NAME
VOP_READ_PGCACHE —
read a file, fast
SYNOPSIS
#include
<sys/param.h>
#include <sys/vnode.h>
#include <sys/uio.h>
int
VOP_READ_PGCACHE(struct vnode
*vp, struct uio *uio, int
ioflag, struct ucred *cred);
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 v_object lifetime, it can use
vn_read_from_obj(9) helper to return data from the resident
vp->v_object pages.
The filesystem indicates support for the
VOP_READ_PGCACHE on specific vnode by setting the
VIRF_PGREAD flag in
vp->v_irflag.
The function does not need to satisfy the whole request; it also
might choose to not provide any data. In these cases, the
uio must be advanced by the amount of read data,
VOP_READ_PGCACHE should return
EJUSTRETURN, and VFS would handle the rest of the
read operation using the
VOP_READ(9).
The VFS layer does the same deadlock avoidance for accessing
userspace pages from VOP_READ_PGCACHE as for
VOP_READ(9).
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 VOP_READ_PGCACHE
(and set VIRF_PGREAD flag) since VFS arranges the
call to
VOP_READ(9) as needed.
The arguments are:
- vp
- The vnode of the file.
- uio
- The location of the data to be read.
- ioflag
- Various flags, see VOP_READ(9) for the list.
- cred
- The credentials of the caller.
VOP_READ_PGCACHE does not handle non-zero
ioflag argument.
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.
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 VOP_READ_PGCACHE was unable to provide
it, EJUSTRETURN must be returned. The
uio records should be updated according to the
partial operation done.
Otherwise an error code is returned, same as from VOP_READ(9)