NAME

     VOP_GETPAGES, VOP_PUTPAGES — read or write VM pages from a file

SYNOPSIS

     #include <sys/param.h>
     #include <sys/vnode.h>
     #include <vm/vm.h>

     int
     VOP_GETPAGES(struct vnode *vp, vm_page_t *ma, int count, int *rbehind, int *rahead);

     int
     VOP_PUTPAGES(struct vnode *vp, vm_page_t *ma, int bytecount, int flags, int *rtvals);

DESCRIPTION

     The 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, VOP_GETPAGES() is requested to
     read those pages as well, although it is not required to do so.  The VOP_PUTPAGES() method does the
     converse; that is to say, it writes out adjacent dirty pages of virtual memory.

     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.

     The arguments are:

     vp       The file to access.

     ma       Pointer to the first element of an array of pages representing a contiguous region of the file to
              be read or written.

     count    The length of the ma array.

     bytecount
              The number of bytes that should be written from the pages of the array.

     flags    A bitfield of flags affecting the function operation.  If 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 VM_PAGER_PUT_INVAL is set, the pages are to be invalidated after being written.  If
              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
              vm_page_deactivate_noreuse(9), which puts such pages onto the head of the inactive queue.  If
              VM_PAGER_CLUSTER_OK is set, writes may be performed asynchronously, so that related writes can be
              coalesced for efficiency, e.g., using the clustering mechanism of the buffer cache.

     rtvals   An array of VM system result codes indicating the status of each page written by VOP_PUTPAGES().

     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.

     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.

     The status of the VOP_PUTPAGES() method is returned on a page-by-page basis in the array rtvals[].  The
     possible status values are as follows:

     VM_PAGER_OK     The page was successfully written.  The implementation must call vm_page_undirty(9) to mark
                     the page as clean.

     VM_PAGER_PEND   The page was scheduled to be written asynchronously.  When the write completes, the
                     completion callback should call vm_object_pip_wakeup(9) and vm_page_sunbusy(9) to clear the
                     busy flag and awaken any other threads waiting for this page, in addition to calling
                     vm_page_undirty(9).

     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.

     VM_PAGER_ERROR  The page could not be written because of an error on the underlying storage medium or
                     protocol.

     VM_PAGER_FAIL   Treated identically to VM_PAGER_ERROR.

     VM_PAGER_AGAIN  The page was not handled by this request.

     The VOP_GETPAGES() method must populate and validate all requested pages in order to return success.  It is
     expected to release any pages in ma that it does not successfully handle, by calling vm_page_free(9).  When
     it succeeds, VOP_GETPAGES() must set the valid bits appropriately.  Upon entry to VOP_GETPAGES(), all pages
     in 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 VOP_GETPAGES() depending on the architecture of the
     particular filesystem.

RETURN VALUES

     If it successfully reads all pages in ma, VOP_GETPAGES() returns VM_PAGER_OK; otherwise, it returns
     VM_PAGER_ERROR.  By convention, the return value of VOP_PUTPAGES() is rtvals[0].

SEE ALSO

     vm_object_pip_wakeup(9), vm_page_free(9), vm_page_sunbusy(9), vm_page_undirty(9), vm_page_xunbusy(9),
     vnode(9)

AUTHORS

     This manual page was written by Doug Rabson and then substantially rewritten by
     Garrett Wollman.