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 delayed, 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.

Debian                                            June 29, 2019                                  VOP_GETPAGES(9)