VM66301: CP SAVE AREA MISUSE

Obtain the fix for this APAR.

APAR status

• Closed as program error.

Error description

• z/VM code for managing guest pages on PAGE space uses a save
  area without proper serialization.
  
  The z/VM control program (CP) storage management
  code for managing guest pages on PAGE space uses a
  save area without proper serialization.
  With certain timing conditions, it could be referencing
  a save area that has already been
  released and/or re-used for another purpose.  This can lead
  to a variety of possible symptoms, such as save area queue
  mismanagement, modification of another task's save area,
  incorrect logic decisions by the paging code, or host page
  faults.
  
  Other Keywords: SAVBK SVGBK CPEBK PPIBK
  

Local fix

• N/A
  

Problem summary

• ****************************************************************
  * USERS AFFECTED: All users of z/VM                            *
  ****************************************************************
  * PROBLEM DESCRIPTION:                                         *
  ****************************************************************
  * RECOMMENDATION: APPLY PTF                                    *
  ****************************************************************
  z/VM Control Program (CP) storage management code for managing
  guest pages on PAGE space (module HCPPAG ASSEMBLE) improperly
  accesses its save area (SAVBK/SVGBK/CPEBK/PPIBK).  It does so
  after turning control of its save area over to I/O code that
  performs the read/write of the guest page.  When the I/O
  completes the interrupt handler will stack the return to
  HCPPAG's caller.
  
  However, in very rare timing windows, it's possible for the
  I/O to complete and return to the caller before HCPPAG
  completes its processing and exits to the dispatcher.  HCPPAG
  continues to use SVGWRK fields in this save area, which might
  have since been re-used.  A further condition required to
  expose this problem is that it must be a system task (SYSTEMMP)
  that invokes the HCPPAG interface to read/write a guest page.
  
  Possible symptoms of this are save area queue mismanagement,
  modification of another task's save area, incorrect logic
  decisions by the paging code, or host page faults.
  

Problem conclusion

• A new PPIBK field (the PPIBK is a remapping/re-labeling of SVGBK
  and SVGWRK fields), is used by HCPPAG to indicate its continued
  use of the save area.  HCPPAG uses TS prior to invoking paging
  I/O routines, and does not release/clear the field until it
  has completed its use of the save area.
  
  The paging I/O interrupt handler (HCPPAH) uses this field to
  detect when HCPPAG is still actively using the save area and
  delays stacking the save area to return to HCPPAG's caller until
  HCPPAG has released/cleared the field.
  
  This ensures that the save area will not be released and re-used
  until after HCPPAG completes its use of it.
  
  Since this only happens in such small/rare timing windows, most
  of the time HCPPAH will be able to stack the save area
  immediately, with no delay, as HCPPAG will already be done with
  it by the time the I/O completes.
  

Temporary fix

• *********
  * HIPER *
  *********
  FOR RELEASE VM/ESACP/ESAR710 :
  PREREQ: VM66263
  CO-REQ: NONE
  IF-REQ: NONE
  

Comments

• ×**** PE19/08/29 FIX IN ERROR. SEE APAR VM66319  FOR DESCRIPTION
  

APAR Information

• APAR is sysrouted FROM one or more of the following:

• APAR is sysrouted TO one or more of the following:

  UM35496

Modules/Macros

• HCPPAG   HCPPAH   HCPPPIBK
  

Fix information

• Fixed component name

  VM CP

• Fixed component ID

  568411202

Applicable component levels

• R710 PSY UM35496

     UP19/07/31 P 2001 ¢

Fix is available

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