xref: /freebsd/share/man/man9/VOP_READ_PGCACHE.9 (revision e5b786625f7f82a1fa91e41823332459ea5550f9)
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28.Dd February 28, 2021
29.Dt VOP_READ_PGCACHE 9
30.Os
31.Sh NAME
32.Nm VOP_READ_PGCACHE
33.Nd read a file, fast
34.Sh SYNOPSIS
35.In sys/param.h
36.In sys/vnode.h
37.In sys/uio.h
38.Ft int
39.Fo VOP_READ_PGCACHE
40.Fa "struct vnode *vp"
41.Fa "struct uio *uio"
42.Fa "int ioflag"
43.Fa "struct ucred *cred"
44.Fc
45.Sh DESCRIPTION
46This entry point reads the contents of a file.
47The intent is to provide the data from caches, which do not require
48expensive operations or any disk IO.
49For instance, if filesystem uses normal VM page cache and maintains
50.Dv v_object
51lifetime, it can use
52.Xr vn_read_from_obj 9
53helper to return data from the resident
54.Dv vp->v_object
55pages.
56.Pp
57The filesystem indicates support for the
58.Nm
59on specific vnode by setting the
60.Dv VIRF_PGREAD
61flag in
62.Dv vp->v_irflag .
63.Pp
64The function does not need to satisfy the whole request; it also might choose
65to not provide any data.
66In these cases, the
67.Fa uio
68must be advanced by the amount of read data,
69.Nm
70should return
71.Er EJUSTRETURN ,
72and VFS would handle the rest of the read operation using the
73.Xr VOP_READ 9 .
74.Pp
75The VFS layer does the same deadlock avoidance for accessing userspace
76pages from
77.Nm
78as for
79.Xr VOP_READ 9 .
80.Pp
81Vnode is not locked on the call entry and should not be locked on return.
82For a filesystem that requires vnode lock to return any data, it does
83not make sense to implement
84.Nm
85(and set
86.Dv VIRF_PGREAD
87flag) since VFS arranges the call to
88.Xr VOP_READ 9
89as needed.
90.Pp
91The arguments are:
92.Bl -tag -width ioflag
93.It Fa vp
94The vnode of the file.
95.It Fa uio
96The location of the data to be read.
97.It Fa ioflag
98Various flags, see
99.Xr VOP_READ 9
100for the list.
101.It Fa cred
102The credentials of the caller.
103.El
104.Pp
105.Nm
106does not handle non-zero
107.Fa ioflag
108argument.
109.Sh LOCKS
110The file should be referenced on entry on entry and will still be
111referenced on exit.
112Rangelock covering the whole read range should be owned around the call.
113.Sh RETURN VALUES
114Zero is returned on success, when the whole request is satisfied, and no
115more data cannot be provided for it by any means.
116If more data can be returned, but
117.Nm
118was unable to provide it,
119.Er EJUSTRETURN
120must be returned.
121The
122.Dv uio
123records should be updated according to the partial operation done.
124.Pp
125Otherwise an error code is returned,
126same as from
127.Xr VOP_READ 9
128.Sh SEE ALSO
129.Xr uiomove 9 ,
130.Xr vnode 9 ,
131.Xr VOP_READ 9
132