cvs commit: src/sys/amd64/amd64 pmap.c src/sys/i386/i386 pmap.c

Alan Cox alc at
Sun Sep 12 13:20:41 PDT 2004

alc         2004-09-12 20:20:40 UTC

  FreeBSD src repository

  Modified files:
    sys/amd64/amd64      pmap.c 
    sys/i386/i386        pmap.c 
  Use an atomic op to update the pte in pmap_protect().  This is to prevent
  the loss of a page modified (PG_M) bit in a race between processors.
  Quoting Tor:
          One scenario where the old code could cause a lost PG_M bit is a
          multithreaded linux program (or FreeBSD program using the
          linuxthreads port) where one thread was starting a subprocess.
          The thread doing fork() would call vmspace_fork(), which would then
          call vm_map_copy_entry() which would call pmap_protect() on an area
          possibly accessed by other threads.
  Additionally, make the clearing of PG_M by pmap_protect() unconditional if
  write permission is removed.  Previously, PG_M could persist on a read-only
  unmanaged page.  That seems inconsistent and confusing.
  In collaboration with: tegge@
  MT5 candidate
  PR: 61852
  Revision  Changes    Path
  1.498     +7 -6      src/sys/amd64/amd64/pmap.c
  1.504     +13 -7     src/sys/i386/i386/pmap.c

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