135 lines
3.8 KiB
Groff
135 lines
3.8 KiB
Groff
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.\" Copyright (c) 2021 The FreeBSD Foundation, Inc.
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.\" All rights reserved.
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.\"
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.\" This documentation was written by
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.\" Konstantin Belousov <kib@FreeBSD.org> under sponsorship
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.\" from the FreeBSD Foundation.
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.\"
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.\" Redistribution and use in source and binary forms, with or without
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.\" modification, are permitted provided that the following conditions
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.\" are met:
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.\" 1. Redistributions of source code must retain the above copyright
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.\" notice, this list of conditions and the following disclaimer.
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.\" 2. Redistributions in binary form must reproduce the above copyright
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.\" notice, this list of conditions and the following disclaimer in the
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.\" documentation and/or other materials provided with the distribution.
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.\"
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.\" THIS SOFTWARE IS PROVIDED BY THE AUTHORS AND CONTRIBUTORS ``AS IS'' AND
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.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
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.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
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.\" ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHORS OR CONTRIBUTORS BE LIABLE
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.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
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.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
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.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
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.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
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.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
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.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
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.\" SUCH DAMAGE.
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.\"
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.\" $FreeBSD$
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.\"
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.Dd February 28, 2021
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.Dt VOP_READ_PGCACHE 9
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.Os
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.Sh NAME
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.Nm VOP_READ_PGCACHE
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.Nd read a file, fast
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.Sh SYNOPSIS
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.In sys/param.h
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.In sys/vnode.h
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.In sys/uio.h
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.Ft int
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.Fo VOP_READ_PGCACHE
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.Fa "struct vnode *vp"
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.Fa "struct uio *uio"
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.Fa "int ioflag"
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.Fa "struct ucred *cred"
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.Fc
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.Sh DESCRIPTION
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This entry point reads the contents of a file.
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The intent is to provide the data from caches, which do not require
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expensive operations or any disk IO.
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For instance, if filesystem uses normal VM page cache and maintains
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.Dv v_object
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lifetime, it can use
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.Xr vn_read_from_obj 9
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helper to return data from the resident
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.Dv vp->v_object
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pages.
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.Pp
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The filesystem indicates support for the
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.Nm
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on specific vnode by setting the
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.Dv VIRF_PGREAD
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flag in
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.Dv vp->v_irflag .
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.Pp
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The function does not need to satisfy the whole request; it also might choose
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to not provide any data.
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In these cases, the
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.Fa uio
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must be advanced by the amount of read data,
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.Nm
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should return
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.Er EJUSTRETURN ,
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and VFS would handle the rest of the read operation using the
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.Xr VOP_READ 9 .
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.Pp
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The VFS layer does the same deadlock avoidance for accessing userspace
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pages from
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.Nm
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as for
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.Xr VOP_READ 9 .
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.Pp
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Vnode is not locked on the call entry and should not be locked on return.
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For a filesystem that requires vnode lock to return any data, it does
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not make sense to implement
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.Nm
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(and set
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.Dv VIRF_PGREAD
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flag) since VFS arranges the call to
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.Xr VOP_READ 9
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as needed.
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.Pp
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The arguments are:
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.Bl -tag -width ioflag
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.It Fa vp
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The vnode of the file.
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.It Fa uio
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The location of the data to be read.
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.It Fa ioflag
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Various flags, see
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.Xr VOP_READ 9
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for the list.
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.It Fa cred
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The credentials of the caller.
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.El
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.Pp
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.Nm
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does not handle non-zero
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.Fa ioflag
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argument.
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.Sh LOCKS
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The file should be referenced on entry on entry and will still be
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referenced on exit.
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Rangelock covering the whole read range should be owned around the call.
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.Sh RETURN VALUES
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Zero is returned on success, when the whole request is satisfied, and no
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more data cannot be provided for it by any means.
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If more data can be returned, but
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.Nm
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was unable to provide it,
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.Er EJUSTRETURN
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must be returned.
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The
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.Dv uio
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records should be updated according to the partial operation done.
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.Pp
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Otherwise an error code is returned,
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same as from
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.Xr VOP_READ 9
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.Sh SEE ALSO
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.Xr uiomove 9 ,
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.Xr vnode 9 ,
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.Xr VOP_READ 9
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