[PATCH] Adding limitation while checking for cardTable

[Tim Bell]

Forwarded message

Please update your address books.  Stop using hotspot-gc-dev at openjdk.dev.java.net
                                                                      ^^^^
and use hotspot-gc-dev at openjdk.java.net instead (note the missing .dev).



-------- Original Message --------
Hi Neo --

Thanks for the heads-up. This seems worth fixing, at least
for hygiene, How about the following instead, where we avoid the
addr_for() translation until we are sure it will be safe,
provided that the mr argument to the method is backed by
card table entries. This avoids the need for the
extra compare-and-branch inside the loop in your patch.

   void do_MemRegion(MemRegion mr) {
     HeapWord* end_of_non_clean = mr.end();
     HeapWord* start_of_non_clean = end_of_non_clean;
     jbyte*       entry = _ct->byte_for(mr.last());
     jbyte* first_entry = _ct->byte_for(mr.start());
     while (entry >= first_entry) {
       jbyte entry_val = *entry;
       HeapWord* cur = _ct->addr_for(entry);
       if (!clear_card(entry)) {
         if (start_of_non_clean < end_of_non_clean) {
           MemRegion mr2(start_of_non_clean, end_of_non_clean);
           _dirty_card_closure->do_MemRegion(mr2);
         }
         end_of_non_clean = cur;
         start_of_non_clean = end_of_non_clean;
       }
       start_of_non_clean = cur;
       entry--;
     }
     if (start_of_non_clean < end_of_non_clean) {
       MemRegion mr2(start_of_non_clean, end_of_non_clean);
       _dirty_card_closure->do_MemRegion(mr2);
     }
   }

-- Ramki Ramakrishna

Neo Jia wrote On 05/09/07 03:31 PM,:

 > hi,
 >
 > Acturally, the existing code works well although it misses the
 > limitation checking, because the current heap layout will make sure
 > the memory region checking failed before it goes beyond the limitation
 > of byte map.
 >
 > The reason is that the nursery space is on the lower address, which
 > will make its committed card table also on the lower address of inside
 > the byte maps. While walking through the card table of the mature
 > space and keeping reduce the entry, we will finally hit the boundary
 > of its byte map before doing the check of memory region, which will
 > fire the assert of the addr_for, if it is on the lower address.
 >
 > So, just adding more safety for this function.
 >
 > Thanks,
 > Neo
 > Index: memory/cardTableRS.cpp
 > ===================================================================
 > --- memory/cardTableRS.cpp  (revision 86)
 > +++ memory/cardTableRS.cpp  (working copy)
 > @@ -163,6 +163,9 @@
 >     start_of_non_clean = end_of_non_clean;
 >        }
 >        entry--;
 > +      // Adding a limit checking for safety.
 > +      if (entry < _ct->base_map())
 > +        break;
 >        start_of_non_clean = cur;
 >        cur = _ct->addr_for(entry);
 >      }
 > Index: memory/cardTableRS.hpp
 > ===================================================================
 > --- memory/cardTableRS.hpp  (revision 86)
 > +++ memory/cardTableRS.hpp  (working copy)
 > @@ -101,6 +101,8 @@
 >
 >    CardTableModRefBS* ct_bs() { return &_ct_bs; }
 >
 > +  jbyte * base_map() { return &(_ct_bs._byte_map[0]); }
 > +
 >    // Override.
 >    void prepare_for_younger_refs_iterate(bool parallel);
 >

-- 

----------------------------------------------------------------------------
Y. Srinivas Ramakrishna               HotSpot JVM
Sun Microsystems, Inc., USCA 14-102   JWS / Software Group
4140 Network Circle                   408 276 7250 (x17250)
Santa Clara, CA  95054, U.S.A.        Y dot S dot Ramakrishna at Sun dot COM
----------------------------------------------------------------------------

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

NOTICE: This email message is for the sole use of the intended recipient(s)
and may contain confidential and privileged information. Any unauthorized
review, use, disclosure or distribution is prohibited. If you are not the
intended recipient, please contact the sender by reply email and destroy
all copies of the original message.

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

-------------- next part --------------
An embedded message was scrubbed...
From: Y Srinivas Ramakrishna <Y.S.Ramakrishna at Sun.COM>
Subject: Re: [PATCH] Adding limitation while checking for cardTable
Date: Thu, 10 May 2007 13:21:02 -0700
Size: 5964
URL: <https://mail.openjdk.org/pipermail/hotspot-gc-dev/attachments/20070510/eb785d37/null.eml>