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+.\"	$NetBSD: ubc.9,v 1.15 2018/05/12 15:03:19 jdolecek Exp $
+.\"
+.\" Copyright (c) 1998 Matthew R. Green
+.\" 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.
+.\"
+.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``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 AUTHOR 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.
+.\"
+.Dd May 12, 2018
+.Dt UBC 9
+.Os
+.Sh NAME
+.Nm ubc
+.Nd unified buffer cache
+.Sh SYNOPSIS
+.In uvm/uvm.h
+.Ft int
+.Fn ubc_uiomove "struct uvm_object *uobj" "struct uio *uio" "vsize_t todo" \
+ "int advice" "int flags"
+.Ft void
+.Fn ubc_zerorange "struct uvm_bject *uobj" "off_t off" "size_t len" \
+ "int flags"
+.Ft void
+.Fn ubc_purge "struct uvm_object *uobj"
+.Sh DESCRIPTION
+.Fn ubc_uiomove
+allocates an UBC memory window, performs I/O on it and unmaps the window.
+The
+.Fa advice
+parameter is the access pattern hint, which must be one of
+.Pp
+.Bl -tag -offset indent -width "UVM_ADV_SEQUENTIAL" -compact
+.It UVM_ADV_NORMAL
+No hint
+.It UVM_ADV_RANDOM
+Random access hint
+.It UVM_ADV_SEQUENTIAL
+Sequential access hint (from lower offset to higher offset)
+.El
+.Pp
+and the
+.Fa flags
+parameter is
+.Pp
+.Bl -tag -offset indent -width "UVM_ADV_SEQUENTIAL" -compact
+.It UBC_READ
+Mapping will be accessed for read.
+.It UBC_WRITE
+Mapping will be accessed for write.
+.It UBC_FAULTBUSY
+Fault in window's pages already during mapping operation.
+Makes sense only for write.
+.It UBC_UNMAP
+Do not cache mapping.
+.It UBC_PARTIALOK
+Indicate that it is acceptable to return if an error occurs mid-transfer.
+.El
+.Pp
+UBC memory window is a kernel mapping of
+.Fa uobj
+starting at offset
+.Fa offset .
+.Pp
+Once the mapping is created, it must be accessed only by methods that can
+handle faults, such as
+.Xr uiomove 9
+or
+.Xr kcopy 9 .
+Page faults on the mapping will result in the object's pager
+method being called to resolve the fault.
+.Pp
+Size of individual UBC memory window is limited to
+.Va ubc_winsize .
+.Fn ubc_uiomove
+sequentially creates the UBC memory windows to eventually process
+the whole range according to
+.Fa offset
+and
+.Fa len
+parameters.
+.Pp
+The mappings may be cached to speed future accesses to the same region
+of the object, unless
+.Dv UBC_UNMAP
+was specified in
+.Fa flags
+parameter.
+.Pp
+.Fn ubc_zerorange
+sets a range of bytes in a UVM object to zero.
+The
+.Fa flags
+parameter takes the same arguments as
+.Fn ubc_uiomove .
+.Pp
+.Fn ubc_purge
+disassociates all UBC structures from an empty UVM object,
+specified by
+.Fa uobj .
+.Sh CODE REFERENCES
+The
+.Nm
+subsystem is implemented within the file
+.Pa sys/uvm/uvm_bio.c .
+.Sh SEE ALSO
+.Xr kcopy 9 ,
+.Xr pmap 9 ,
+.Xr uiomove 9 ,
+.Xr uvm 9 ,
+.Xr vnode 9 ,
+.Xr vnodeops 9
+.Rs
+.%A Chuck Silvers
+.%T "UBC: An Efficient Unified I/O and Memory Caching Subsystem for NetBSD"
+.%I USENIX Association
+.%B Proceedings of the FREENIX Track: 2000 USENIX Annual Technical Conference
+.%P 285-290
+.%D June 18-23, 2000
+.%U http://www.usenix.org/event/usenix2000/freenix/full_papers/silvers/silvers.pdf
+.Re
+.Sh HISTORY
+UBC first appeared in
+.Nx 1.6 .
+.Sh AUTHORS
+.An Chuck Silvers
+.Aq Mt chuq@chuq.com
+designed and implemented the UBC part of UVM, which uses UVM pages
+to cache vnode data rather than the traditional buffer cache buffers.