commit f2b97c01875913c7fe47f6247d666d97a4c5952e
author Chia-hung Duan <chiahungduan@google.com> Wed Mar 08 23:08:50 2023 +0000
committer Copybara-Service <copybara-worker@google.com> Thu Mar 09 08:21:16 2023 -0800
tree 3dea425ccb835c06bf24a74af342b1ce22028deb
parent 47ca44164b55a93bd866a3bdcf2312ebc625efce

[scudo] Slightly improve the handling of last block in a region

Instead of going through all those trailing blocks, just count the
number and increase the counter at once.

Reviewed By: cferris

Differential Revision: https://reviews.llvm.org/D145419

GitOrigin-RevId: e4efa885387e2df3a5e9abb3d64f232d2b4e9025
Change-Id: I4ac5eb1992b3db54c715a2f04d099a5fb14a3572

standalone/primary32.h

diff --git a/standalone/primary32.h b/standalone/primary32.h
index 1f06214..5d157f0 100644
--- a/standalone/primary32.h
+++ b/standalone/primary32.h
@@ -788,8 +788,12 @@
         continue;
 
       const uptr GroupBase = decompactGroupBase(BG.CompactPtrGroupBase);
-      uptr AllocatedGroupSize =
-          GroupBase == CurGroupBase ? Sci->CurrentRegionAllocated : GroupSize;
+      // The `GroupSize` may not be divided by `BlockSize`, which means there is
+      // an unused space at the end of Region. Exclude that space to avoid
+      // unused page map entry.
+      uptr AllocatedGroupSize = GroupBase == CurGroupBase
+                                    ? Sci->CurrentRegionAllocated
+                                    : roundDownSlow(GroupSize, BlockSize);
       if (AllocatedGroupSize == 0)
         continue;
 

standalone/release.h

diff --git a/standalone/release.h b/standalone/release.h
index c3b0a77..bb66a5d2 100644
--- a/standalone/release.h
+++ b/standalone/release.h
@@ -498,12 +498,28 @@
           ((RegionSize / BlockSize) - 1U) * BlockSize;
       // The last block in a region may not use the entire page, we mark the
       // following "pretend" memory block(s) as free in advance.
+      //
+      //     Region Boundary
+      //         v
+      //  -----+-----------------------+
+      //       |      Last Page        | <- Rounded Region Boundary
+      //  -----+-----------------------+
+      //   |-----||- trailing blocks  -|
+      //      ^
+      //   last block
       const uptr RoundedRegionSize = roundUp(RegionSize, PageSize);
-      uptr PInRegion = LastBlockInRegion + BlockSize;
-      while (PInRegion < RoundedRegionSize) {
-        PageMap.incRange(RegionIndex, getPageIndex(PInRegion),
-                         getPageIndex(PInRegion + BlockSize - 1));
-        PInRegion += BlockSize;
+      const uptr TrailingBlockBase = LastBlockInRegion + BlockSize;
+      // Only the last page touched by the last block needs to mark the trailing
+      // blocks. If the difference between `RoundedRegionSize` and
+      // `TrailingBlockBase` is larger than a page, that implies the reported
+      // `RegionSize` may not be accurate.
+      DCHECK_LT(RoundedRegionSize - TrailingBlockBase, PageSize);
+      uptr NumTrailingBlocks =
+          roundUpSlow(RoundedRegionSize - TrailingBlockBase, BlockSize) /
+          BlockSize;
+      if (NumTrailingBlocks > 0) {
+        PageMap.incN(RegionIndex, getPageIndex(TrailingBlockBase),
+                     NumTrailingBlocks);
       }
     }