hack and it would be nice if we could get rid 77of it completely. 78.Pp 79A VM buffer is capable of mapping the underlying VM cache pages into KVM in 80order to allow the kernel to directly manipulate the data associated with 81the (vnode,b_offset,b_size). The kernel typically unmaps VM buffers the moment 82they are no longer needed but often keeps the 'struct buf' structure 83instantiated and even bp->b_pages array instantiated despite having unmapped 84them from KVM. If a page making up a VM buffer is about to undergo I/O, the 85system typically unmaps it from KVM and replaces the page in the b_pages[] 86array with a placemarker called bogus_page. The placemarker forces any kernel 87subsystems referencing the associated struct buf to re-lookup the associated 88page. I believe the placemarker hack is used to allow sophisticated devices 89such as filesystem devices to remap underlying pages in order to deal with, 90for example, remapping a file fragment into a file block. 91.Pp 92VM buffers are used to track I/O operations within the kernel. Unfortunately, 93the I/O implementation is also somewhat of a hack because the kernel wants 94to clear the dirty bit on the underlying pages the moment it queues the I/O 95to the VFS device, not when the physical I/O is actually initiated. This 96can create confusion within filesystem devices that use delayed-writes because 97you wind up with pages marked clean that are actually still dirty. If not 98treated carefully, these pages could be thrown away! Indeed, a number of 99serious bugs related to this hack were not fixed until the 2.2.8/3.0 release. 100The kernel uses an instantiated VM buffer (i.e. struct buf) to placemark pages 101in this special state. The buffer is typically flagged B_DELWRI. When a 102device no longer needs a buffer it typically flags it as B_RELBUF. Due to 103the underlying pages being marked clean, the B_DELWRI|B_RELBUF combination must 104be interpreted to mean that the buffer is still actually dirty and must be 105written to its backing store before it can actually be released. In the case 106where B_DELWRI is not set, the underlying dirty pages are still properly 107marked as dirty and the buffer can be completely freed without losing that 108clean/dirty state information. ( XXX do we have to check other flags in 109regards to this situation ??? ). 110.Pp 111The kernel reserves a portion of its KVM space to hold VM Buffer's data 112maps. Even though this is virtual space (since the buffers are mapped 113from the buffer cache), we cannot make it arbitrarily large because 114instantiated VM Buffers (struct buf's) prevent their underlying pages in the 115buffer cache from being freed. This can complicate the life of the paging 116system. 117.Pp 118.\" .Sh SEE ALSO 119.\" .Xr <fillmein> 9 120.Sh HISTORY 121The 122.Nm 123manual page was originally written by 124.An Matthew Dillon 125and first appeared in 126.Fx 3.1 , 127December 1998. 128 129