man/man9/VOP_GETPAGES.9

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.Dd June 29, 2019
.Dt VOP_GETPAGES 9
.Os
.Sh NAME
.Nm VOP_GETPAGES ,
.Nm VOP_PUTPAGES
.Nd read or write VM pages from a file
.Sh SYNOPSIS
.In sys/param.h
.In sys/vnode.h
.In vm/vm.h
.Ft int
.Fo VOP_GETPAGES
.Fa "struct vnode *vp"
.Fa "vm_page_t *ma"
.Fa "int count"
.Fa "int *rbehind"
.Fa "int *rahead"
.Fc
.Ft int
.Fo VOP_PUTPAGES
.Fa "struct vnode *vp"
.Fa "vm_page_t *ma"
.Fa "int bytecount"
.Fa "int flags"
.Fa "int *rtvals"
.Fc
.Sh DESCRIPTION
The
.Fn VOP_GETPAGES
method is called to read in pages of virtual memory which are backed by
ordinary files.
If other adjacent pages are backed by adjacent regions of the same file,
.Fn VOP_GETPAGES
is requested to read those pages as well, although it is not required to
do so.
The
.Fn VOP_PUTPAGES
method does the converse; that is to say, it writes out adjacent dirty
pages of virtual memory.
.Pp
On entry, the vnode lock is held but neither the page queue nor VM object
locks are held.
Both methods return in the same state on both success and error returns.
.Pp
The arguments are:
.Bl -tag -width rbehind
.It Fa vp
The file to access.
.It Fa ma
Pointer to the first element of an array of pages representing a
contiguous region of the file to be read or written.
.It Fa count
The length of the
.Fa ma
array.
.It Fa bytecount
The number of bytes that should be written from the pages of the array.
.It Fa flags
A bitfield of flags affecting the function operation.
If
.Dv VM_PAGER_PUT_SYNC
is set, the write should be synchronous; control must not be returned
to the caller until after the write is finished.
If
.Dv VM_PAGER_PUT_INVAL
is set, the pages are to be invalidated after being written.
If
.Dv VM_PAGER_PUT_NOREUSE
is set, the I/O performed should set the IO_NOREUSE flag, to indicate
to the filesystem that pages should be marked for fast reuse if needed.
This could occur via a call to
.Xr vm_page_deactivate_noreuse 9 ,
which puts such pages onto the head of the inactive queue.
If
.Dv VM_PAGER_CLUSTER_OK
is set, writes may be delayed, so that related writes
can be coalesced for efficiency, e.g.,
using the clustering mechanism of the buffer cache.
.It Fa rtvals
An array of VM system result codes indicating the status of each
page written by
.Fn VOP_PUTPAGES .
.It Fa rbehind
Optional pointer to integer specifying number of pages to be read behind, if
possible.
If the filesystem supports that feature, number of actually read pages is
reported back, otherwise zero is returned.
.It Fa rahead
Optional pointer to integer specifying number of pages to be read ahead, if
possible.
If the filesystem supports that feature, number of actually read pages is
reported back, otherwise zero is returned.
.El
.Pp
The status of the
.Fn VOP_PUTPAGES
method is returned on a page-by-page basis in the array
.Fa rtvals[] .
The possible status values are as follows:
.Bl -tag -width VM_PAGER_ERROR
.It Dv VM_PAGER_OK
The page was successfully written.
The implementation must call
.Xr vm_page_undirty 9
to mark the page as clean.
.It Dv VM_PAGER_PEND
The page was scheduled to be written asynchronously.
When the write completes, the completion callback should
call
.Xr vm_object_pip_wakeup 9
and
.Xr vm_page_sunbusy 9
to clear the busy flag and awaken any other threads waiting for this page,
in addition to calling
.Xr vm_page_undirty 9 .
.It Dv VM_PAGER_BAD
The page was entirely beyond the end of the backing file.
This condition should not be possible if the vnode's file system
is correctly implemented.
.It Dv VM_PAGER_ERROR
The page could not be written because of an error on the underlying storage
medium or protocol.
.It Dv VM_PAGER_FAIL
Treated identically to
.Dv VM_PAGER_ERROR .
.It Dv VM_PAGER_AGAIN
The page was not handled by this request.
.El
.Pp
The
.Fn VOP_GETPAGES
method must populate and validate all requested pages in order to
return success.
It is expected to release any pages in
.Fa ma
that it does not successfully handle, by calling
.Xr vm_page_free 9 .
When it succeeds,
.Fn VOP_GETPAGES
must set the valid bits appropriately.
Upon entry to
.Fn VOP_GETPAGES ,
all pages in
.Fa ma
are busied exclusively.
Upon successful return, the pages must all be busied exclusively
as well, but pages may be unbusied during processing.
The filesystem is responsible for activating paged-out pages, but this
does not necessarily need to be done within
.Fn VOP_GETPAGES
depending on the architecture of the particular filesystem.
.Sh RETURN VALUES
If it successfully reads all pages in
.Fa ma ,
.Fn VOP_GETPAGES
returns
.Dv VM_PAGER_OK ;
otherwise, it returns
.Dv VM_PAGER_ERROR .
By convention, the return value of
.Fn VOP_PUTPAGES
is
.Fa rtvals[0] .
.Sh SEE ALSO
.Xr vm_object_pip_wakeup 9 ,
.Xr vm_page_free 9 ,
.Xr vm_page_sunbusy 9 ,
.Xr vm_page_undirty 9 ,
.Xr vm_page_xunbusy 9 ,
.Xr vnode 9
.Sh AUTHORS
This manual page was written by
.An Doug Rabson
and then substantially rewritten by
.An Garrett Wollman .