aboutsummaryrefslogtreecommitdiff
path: root/share/man/man9/VOP_READ_PGCACHE.9
blob: 3e6ae9be4fc7252f1b86113226122de5ddcc3fd6 (plain) (blame)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
.\" Copyright (c) 2021 The FreeBSD Foundation, Inc.
.\" All rights reserved.
.\"
.\" This documentation was written by
.\" Konstantin Belousov <kib@FreeBSD.org> under sponsorship
.\" from the FreeBSD Foundation.
.\"
.\" 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 AUTHORS 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 AUTHORS 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.
.\"
.\" $FreeBSD$
.\"
.Dd February 28, 2021
.Dt VOP_READ_PGCACHE 9
.Os
.Sh NAME
.Nm VOP_READ_PGCACHE
.Nd read a file, fast
.Sh SYNOPSIS
.In sys/param.h
.In sys/vnode.h
.In sys/uio.h
.Ft int
.Fo VOP_READ_PGCACHE
.Fa "struct vnode *vp"
.Fa "struct uio *uio"
.Fa "int ioflag"
.Fa "struct ucred *cred"
.Fc
.Sh DESCRIPTION
This entry point reads the contents of a file.
The intent is to provide the data from caches, which do not require
expensive operations or any disk IO.
For instance, if filesystem uses normal VM page cache and maintains
.Dv v_object
lifetime, it can use
.Xr vn_read_from_obj 9
helper to return data from the resident
.Dv vp->v_object
pages.
.Pp
The filesystem indicates support for the
.Nm
on specific vnode by setting the
.Dv VIRF_PGREAD
flag in
.Dv vp->v_irflag .
.Pp
The function does not need to satisfy the whole request; it also might choose
to not provide any data.
In these cases, the
.Fa uio
must be advanced by the amount of read data,
.Nm
should return
.Er EJUSTRETURN ,
and VFS would handle the rest of the read operation using the
.Xr VOP_READ 9 .
.Pp
The VFS layer does the same deadlock avoidance for accessing userspace
pages from
.Nm
as for
.Xr VOP_READ 9 .
.Pp
Vnode is not locked on the call entry and should not be locked on return.
For a filesystem that requires vnode lock to return any data, it does
not make sense to implement
.Nm
(and set
.Dv VIRF_PGREAD
flag) since VFS arranges the call to
.Xr VOP_READ 9
as needed.
.Pp
The arguments are:
.Bl -tag -width ioflag
.It Fa vp
The vnode of the file.
.It Fa uio
The location of the data to be read.
.It Fa ioflag
Various flags, see
.Xr VOP_READ 9
for the list.
.It Fa cred
The credentials of the caller.
.El
.Pp
.Nm
does not handle non-zero
.Fa ioflag
argument.
.Sh LOCKS
The file should be referenced on entry on entry and will still be
referenced on exit.
Rangelock covering the whole read range should be owned around the call.
.Sh RETURN VALUES
Zero is returned on success, when the whole request is satisfied, and no
more data cannot be provided for it by any means.
If more data can be returned, but
.Nm
was unable to provide it,
.Er EJUSTRETURN
must be returned.
The
.Dv uio
records should be updated according to the partial operation done.
.Pp
Otherwise an error code is returned,
same as from
.Xr VOP_READ 9
.Sh SEE ALSO
.Xr uiomove 9 ,
.Xr vnode 9 ,
.Xr VOP_READ 9