lazyrite.c

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/*++

Copyright (c) 1990  Microsoft Corporation

Module Name:

    lazyrite.c

Abstract:

    This module implements the lazy writer for the Cache subsystem.

Author:

    Tom Miller      [TomM]      22-July-1990

Revision History:

--*/

#include "cc.h"

//
//  The Bug check file id for this module
//

#define BugCheckFileId                   (CACHE_BUG_CHECK_LAZYRITE)

//
//  Define our debug constant
//

#define me 0x00000020

//
//  Local support routines
//

PWORK_QUEUE_ENTRY
CcReadWorkQueue (
    );

VOID
CcLazyWriteScan (
    );


VOID
CcScheduleLazyWriteScan (
    )

/*++

Routine Description:

    This routine may be called to schedule the next lazy writer scan,
    during which lazy write and lazy close activity is posted to other
    worker threads.  Callers should acquire the lazy writer spin lock
    to see if the scan is currently active, and then call this routine
    still holding the spin lock if not.  One special call is used at
    the end of the lazy write scan to propagate lazy write active once
    we go active.  This call is "the" scan thread, and it can therefore
    safely schedule the next scan without taking out the spin lock.

Arguments:

    None

Return Value:

    None.

--*/

{
    //
    //  It is important to set the active flag TRUE first for the propagate
    //  case, because it is conceivable that once the timer is set, another
    //  thread could actually run and make the scan go idle before we then
    //  jam the flag TRUE.
    //
    //  When going from idle to active, we delay a little longer to let the
    //  app finish saving its file.
    //

    if (LazyWriter.ScanActive) {

        KeSetTimer( &LazyWriter.ScanTimer, CcIdleDelay, &LazyWriter.ScanDpc );

    } else {

        LazyWriter.ScanActive = TRUE;
        KeSetTimer( &LazyWriter.ScanTimer, CcFirstDelay, &LazyWriter.ScanDpc );
    }
}


VOID
CcScanDpc (
    IN PKDPC Dpc,
    IN PVOID DeferredContext,
    IN PVOID SystemArgument1,
    IN PVOID SystemArgument2
    )

/*++

Routine Description:

    This is the Dpc routine which runs when the scan timer goes off.  It
    simply posts an element for an Ex Worker thread to do the scan.

Arguments:

    (All are ignored)

Return Value:

    None.

--*/

{
    PWORK_QUEUE_ENTRY WorkQueueEntry;

    UNREFERENCED_PARAMETER(Dpc);
    UNREFERENCED_PARAMETER(DeferredContext);
    UNREFERENCED_PARAMETER(SystemArgument1);
    UNREFERENCED_PARAMETER(SystemArgument2);

    WorkQueueEntry = CcAllocateWorkQueueEntry();

    //
    //  If we failed to allocate a WorkQueueEntry, things must
    //  be in pretty bad shape.  However, all we have to do is
    //  say we are not active, and wait for another event to
    //  wake things up again.
    //

    if (WorkQueueEntry == NULL) {

        LazyWriter.ScanActive = FALSE;

    } else {

        //
        //  Otherwise post a work queue entry to do the scan.
        //

        WorkQueueEntry->Function = (UCHAR)LazyWriteScan;

        CcPostWorkQueue( WorkQueueEntry, &CcRegularWorkQueue );
    }
}


NTSTATUS
CcWaitForCurrentLazyWriterActivity (
    )

/*++

Routine Description:

    This routine allows a thread to receive notification when the current tick
    of lazy writer work has completed.  It must not be called within a lazy
    writer workitem!  The caller must not be holding synchronization that could
    block a Cc workitem!
    
    In particular, this lets a caller insure that all avaliable lazy closes at
    the time of the call have completed.

Arguments:

    None.

Return Value:

    Final result of the wait.    

--*/

{
    KIRQL OldIrql;
    KEVENT Event;
    PWORK_QUEUE_ENTRY WorkQueueEntry;

    WorkQueueEntry = CcAllocateWorkQueueEntry();

    if (WorkQueueEntry == NULL) {
        return STATUS_INSUFFICIENT_RESOURCES;
    }

    WorkQueueEntry->Function = (UCHAR)EventSet;
    KeInitializeEvent( &Event, NotificationEvent, FALSE );
    WorkQueueEntry->Parameters.Event.Event = &Event;

    //
    //  Add this to the post-tick work queue and wake the lazy writer for it.
    //  The lazy writer will add this to the end of the next batch of work
    //  he issues.
    //

    CcAcquireMasterLock( &OldIrql );

    InsertTailList( &CcPostTickWorkQueue, &WorkQueueEntry->WorkQueueLinks );

    LazyWriter.OtherWork = TRUE;
    if (!LazyWriter.ScanActive) {
        CcScheduleLazyWriteScan();
    }

    CcReleaseMasterLock( OldIrql );

    return KeWaitForSingleObject( &Event, Executive, KernelMode, FALSE, NULL );
}



VOID
CcLazyWriteScan (
    )

/*++

Routine Description:

    This routine implements the Lazy Writer scan for dirty data to flush
    or any other work to do (lazy close).  This routine is scheduled by
    calling CcScheduleLazyWriteScan.

Arguments:

    None.

Return Value:

    None.

--*/

{
    ULONG PagesToWrite, ForegroundRate, EstimatedDirtyNextInterval;
    PSHARED_CACHE_MAP SharedCacheMap, FirstVisited;
    KIRQL OldIrql;
    ULONG LoopsWithLockHeld = 0;
    BOOLEAN AlreadyMoved = FALSE;

    LIST_ENTRY PostTickWorkQueue;

    //
    //  Top of Lazy Writer scan.
    //

    try {

        //
        //  If there is no work to do, then we will go inactive, and return.
        //

        CcAcquireMasterLock( &OldIrql );

        if ((CcTotalDirtyPages == 0) && !LazyWriter.OtherWork) {

            //
            //  Sleep if there are no deferred writes.  It is important to check
            //  proactively because writes may be blocked for reasons external
            //  to the cache manager.  The lazy writer must keep poking since it
            //  may have no bytes to write itself.
            //
        
            if (IsListEmpty(&CcDeferredWrites)) {
                
                LazyWriter.ScanActive = FALSE;
                CcReleaseMasterLock( OldIrql );
            
            } else {

                CcReleaseMasterLock( OldIrql );
        
                //
                //  Check for writes and schedule the next scan.
                //
        
                CcPostDeferredWrites();
                CcScheduleLazyWriteScan();
            }

            return;
        }

        //
        //  Pull out the post tick workitems for this pass.  It is important that
        //  we are doing this at the top since more could be queued as we rummage
        //  for work to do.  Post tick workitems are guaranteed to occur after all
        //  work generated in a complete scan.
        //

        InitializeListHead( &PostTickWorkQueue );
        while (!IsListEmpty( &CcPostTickWorkQueue )) {

            PLIST_ENTRY Entry = RemoveHeadList( &CcPostTickWorkQueue );
            InsertTailList( &PostTickWorkQueue, Entry );
        }

        //
        //  Calculate the next sweep time stamp, then update all relevant fields for
        //  the next time around.  Also we can clear the OtherWork flag.
        //

        LazyWriter.OtherWork = FALSE;

        //
        //  Assume we will write our usual fraction of dirty pages.  Do not do the
        //  divide if there is not enough dirty pages, or else we will never write
        //  the last few pages.
        //

        PagesToWrite = CcTotalDirtyPages;
        if (PagesToWrite > LAZY_WRITER_MAX_AGE_TARGET) {
            PagesToWrite /= LAZY_WRITER_MAX_AGE_TARGET;
        }

        //
        //  Estimate the rate of dirty pages being produced in the foreground.
        //  This is the total number of dirty pages now plus the number of dirty
        //  pages we scheduled to write last time, minus the number of dirty
        //  pages we have now.  Throw out any cases which would not produce a
        //  positive rate.
        //

        ForegroundRate = 0;

        if ((CcTotalDirtyPages + CcPagesWrittenLastTime) > CcDirtyPagesLastScan) {
            ForegroundRate = (CcTotalDirtyPages + CcPagesWrittenLastTime) -
                             CcDirtyPagesLastScan;
        }

        //
        //  If we estimate that we will exceed our dirty page target by the end
        //  of this interval, then we must write more.  Try to arrive on target.
        //

        EstimatedDirtyNextInterval = CcTotalDirtyPages - PagesToWrite + ForegroundRate;

        if (EstimatedDirtyNextInterval > CcDirtyPageTarget) {
            PagesToWrite += EstimatedDirtyNextInterval - CcDirtyPageTarget;
        }

        //
        //  Now save away the number of dirty pages and the number of pages we
        //  just calculated to write.
        //

        CcDirtyPagesLastScan = CcTotalDirtyPages;
        CcPagesYetToWrite = CcPagesWrittenLastTime = PagesToWrite;

        //
        //  Loop to flush enough Shared Cache Maps to write the number of pages
        //  we just calculated.
        //

        SharedCacheMap = CONTAINING_RECORD( CcLazyWriterCursor.SharedCacheMapLinks.Flink,
                                            SHARED_CACHE_MAP,
                                            SharedCacheMapLinks );

        DebugTrace( 0, me, "Start of Lazy Writer Scan\n", 0 );

        //
        //  Normally we would just like to visit every Cache Map once on each scan,
        //  so the scan will terminate normally when we return to FirstVisited.  But
        //  in the off chance that FirstVisited gets deleted, we are guaranteed to stop
        //  when we get back to our own listhead.
        //

        FirstVisited = NULL;
        while ((SharedCacheMap != FirstVisited) &&
               (&SharedCacheMap->SharedCacheMapLinks != &CcLazyWriterCursor.SharedCacheMapLinks)) {

            if (FirstVisited == NULL) {
                FirstVisited = SharedCacheMap;
            }

            //
            //  Skip the SharedCacheMap if a write behind request is
            //  already queued, write behind has been disabled, or
            //  if there is no work to do (either dirty data to be written
            //  or a delete is required).
            //
            //  Note that for streams where modified writing is disabled, we
            //  need to take out Bcbs exclusive, which serializes with foreground
            //  activity.  Therefore we use a special counter in the SharedCacheMap
            //  to only service these once every n intervals.
            //
            //  Skip temporary files unless we currently could not write as many
            //  bytes as we might charge some hapless thread for throttling.
            //
            //  When considering lazy closes, decline to work on SharedCacheMaps
            //  that would require additional IO already prohibited by our normal
            //  tests.
            //

            if (!FlagOn(SharedCacheMap->Flags, WRITE_QUEUED | IS_CURSOR)

                    &&

                (((PagesToWrite != 0) && (SharedCacheMap->DirtyPages != 0) &&
                  (((++SharedCacheMap->LazyWritePassCount & 0xF) == 0) ||
                   !FlagOn(SharedCacheMap->Flags, MODIFIED_WRITE_DISABLED) ||
                   (CcCapturedSystemSize == MmSmallSystem) ||
                   (SharedCacheMap->DirtyPages >= (4 * (MAX_WRITE_BEHIND / PAGE_SIZE)))) &&
                  (!FlagOn(SharedCacheMap->FileObject->Flags, FO_TEMPORARY_FILE) ||
                   !CcCanIWrite(SharedCacheMap->FileObject, WRITE_CHARGE_THRESHOLD, FALSE, MAXUCHAR)))

                    ||

                 ((SharedCacheMap->OpenCount == 0) &&
                  ((SharedCacheMap->DirtyPages == 0) ||
                   (SharedCacheMap->FileSize.QuadPart == 0))))) {

                PWORK_QUEUE_ENTRY WorkQueueEntry;

                //
                //  If this is a metadata stream with at least 4 times
                //  the maximum write behind I/O size, then let's tell
                //  this guy to write 1/8 of his dirty data on this pass
                //  so it doesn't build up.
                //
                //  Else assume we can write everything (PagesToWrite only affects
                //  metadata streams - otherwise writing is controlled by the Mbcb).
                //

                SharedCacheMap->PagesToWrite = SharedCacheMap->DirtyPages;

                if (FlagOn(SharedCacheMap->Flags, MODIFIED_WRITE_DISABLED) &&
                    (SharedCacheMap->PagesToWrite >= (4 * (MAX_WRITE_BEHIND / PAGE_SIZE))) &&
                    (CcCapturedSystemSize != MmSmallSystem)) {

                    SharedCacheMap->PagesToWrite /= 8;
                }

                //
                //  If still searching for pages to write, adjust our targets.
                //

                if (!AlreadyMoved) {

                    //
                    //  See if he exhausts the number of pages to write.  (We
                    //  keep going in case there are any closes to do.)
                    //

                    if (SharedCacheMap->PagesToWrite >= PagesToWrite) {

                        //
                        //  If we met our write quota on a given SharedCacheMap, then make sure
                        //  we start at him on the next scan, unless it is a metadata stream.
                        //

                        RemoveEntryList( &CcLazyWriterCursor.SharedCacheMapLinks );

                        //
                        //  For Metadata streams, set up to resume on the next stream on the
                        //  next scan.
                        //

                        if (FlagOn(SharedCacheMap->Flags, MODIFIED_WRITE_DISABLED)) {
                            InsertHeadList( &SharedCacheMap->SharedCacheMapLinks, &CcLazyWriterCursor.SharedCacheMapLinks );

                            //
                            //  For other streams, set up to resume on the same stream on the
                            //  next scan.
                            //

                        } else {
                            InsertTailList( &SharedCacheMap->SharedCacheMapLinks, &CcLazyWriterCursor.SharedCacheMapLinks );
                        }

                        PagesToWrite = 0;
                        AlreadyMoved = TRUE;

                    } else {

                        PagesToWrite -= SharedCacheMap->PagesToWrite;
                    }
                }

                //
                //  Otherwise show we are actively writing, and keep it in the dirty
                //  list.
                //

                SetFlag(SharedCacheMap->Flags, WRITE_QUEUED);
                SharedCacheMap->DirtyPages += 1;

                CcReleaseMasterLock( OldIrql );

                //
                //  Queue the request to do the work to a worker thread.
                //

                WorkQueueEntry = CcAllocateWorkQueueEntry();

                //
                //  If we failed to allocate a WorkQueueEntry, things must
                //  be in pretty bad shape.  However, all we have to do is
                //  break out of our current loop, and try to go back and
                //  delay a while.  Even if the current guy should have gone
                //  away when we clear WRITE_QUEUED, we will find him again
                //  in the LW scan.
                //

                if (WorkQueueEntry == NULL) {

                    CcAcquireMasterLock( &OldIrql );
                    ClearFlag(SharedCacheMap->Flags, WRITE_QUEUED);
                    SharedCacheMap->DirtyPages -= 1;
                    break;
                }

                WorkQueueEntry->Function = (UCHAR)WriteBehind;
                WorkQueueEntry->Parameters.Write.SharedCacheMap = SharedCacheMap;

                //
                //  Post it to the regular work queue.
                //

                CcAcquireMasterLock( &OldIrql );
                SharedCacheMap->DirtyPages -= 1;
                CcPostWorkQueue( WorkQueueEntry, &CcRegularWorkQueue );

                LoopsWithLockHeld = 0;

            //
            //  Make sure we occassionally drop the lock.  Set WRITE_QUEUED
            //  to keep the guy from going away.
            //

            } else if ((++LoopsWithLockHeld >= 20) &&
                       !FlagOn(SharedCacheMap->Flags, WRITE_QUEUED | IS_CURSOR)) {

                SetFlag(SharedCacheMap->Flags, WRITE_QUEUED);
                SharedCacheMap->DirtyPages += 1;
                CcReleaseMasterLock( OldIrql );
                LoopsWithLockHeld = 0;
                CcAcquireMasterLock( &OldIrql );
                ClearFlag(SharedCacheMap->Flags, WRITE_QUEUED);
                SharedCacheMap->DirtyPages -= 1;
            }

            //
            //  Now loop back.
            //

            SharedCacheMap =
                CONTAINING_RECORD( SharedCacheMap->SharedCacheMapLinks.Flink,
                                   SHARED_CACHE_MAP,
                                   SharedCacheMapLinks );
        }

        DebugTrace( 0, me, "End of Lazy Writer Scan\n", 0 );

        //
        //  Queue up our  post tick workitems for this pass.
        //

        while (!IsListEmpty( &PostTickWorkQueue )) {

            PLIST_ENTRY Entry = RemoveHeadList( &PostTickWorkQueue );
            CcPostWorkQueue( CONTAINING_RECORD( Entry, WORK_QUEUE_ENTRY, WorkQueueLinks ),
                             &CcRegularWorkQueue );
        }
        
        //
        //  Now we can release the global list and loop back, per chance to sleep.
        //

        CcReleaseMasterLock( OldIrql );

        //
        //  Once again we need to give the deferred writes a poke.  We can have all dirty
        //  pages on disable_write_behind files but also have an external condition that
        //  caused the cached IO to be deferred. If so, this serves as our only chance to
        //  issue it when the condition clears.
        //
        //  Case hit on ForrestF's 5gb Alpha, 1/12/99.
        //
        
        if (!IsListEmpty(&CcDeferredWrites)) {
            
            CcPostDeferredWrites();
        }

        //
        //  Now go ahead and schedule the next scan.
        //

        CcScheduleLazyWriteScan();

    //
    //  Basically, the Lazy Writer thread should never get an exception,
    //  so we put a try-except around it that bug checks one way or the other.
    //  Better we bug check here than worry about what happens if we let one
    //  get by.
    //

    } except( CcExceptionFilter( GetExceptionCode() )) {

        CcBugCheck( GetExceptionCode(), 0, 0 );
    }
}


//
//  Internal support routine
//

LONG
CcExceptionFilter (
    IN NTSTATUS ExceptionCode
    )

/*++

Routine Description:

    This is the standard exception filter for worker threads which simply
    calls an FsRtl routine to see if an expected status is being raised.
    If so, the exception is handled, else we bug check.

Arguments:

    ExceptionCode - the exception code which was raised.

Return Value:

    EXCEPTION_EXECUTE_HANDLER if expected, else a Bug Check occurs.

--*/

{
    DebugTrace(0, 0, "CcExceptionFilter %08lx\n", ExceptionCode);

    if (FsRtlIsNtstatusExpected( ExceptionCode )) {

        return EXCEPTION_EXECUTE_HANDLER;

    } else {

        return EXCEPTION_CONTINUE_SEARCH;
    }
}



//
//  Internal support routine
//

VOID
FASTCALL
CcPostWorkQueue (
    IN PWORK_QUEUE_ENTRY WorkQueueEntry,
    IN PLIST_ENTRY WorkQueue
    )

/*++

Routine Description:

    This routine queues a WorkQueueEntry, which has been allocated and
    initialized by the caller, to the WorkQueue for FIFO processing by
    the work threads.

Arguments:

    WorkQueueEntry - supplies a pointer to the entry to queue

Return Value:

    None

--*/

{
    KIRQL OldIrql;
    PLIST_ENTRY WorkerThreadEntry = NULL;

    ASSERT(FIELD_OFFSET(WORK_QUEUE_ITEM, List) == 0);

    DebugTrace(+1, me, "CcPostWorkQueue:\n", 0 );
    DebugTrace( 0, me, "    WorkQueueEntry = %08lx\n", WorkQueueEntry );

    //
    //  Queue the entry to the respective work queue.
    //

    ExAcquireFastLock( &CcWorkQueueSpinlock, &OldIrql );
    InsertTailList( WorkQueue, &WorkQueueEntry->WorkQueueLinks );

    //
    //  Now, if we aren't throttled and have any more idle threads we can
    //  use, activate one.
    //

    if (!CcQueueThrottle && !IsListEmpty(&CcIdleWorkerThreadList)) {
        WorkerThreadEntry = RemoveHeadList( &CcIdleWorkerThreadList );
        CcNumberActiveWorkerThreads += 1;
    }
    ExReleaseFastLock( &CcWorkQueueSpinlock, OldIrql );

    if (WorkerThreadEntry != NULL) {

        //
        //  I had to peak in the sources to verify that this routine
        //  is a noop if the Flink is not NULL.  Sheeeeit!
        //

        ((PWORK_QUEUE_ITEM)WorkerThreadEntry)->List.Flink = NULL;
        ExQueueWorkItem( (PWORK_QUEUE_ITEM)WorkerThreadEntry, CriticalWorkQueue );
    }

    //
    //  And return to our caller
    //

    DebugTrace(-1, me, "CcPostWorkQueue -> VOID\n", 0 );

    return;
}


//
//  Internal support routine
//

VOID
CcWorkerThread (
    PVOID ExWorkQueueItem
    )

/*++

Routine Description:

    This is worker thread routine for processing cache manager work queue
    entries.

Arguments:

    ExWorkQueueItem - The work item used for this thread

Return Value:

    None

--*/

{
    KIRQL OldIrql;
    PLIST_ENTRY WorkQueue;
    PWORK_QUEUE_ENTRY WorkQueueEntry;
    BOOLEAN RescanOk = FALSE;
    BOOLEAN DropThrottle = FALSE;
    IO_STATUS_BLOCK IoStatus;

    IoStatus.Status = STATUS_SUCCESS;
    IoStatus.Information = 0;

    ASSERT(FIELD_OFFSET(WORK_QUEUE_ENTRY, WorkQueueLinks) == 0);

    while (TRUE) {

        ExAcquireFastLock( &CcWorkQueueSpinlock, &OldIrql );

        //
        //  If we just processed a throttled operation, drop the flag.
        //

        if (DropThrottle) {

            DropThrottle = CcQueueThrottle = FALSE;
        }

        //
        //  On requeue, push at end of the regular queue and clear hint.
        //
        
        if (IoStatus.Information == CC_REQUEUE) {

            InsertTailList( WorkQueue, &WorkQueueEntry->WorkQueueLinks );
            IoStatus.Information = 0;
        }
        
        //
        //  First see if there is something in the express queue.
        //

        if (!IsListEmpty(&CcExpressWorkQueue)) {
            WorkQueue = &CcExpressWorkQueue;

        //
        //  If there was nothing there, then try the regular queue.
        //

        } else if (!IsListEmpty(&CcRegularWorkQueue)) {
            WorkQueue = &CcRegularWorkQueue;

        //
        //  Else we can break and go idle.
        //
        
        } else {

            break;
        }

        WorkQueueEntry = CONTAINING_RECORD( WorkQueue->Flink, WORK_QUEUE_ENTRY, WorkQueueLinks );

        //
        //  If this is an EventSet, throttle down to a single thread to be sure
        //  that this event fires after all preceeding workitems have completed.
        //

        if (WorkQueueEntry->Function == EventSet && CcNumberActiveWorkerThreads > 1) {

            CcQueueThrottle = TRUE;
            break;
        }

        //
        //  Pop the workitem off: we will execute it now.
        //

        RemoveHeadList( WorkQueue );

        ExReleaseFastLock( &CcWorkQueueSpinlock, OldIrql );

        //
        //  Process the entry within a try-except clause, so that any errors
        //  will cause us to continue after the called routine has unwound.
        //

        try {

            switch (WorkQueueEntry->Function) {

            //
            //  Perform read ahead
            //

            case ReadAhead:

                DebugTrace( 0, me, "CcWorkerThread Read Ahead FileObject = %08lx\n",
                            WorkQueueEntry->Parameters.Read.FileObject );

                CcPerformReadAhead( WorkQueueEntry->Parameters.Read.FileObject );

                break;

            //
            //  Perform write behind
            //

            case WriteBehind:

                DebugTrace( 0, me, "CcWorkerThread WriteBehind SharedCacheMap = %08lx\n",
                            WorkQueueEntry->Parameters.Write.SharedCacheMap );

                CcWriteBehind( WorkQueueEntry->Parameters.Write.SharedCacheMap, &IoStatus );
                RescanOk = (BOOLEAN)NT_SUCCESS(IoStatus.Status);
                break;

            
            //
            //  Perform set event
            //
            
            case EventSet:
                
                DebugTrace( 0, me, "CcWorkerThread SetEvent Event = %08lx\n",
                            WorkQueueEntry->Parameters.Event.Event );

                KeSetEvent( WorkQueueEntry->Parameters.Event.Event, 0, FALSE );
                DropThrottle = TRUE;
                break;

            //
            //  Perform Lazy Write Scan
            //

            case LazyWriteScan:

                DebugTrace( 0, me, "CcWorkerThread Lazy Write Scan\n", 0 );

                CcLazyWriteScan();
                break;
            }

        }
        except( CcExceptionFilter( GetExceptionCode() )) {

            NOTHING;
        }

        //
        //  If not a requeue request, free the workitem.
        //
        
        if (IoStatus.Information != CC_REQUEUE) {
            
            CcFreeWorkQueueEntry( WorkQueueEntry );
        }
    }

    //
    //  No more work.  Requeue our worker thread entry and get out.
    //

    InsertTailList( &CcIdleWorkerThreadList,
                    &((PWORK_QUEUE_ITEM)ExWorkQueueItem)->List );
    CcNumberActiveWorkerThreads -= 1;

    ExReleaseFastLock( &CcWorkQueueSpinlock, OldIrql );

    if (!IsListEmpty(&CcDeferredWrites) && (CcTotalDirtyPages >= 20) && RescanOk) {
        CcLazyWriteScan();
    }

    return;
}

---