[arty] 56267: [NTOSKRKNL] Write a lot of general prose about the operation of the Mm parts of NewCC. Dedicate to timo and chongo. No code changes.

Author: arty
Date: Thu Mar 29 04:43:44 2012
New Revision: 56267
URL: http://svn.reactos.org/svn/reactos?rev=56267&view=rev
Log:
[NTOSKRKNL]
Write a lot of general prose about the operation of the Mm parts of NewCC.
Dedicate to timo and chongo.
No code changes.
Modified:
    trunk/reactos/ntoskrnl/cache/section/data.c
    trunk/reactos/ntoskrnl/cache/section/fault.c
    trunk/reactos/ntoskrnl/cache/section/io.c
    trunk/reactos/ntoskrnl/cache/section/reqtools.c
    trunk/reactos/ntoskrnl/cache/section/sptab.c
    trunk/reactos/ntoskrnl/cache/section/swapout.c
    trunk/reactos/ntoskrnl/mm/rmap.c
Modified: trunk/reactos/ntoskrnl/cache/section/data.c
URL:
http://svn.reactos.org/svn/reactos/trunk/reactos/ntoskrnl/cache/section/dat…
==============================================================================
--- trunk/reactos/ntoskrnl/cache/section/data.c [iso-8859-1] (original)
+++ trunk/reactos/ntoskrnl/cache/section/data.c [iso-8859-1] Thu Mar 29 04:43:44 2012
@@ -42,6 +42,36 @@
  *                  Herve Poussineau
  */
+/*
+
+A note on this code:
+
+Unlike the previous section code, this code does not rely on an active map
+for a page to exist in a data segment.  Each mapping contains a large integer
+offset to map at, and the segment always represents the entire section space
+from zero to the maximum long long.  This allows us to associate one single
+page map with each file object, and to let each mapping view an offset into
+the overall mapped file.  Temporarily unmapping the file has no effect on the
+section membership.
+
+This necessitates a change in the section page table implementation, which is
+now an RtlGenericTable.  This will be elaborated more in sptab.c.  One upshot
+of this change is that a mapping of a small files takes a bit more than 1/4
+of the size in nonpaged kernel space as it did previously.
+
+When we need other threads that may be competing for the same page fault to
+wait, we have a mechanism seperate from PageOps for dealing with that, which
+was suggested by Travis Geiselbrecht after a conversation I had with Alex
+Ionescu.  That mechanism is the MM_WAIT_ENTRY, which is the all-ones SWAPENTRY.
+
+When we wish for other threads to know that we're waiting and will finish
+handling a page fault, we place the swap entry MM_WAIT_ENTRY in the page table
+at the fault address (this works on either the section page table or a process
+address space), perform any blocking operations required, then replace the
+entry with
+
+*/
+
 /* INCLUDES *****************************************************************/
 #include <ntoskrnl.h>
@@ -163,6 +193,17 @@
     return STATUS_SUCCESS;
 }
+/*
+
+MiFlushMappedSection
+
+Called from cache code to cause dirty pages of a section
+to be written back.  This doesn't affect the mapping.
+
+BaseOffset is the base at which to start writing in file space.
+FileSize is the length of the file as understood by the cache.
+
+ */
 NTSTATUS
 NTAPI
 _MiFlushMappedSection(PVOID BaseAddress,
@@ -299,6 +340,13 @@
     return Status;
 }
+/*
+
+This deletes a segment entirely including its page map.
+It must have been unmapped in every address space.
+
+ */
+
 VOID
 NTAPI
 MmFinalizeSegment(PMM_SECTION_SEGMENT Segment)
@@ -346,7 +394,7 @@
                      ULONG AllocationAttributes,
                      PFILE_OBJECT FileObject)
 /*
- * Create a section backed by a data file
+ * Create a section backed by a data file.
  */
 {
     PROS_SECTION_OBJECT Section;
@@ -641,6 +689,13 @@
     return STATUS_SUCCESS;
 }
+/*
+
+Completely remove the page at FileOffset in Segment.  The page must not
+be mapped.
+
+*/
+
 VOID
 NTAPI
 MiFreeSegmentPage(PMM_SECTION_SEGMENT Segment,
Modified: trunk/reactos/ntoskrnl/cache/section/fault.c
URL:
http://svn.reactos.org/svn/reactos/trunk/reactos/ntoskrnl/cache/section/fau…
==============================================================================
--- trunk/reactos/ntoskrnl/cache/section/fault.c [iso-8859-1] (original)
+++ trunk/reactos/ntoskrnl/cache/section/fault.c [iso-8859-1] Thu Mar 29 04:43:44 2012
@@ -43,6 +43,34 @@
  *                  Herve Poussineau
  */
+/*
+
+I've generally organized fault handling code in newmm as handlers that run
+under a single lock acquisition, check the state, and either take necessary
+action atomically, or place a wait entry and return a continuation to the
+caller.  This lends itself to code that has a simple, structured form,
+doesn't make assumptions about lock taking and breaking, and provides an
+obvious, graphic seperation between code that may block and code that isn't
+allowed to.  This file contains the non-blocking half.
+
+In order to request a blocking operation to happen outside locks, place a
+function pointer in the provided MM_REQUIRED_RESOURCES struct and return
+STATUS_MORE_PROCESSING_REQUIRED.  The function indicated will receive the
+provided struct and take action outside of any mm related locks and at
+PASSIVE_LEVEL.  The same fault handler will be called again after the
+blocking operation succeeds.  In this way, the fault handler can accumulate
+state, but will freely work while competing with other threads.
+
+Fault handlers in this file should check for an MM_WAIT_ENTRY in a page
+table they're using and return STATUS_SUCCESS + 1 if it's found.  In that
+case, the caller will wait on the wait entry event until the competing thread
+is finished, and recall this handler in the current thread.
+
+Another thing to note here is that we require mappings to exactly mirror
+rmaps, so each mapping should be immediately followed by an rmap addition.
+
+*/
+
 /* INCLUDES *****************************************************************/
 #include <ntoskrnl.h>
@@ -55,6 +83,22 @@
 extern KEVENT MmWaitPageEvent;
 extern PMMWSL MmWorkingSetList;
+
+/*
+
+Multiple stage handling of a not-present fault in a data section.
+
+Required->State is used to accumulate flags that indicate the next action
+the handler should take.
+
+State & 2 is currently used to indicate that the page acquired by a previous
+callout is a global page to the section and should be placed in the section
+page table.
+
+Note that the primitive tail recursion done here reaches the base case when
+the page is present.
+
+*/
 NTSTATUS
 NTAPI
@@ -168,6 +212,10 @@
     }
     else if (MM_IS_WAIT_PTE(Entry))
     {
+        // Whenever MM_WAIT_ENTRY is required as a swap entry, we need to
+        // ask the fault handler to wait until we should continue.  Rathern
+        // than recopy this boilerplate code everywhere, we just ask them
+        // to wait.
         MmUnlockSectionSegment(Segment);
         return STATUS_SUCCESS + 1;
     }
@@ -254,6 +302,18 @@
     MiUnmapPageInHyperSpace(Process, TempAddress, Irql);
     return STATUS_SUCCESS;
 }
+
+/*
+
+This function is deceptively named, in that it does the actual work of handling
+access faults on data sections.  In the case of the code that's present here,
+we don't allow cow sections, but we do need this to unset the initial
+PAGE_READONLY condition of pages faulted into the cache so that we can add
+a dirty bit in the section page table on the first modification.
+
+In the ultimate form of this code, CoW is reenabled.
+
+*/
 NTSTATUS
 NTAPI
@@ -344,6 +404,8 @@
             else
                 return STATUS_SUCCESS; // Nonwait swap entry ... handle elsewhere
         }
+        /* Call out to acquire a page to copy to.  We'll be re-called when
+         * the page has been allocated. */
         Required->Page[1] = MmGetPfnForProcess(Process, Address);
         Required->Consumer = MC_CACHE;
         Required->Amount = 1;
@@ -402,6 +464,15 @@
     KEVENT Wait;
     AcquireResource DoAcquisition;
 } WORK_QUEUE_WITH_CONTEXT, *PWORK_QUEUE_WITH_CONTEXT;
+
+/*
+
+This is the work item used do blocking resource acquisition when a fault
+handler returns STATUS_MORE_PROCESSING_REQUIRED.  It's used to allow resource
+acquisition to take place on a different stack, and outside of any locks used
+by fault handling, making recursive fault handling possible when required.
+
+*/
 VOID
 NTAPI
@@ -414,6 +485,38 @@
     DPRINT("Status %x\n", WorkItem->Status);
     KeSetEvent(&WorkItem->Wait, IO_NO_INCREMENT, FALSE);
 }
+
+/*
+
+This code seperates the action of fault handling into an upper and lower
+handler to allow the inner handler to optionally be called in work item
+if the stack is getting too deep.  My experiments show that the third
+recursive page fault taken at PASSIVE_LEVEL must be shunted away to a
+worker thread.  In the ultimate form of this code, the primary fault handler
+makes this decision by using a thread-local counter to detect a too-deep
+fault stack and call the inner fault handler in a worker thread if required.
+
+Note that faults are taken at passive level and have access to ordinary
+driver entry points such as those that read and write files, and filesystems
+should use paged structures whenever possible.  This makes recursive faults
+both a perfectly normal occurrance, and a worthwhile case to handle.
+
+The code below will repeatedly call MiCowSectionPage as long as it returns
+either STATUS_SUCCESS + 1 or STATUS_MORE_PROCESSING_REQUIRED.  In the more
+processing required case, we call out to a blocking resource acquisition
+function and then recall the faut handler with the shared state represented
+by the MM_REQUIRED_RESOURCES struct.
+
+In the other case, we wait on the wait entry event and recall the handler.
+Each time the wait entry event is signalled, one thread has removed an
+MM_WAIT_ENTRY from a page table.
+
+In the ultimate form of this code, there is a single system wide fault handler
+for each of access fault and not present and each memory area contains a
+function pointer that indicates the active fault handler.  Since the mm code
+in reactos is currently fragmented, I didn't bring this change to trunk.
+
+*/
 NTSTATUS
 NTAPI
@@ -564,6 +667,17 @@
     return Status;
 }
+/*
+
+This is the outer fault handler mentioned in the description of
+MmpSectionAccsesFaultInner.  It increments a fault depth count in the current
+thread.
+
+In the ultimate form of this code, the lower fault handler will optionally
+use the count to keep the kernel stack from overflowing.
+
+*/
+
 NTSTATUS
 NTAPI
 MmAccessFaultCacheSection(KPROCESSOR_MODE Mode,
@@ -612,6 +726,16 @@
     return Status;
 }
+
+/*
+
+As above, this code seperates the active part of fault handling from a carrier
+that can use the thread's active fault count to determine whether a work item
+is required.  Also as above, this function repeatedly calls the active not
+present fault handler until a clear success or failure is received, using a
+return of STATUS_MORE_PROCESSING_REQUIRED or STATUS_SUCCESS + 1.
+
+*/
 NTSTATUS
 NTAPI
@@ -765,6 +889,14 @@
     return Status;
 }
+/*
+
+Call the inner not present fault handler, keeping track of the fault count.
+In the ultimate form of this code, optionally use a worker thread the handle
+the fault in order to sidestep stack overflow in the multiple fault case.
+
+*/
+
 NTSTATUS
 NTAPI
 MmNotPresentFaultCacheSection(KPROCESSOR_MODE Mode,
Modified: trunk/reactos/ntoskrnl/cache/section/io.c
URL:
http://svn.reactos.org/svn/reactos/trunk/reactos/ntoskrnl/cache/section/io.…
==============================================================================
--- trunk/reactos/ntoskrnl/cache/section/io.c [iso-8859-1] (original)
+++ trunk/reactos/ntoskrnl/cache/section/io.c [iso-8859-1] Thu Mar 29 04:43:44 2012
@@ -64,9 +64,13 @@
     return IoGetRelatedDeviceObject(FileObject);
 }
-/* Note:
-   This completion function is really required. Paging io completion does almost
-   nothing, including freeing the mdls. */
+/*
+
+Note:
+This completion function is really required. Paging io completion does almost
+nothing, including freeing the mdls.
+
+*/
 NTSTATUS
 NTAPI
 MiSimpleReadComplete(PDEVICE_OBJECT DeviceObject,
@@ -94,6 +98,15 @@
     return STATUS_SUCCESS;
 }
+
+/*
+
+MiSimpleRead is a convenience function that provides either paging or non
+paging reads.  The caching and mm systems use this in paging mode, where
+a completion function is required as above.  The Paging BOOLEAN determines
+whether the read is issued as a paging read or as an ordinary buffered read.
+
+*/
 NTSTATUS
 NTAPI
@@ -177,6 +190,13 @@
     return Status;
 }
+/*
+
+Convenience function for writing from kernel space.  This issues a paging
+write in all cases.
+
+*/
+
 NTSTATUS
 NTAPI
 _MiSimpleWrite(PFILE_OBJECT FileObject,
@@ -259,6 +279,15 @@
 extern KEVENT MpwThreadEvent;
 FAST_MUTEX MiWriteMutex;
+/*
+
+Function which uses MiSimpleWrite to write back a single page to a file.
+The page in question does not need to be mapped.  This function could be
+made a bit more efficient by avoiding the copy and making a system space
+mdl.
+
+*/
+
 NTSTATUS
 NTAPI
 _MiWriteBackPage(PFILE_OBJECT FileObject,
Modified: trunk/reactos/ntoskrnl/cache/section/reqtools.c
URL:
http://svn.reactos.org/svn/reactos/trunk/reactos/ntoskrnl/cache/section/req…
==============================================================================
--- trunk/reactos/ntoskrnl/cache/section/reqtools.c [iso-8859-1] (original)
+++ trunk/reactos/ntoskrnl/cache/section/reqtools.c [iso-8859-1] Thu Mar 29 04:43:44 2012
@@ -42,6 +42,13 @@
  *                  Herve Poussineau
  */
+/*
+  This file contains functions used by fault.c to do blocking resource
+  acquisition.  To call one of these functions, fill out your
+  MM_REQUIRED_RESOURCES with a pointer to the desired function and configure
+  the other members as below.
+ */
+
 /* INCLUDES *****************************************************************/
 #include <ntoskrnl.h>
@@ -55,6 +62,22 @@
 NTAPI
 MmBuildMdlFromPages(PMDL Mdl, PPFN_NUMBER Pages);
+/*
+
+Blocking function to acquire zeroed pages from the balancer.
+
+Upon entry:
+
+Required->Amount: Number of pages to acquire
+Required->Consumer: consumer to charge the page to
+
+Upon return:
+
+Required->Pages[0..Amount]: Allocated pages.
+
+The function fails unless all requested pages can be allocated.
+
+ */
 NTSTATUS
 NTAPI
 MiGetOnePage(PMMSUPPORT AddressSpace,
@@ -85,6 +108,26 @@
     return Status;
 }
+/*
+
+Blocking function to read (part of) a page from a file.
+
+Upon entry:
+
+Required->Context: a FILE_OBJECT to read
+Required->Consumer: consumer to charge the page to
+Required->FileOffset: Offset to read at
+Required->Amount: Number of bytes to read (0 -> 4096)
+
+Upon return:
+
+Required->Page[Required->Offset]: The allocated and read in page
+
+The indicated page is filled to Required->Amount with file data and zeroed
+afterward.
+
+ */
+
 NTSTATUS
 NTAPI
 MiReadFilePage(PMMSUPPORT AddressSpace,
@@ -158,6 +201,21 @@
     return STATUS_SUCCESS;
 }
+/*
+
+Blocking function to read a swap page into a memory page.
+
+Upon entry:
+
+Required->Consumer: consumer to charge the page to
+Required->SwapEntry: swap entry to use
+
+Upon return:
+
+Required->Page[Required->Offset]: Populated page
+
+*/
+
 NTSTATUS
 NTAPI
 MiSwapInPage(PMMSUPPORT AddressSpace,
@@ -192,6 +250,22 @@
     return Status;
 }
+
+/*
+
+A way to write a page without a lock acquired using the same blocking mechanism
+as resource acquisition.
+
+Upon entry:
+
+Required->Page[Required->Offset]: Page to write
+Required->Context: FILE_OBJECT to write to
+Required->FileOffset: offset to write at
+
+This always does a paging write with whole page size.  Note that paging IO
+doesn't change the valid data length of a file.
+
+*/
 NTSTATUS
 NTAPI
Modified: trunk/reactos/ntoskrnl/cache/section/sptab.c
URL:
http://svn.reactos.org/svn/reactos/trunk/reactos/ntoskrnl/cache/section/spt…
==============================================================================
--- trunk/reactos/ntoskrnl/cache/section/sptab.c [iso-8859-1] (original)
+++ trunk/reactos/ntoskrnl/cache/section/sptab.c [iso-8859-1] Thu Mar 29 04:43:44 2012
@@ -22,6 +22,34 @@
  *
  * PROGRAMMERS:     arty
  */
+
+/*
+
+This file implements the section page table.  It relies on rtl generic table
+functionality to provide access to 256-page chunks.  Calls to
+MiSetPageEntrySectionSegment and MiGetPageEntrySectionSegment must be
+synchronized by holding the segment lock.
+
+Each page table entry is a ULONG as in x86.
+
+Bit 1 is used as a swap entry indication as in the main page table.
+Bit 2 is used as a dirty indication.  A dirty page will eventually be written
+back to the file.
+Bits 3-11 are used as a map count in the legacy mm code, Note that zero is
+illegal, as the legacy code does not take advantage of segment rmaps.
+Therefore, every segment page is mapped in at least one address space, and
+MmUnsharePageEntry is quite complicated.  In addition, the page may also be
+owned by the legacy cache manager, giving an implied additional reference.
+Upper bits are a PFN_NUMBER.
+
+These functions, in addition to maintaining the segment page table also
+automatically maintain the segment rmap by calling MmSetSectionAssociation
+and MmDeleteSectionAssociation.  Segment rmaps are discussed in rmap.c.  The
+upshot is that it is impossible to have a page properly registered in a segment
+page table and not also found in a segment rmap that can be found from the
+paging machinery.
+
+*/
 /* INCLUDES *****************************************************************/
@@ -232,6 +260,17 @@
     return Result;
 }
+/*
+
+Destroy the rtl generic table that serves as the section's page table.  Call
+the FreePage function for each non-zero entry in the section page table as
+we go.  Note that the page table is still techinally valid until after all
+pages are destroyed, as we don't finally destroy the table until we've free
+each slice.  There is no order guarantee for deletion of individual elements
+although it's in-order as written now.
+
+*/
+
 VOID
 NTAPI
 MmFreePageTablesSectionSegment(PMM_SECTION_SEGMENT Segment,
@@ -271,6 +310,21 @@
     DPRINT("Done\n");
 }
+/*
+
+Retrieves the MM_SECTION_SEGMENT and fills in the LARGE_INTEGER Offset given
+by the caller that corresponds to the page specified.  This uses
+MmGetSegmentRmap to find the rmap belonging to the segment itself, and uses
+the result as a pointer to a 256-entry page table structure.  The rmap also
+includes 8 bits of offset information indication one of 256 page entries that
+the rmap corresponds to.  This information together gives us an exact offset
+into the file, as well as the MM_SECTION_SEGMENT pointer stored in the page
+table slice.
+
+NULL is returned is there is no segment rmap for the page.
+
+*/
+
 PMM_SECTION_SEGMENT
 NTAPI
 MmGetSectionAssociation(PFN_NUMBER Page,
Modified: trunk/reactos/ntoskrnl/cache/section/swapout.c
URL:
http://svn.reactos.org/svn/reactos/trunk/reactos/ntoskrnl/cache/section/swa…
==============================================================================
--- trunk/reactos/ntoskrnl/cache/section/swapout.c [iso-8859-1] (original)
+++ trunk/reactos/ntoskrnl/cache/section/swapout.c [iso-8859-1] Thu Mar 29 04:43:44 2012
@@ -43,6 +43,21 @@
  *                  Herve Poussineau
  */
+/*
+
+This file implements page out infrastructure for cache type sections.  This
+is implemented a little differently from the legacy mm because mapping in an
+address space and membership in a segment are considered separate.
+
+The general strategy here is to try to remove all mappings as gently as
+possible, then to remove the page entry from the section itself as a final
+step.  If at any time during the page out operation, the page is mapped in
+a new address space by a competing thread, the operation will abort before
+the segment page is finally removed, and the page will be naturally faulted
+back into any address spaces required in the normal way.
+
+*/
+
 /* INCLUDES *****************************************************************/
 #include <ntoskrnl.h>
@@ -58,6 +73,15 @@
 FAST_MUTEX MiGlobalPageOperation;
+/*
+
+MmWithdrawSectionPage removes a page entry from the section segment, replacing
+it with a wait entry.  The caller must replace the wait entry with a 0, when
+any required writing is done.  The wait entry must remain until the page is
+written to protect against cases where a fault brings a stale copy of the page
+back before writing is complete.
+
+*/
 PFN_NUMBER
 NTAPI
 MmWithdrawSectionPage(PMM_SECTION_SEGMENT Segment,
@@ -119,6 +143,22 @@
         return 0;
     }
 }
+
+/*
+
+This function determines whether the segment holds the very last reference to
+the page being considered and if so, writes it back or discards it as
+approriate.  One small niggle here is that we might be holding the last
+reference to the section segment associated with this page.  That happens
+when the segment is destroyed at the same time that an active swap operation
+is occurring, and all maps were already withdrawn.  In that case, it's our
+responsiblity for finalizing the segment.
+
+Note that in the current code, WriteZero is always TRUE because the section
+always backs a file.  In the ultimate form of this code, it also writes back
+pages without necessarily evicting them.  In reactos' trunk, this is vestigal.
+
+*/
 NTSTATUS
 NTAPI
@@ -219,6 +259,20 @@
     DPRINT("Status %x\n", Status);
     return Status;
 }
+
+/*
+
+The slightly misnamed MmPageOutCacheSection removes a page from an address
+space in the manner of fault handlers found in fault.c.  In the ultimate form
+of the code, this is one of the function pointers stored in a memory area
+to control how pages in that memory area are managed.
+
+Also misleading is the call to MmReleasePageMemoryConsumer, which releases
+the reference held by this address space only.  After all address spaces
+have had MmPageOutCacheSection succeed on them for the indicated page,
+then paging out of a cache page can continue.
+
+*/
 NTSTATUS
 NTAPI
@@ -257,12 +311,32 @@
     MmDeleteVirtualMapping(Process, Address, FALSE, Dirty, &OurPage);
     ASSERT(OurPage == Required->Page[0]);
+    /* Note: this releases the reference held by this address space only. */
     MmReleasePageMemoryConsumer(MC_CACHE, Required->Page[0]);
     MmUnlockSectionSegment(Segment);
     MiSetPageEvent(Process, Address);
     return STATUS_SUCCESS;
 }
+
+/*
+
+This function is called by rmap when spare pages are needed by the blancer.
+It attempts first to release the page from every address space in which it
+appears, and, after a final check that no competing thread has mapped the
+page again, uses MmFinalizeSectionPageOut to completely evict the page.  If
+that's successful, then a suitable non-page map will be left in the segment
+page table, otherwise, the original page is replaced in the section page
+map.  Failure may result from a variety of conditions, but always leaves
+the page mapped.
+
+This code is like the other fault handlers, in that MmPageOutCacheSection has
+the option of returning either STATUS_SUCCESS + 1 to wait for a wait entry
+to disppear or to use the blocking callout facility by returning
+STATUS_MORE_PROCESSING_REQUIRED and placing a pointer to a function from
+reqtools.c in the MM_REQUIRED_RESOURCES struct.
+
+*/
 NTSTATUS
 NTAPI
Modified: trunk/reactos/ntoskrnl/mm/rmap.c
URL:
http://svn.reactos.org/svn/reactos/trunk/reactos/ntoskrnl/mm/rmap.c?rev=562…
==============================================================================
--- trunk/reactos/ntoskrnl/mm/rmap.c [iso-8859-1] (original)
+++ trunk/reactos/ntoskrnl/mm/rmap.c [iso-8859-1] Thu Mar 29 04:43:44 2012
@@ -418,6 +418,20 @@
    KeBugCheck(MEMORY_MANAGEMENT);
 }
+/*
+
+Return the process pointer given when a previous call to MmInsertRmap was
+called with a process and address pointer that conform to the segment rmap
+schema.  In short, this requires the address part to be 0xffffff00 + n
+where n is between 0 and 255.  When such an rmap exists, it specifies a
+segment rmap in which the process part is a pointer to a slice of a section
+page table, and the low 8 bits of the address represent a page index in the
+page table slice.  Together, this information is used by
+MmGetSectionAssociation to determine which page entry points to this page in
+the segment page table.
+
+*/
+
 PVOID
 NTAPI
 MmGetSegmentRmap(PFN_NUMBER Page, PULONG RawOffset)
@@ -445,6 +459,12 @@
    return NULL;
 }
+/*
+
+Remove the section rmap associated with the indicated page, if it exists.
+
+*/
+
 VOID
 NTAPI
 MmDeleteSectionAssociation(PFN_NUMBER Page)