VOP_READ_PGCACHE.9

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.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