Skip decommit for large pages and add fake large pages test mode

src/coreclr/gc/gc.cpp

@@ -2619,6 +2619,7 @@ size_t gc_heap::eph_gen_starts_size = 0;
 heap_segment* gc_heap::segment_standby_list;
 #endif //USE_REGIONS
 bool          gc_heap::use_large_pages_p = 0;
+bool          gc_heap::large_pages_fake_mode_p = 0;
 #ifdef HEAP_BALANCE_INSTRUMENTATION
 size_t        gc_heap::last_gc_end_time_us = 0;
 #endif //HEAP_BALANCE_INSTRUMENTATION

src/coreclr/gc/gcconfig.h

@@ -76,7 +76,7 @@ class GCConfigStringHolder
     BOOL_CONFIG  (ConfigLogEnabled,          "GCConfigLogEnabled",        NULL,                                false,              "Specifies the name of the GC config log file")                                           \
     BOOL_CONFIG  (GCNumaAware,               "GCNumaAware",               NULL,                                true,               "Enables numa allocations in the GC")                                                     \
     BOOL_CONFIG  (GCCpuGroup,                "GCCpuGroup",                "System.GC.CpuGroup",                false,              "Enables CPU groups in the GC")                                                            \
-    BOOL_CONFIG  (GCLargePages,              "GCLargePages",              "System.GC.LargePages",              false,              "Enables using Large Pages in the GC")                                                     \
+    INT_CONFIG   (GCLargePages,              "GCLargePages",              "System.GC.LargePages",              0,                  "Enables Large Pages in the GC (1=real large pages, 2=fake mode for testing)")             \
     INT_CONFIG   (HeapVerifyLevel,           "HeapVerify",                NULL,                                HEAPVERIFY_NONE,    "When set verifies the integrity of the managed heap on entry and exit of each GC")       \
     INT_CONFIG   (LOHCompactionMode,         "GCLOHCompact",              NULL,                                0,                  "Specifies the LOH compaction mode")                                                      \
     INT_CONFIG   (LOHThreshold,              "GCLOHThreshold",            "System.GC.LOHThreshold",            LARGE_OBJECT_SIZE,  "Specifies the size that will make objects go on LOH")                                    \

src/coreclr/gc/gcpriv.h

@@ -2485,7 +2485,7 @@ class gc_heap
     PER_HEAP_METHOD void decommit_heap_segment (heap_segment* seg);
     PER_HEAP_ISOLATED_METHOD bool virtual_alloc_commit_for_heap (void* addr, size_t size, int h_number);
     PER_HEAP_ISOLATED_METHOD bool virtual_commit (void* address, size_t size, int bucket, int h_number=-1, bool* hard_limit_exceeded_p=NULL);
-    PER_HEAP_ISOLATED_METHOD bool virtual_decommit (void* address, size_t size, int bucket, int h_number=-1, void* end_of_data=nullptr);
+    PER_HEAP_ISOLATED_METHOD bool virtual_decommit (void* address, size_t size, int bucket, int h_number=-1);
     PER_HEAP_ISOLATED_METHOD void reduce_committed_bytes (void* address, size_t size, int bucket, int h_number, bool decommit_succeeded_p);
     friend void destroy_card_table (uint32_t*);
     PER_HEAP_ISOLATED_METHOD void destroy_card_table_helper (uint32_t* c_table);
@@ -5369,7 +5369,9 @@ class gc_heap
 #endif
 
     // Indicate to use large pages. This only works if hardlimit is also enabled.
+    // GCLargePages=1 uses real OS large pages, GCLargePages=2 fakes it for testing.
     PER_HEAP_ISOLATED_FIELD_INIT_ONLY bool use_large_pages_p;
+    PER_HEAP_ISOLATED_FIELD_INIT_ONLY bool large_pages_fake_mode_p;
 
 #ifdef MULTIPLE_HEAPS
     // Init-ed in gc_heap::initialize_gc

src/coreclr/gc/init.cpp

@@ -882,13 +882,23 @@ HRESULT gc_heap::initialize_gc (size_t soh_segment_size,
         // Right now all the non mark array portions are commmitted since I'm calling make_card_table
         // on the whole range. This can be committed as needed.
         size_t reserve_size = regions_range;
-        uint8_t* reserve_range = (uint8_t*)virtual_alloc (reserve_size, use_large_pages_p);
+        // In fake large pages mode, use normal reserve (not real large pages) then
+        // commit all upfront to simulate the "always committed" property.
+        bool use_real_large_pages = use_large_pages_p && !large_pages_fake_mode_p;
+        uint8_t* reserve_range = (uint8_t*)virtual_alloc (reserve_size, use_real_large_pages);
         if (!reserve_range)
         {
             log_init_error_to_host ("Reserving %zd bytes (%zd GiB) for the regions range failed, do you have a virtual memory limit set on this process?",
                 reserve_size, gib (reserve_size));
             return E_OUTOFMEMORY;
         }
+        if (large_pages_fake_mode_p)
+        {
+            if (!GCToOSInterface::VirtualCommit (reserve_range, reserve_size))
+            {
+                return E_OUTOFMEMORY;
+            }
+        }
 
         if (!global_region_allocator.init (reserve_range, (reserve_range + reserve_size),
                                            ((size_t)1 << min_segment_size_shr),
@@ -909,8 +919,9 @@ HRESULT gc_heap::initialize_gc (size_t soh_segment_size,
                            heap_hard_limit_oh[soh] &&
                            (GCConfig::GetGCHeapHardLimitPOH() == 0) &&
                            (GCConfig::GetGCHeapHardLimitPOHPercent() == 0);
+    bool use_real_large_pages = use_large_pages_p && !large_pages_fake_mode_p;
     if (!reserve_initial_memory (soh_segment_size, loh_segment_size, poh_segment_size, number_of_heaps,
-                                 use_large_pages_p, separated_poh_p, heap_no_to_numa_node))
+                                 use_real_large_pages, separated_poh_p, heap_no_to_numa_node))
         return E_OUTOFMEMORY;
     if (use_large_pages_p)
     {
@@ -1279,7 +1290,9 @@ bool gc_heap::compute_hard_limit()
     heap_hard_limit_oh[poh] = (size_t)GCConfig::GetGCHeapHardLimitPOH();
 
 #ifdef HOST_64BIT
-    use_large_pages_p = GCConfig::GetGCLargePages();
+    int64_t large_pages_config = GCConfig::GetGCLargePages();
+    use_large_pages_p = (large_pages_config != 0);
+    large_pages_fake_mode_p = (large_pages_config == 2);
 #endif //HOST_64BIT
 
     if (heap_hard_limit_oh[soh] || heap_hard_limit_oh[loh] || heap_hard_limit_oh[poh])

src/coreclr/gc/interface.cpp

@@ -275,7 +275,7 @@ HRESULT GCHeap::Initialize()
     {
         return CLR_E_GC_LARGE_PAGE_MISSING_HARD_LIMIT;
     }
-    GCConfig::SetGCLargePages(gc_heap::use_large_pages_p);
+    GCConfig::SetGCLargePages(gc_heap::use_large_pages_p ? (gc_heap::large_pages_fake_mode_p ? 2 : 1) : 0);
 
 #ifdef USE_REGIONS
     gc_heap::regions_range = (size_t)GCConfig::GetGCRegionRange();

src/coreclr/gc/memory.cpp

@@ -161,7 +161,7 @@ void gc_heap::reduce_committed_bytes (void* address, size_t size, int bucket, in
     }
 }
 
-bool gc_heap::virtual_decommit (void* address, size_t size, int bucket, int h_number, void* end_of_data)
+bool gc_heap::virtual_decommit (void* address, size_t size, int bucket, int h_number)
 {
     /**
      * Here are all possible cases for the decommits:
@@ -171,15 +171,12 @@ bool gc_heap::virtual_decommit (void* address, size_t size, int bucket, int h_nu
      * Case 3: This is for free - the bucket will be recorded_committed_free_bucket, and the h_number will be -1
      */
 
-    bool decommit_succeeded_p = ((bucket != recorded_committed_bookkeeping_bucket) && use_large_pages_p) ? true : GCToOSInterface::VirtualDecommit (address, size);
+    // With large pages, VirtualDecommit on heap memory is a no-op. All such callers
+    // should either skip the decommit or handle stale data themselves (decommit_region
+    // does the latter by calling reduce_committed_bytes directly and clearing memory).
+    assert (!use_large_pages_p || bucket == recorded_committed_bookkeeping_bucket);
 
-    // Large pages: the decommit above is a no-op so memory retains stale data.
-    // Clear up to end_of_data if the caller provided it so that the heap never
-    // observes leftover object references after the region is reused.
-    if (use_large_pages_p && (end_of_data != nullptr) && (end_of_data > address))
-    {
-        memclr ((uint8_t*)address, (uint8_t*)end_of_data - (uint8_t*)address);
-    }
+    bool decommit_succeeded_p = GCToOSInterface::VirtualDecommit (address, size);
 
     reduce_committed_bytes (address, size, bucket, h_number, decommit_succeeded_p);
 
@@ -347,7 +344,18 @@ size_t gc_heap::decommit_region (heap_segment* region, int bucket, int h_number)
     uint8_t* page_start = align_lower_page (get_region_start (region));
     uint8_t* decommit_end = heap_segment_committed (region);
     size_t decommit_size = decommit_end - page_start;
-    bool decommit_succeeded_p = virtual_decommit (page_start, decommit_size, bucket, h_number);
+    bool decommit_succeeded_p;
+    if (use_large_pages_p)
+    {
+        // VirtualDecommit is a no-op for large pages so skip it and update
+        // committed bookkeeping directly. Memory clearing is handled below.
+        decommit_succeeded_p = true;
+        reduce_committed_bytes (page_start, decommit_size, bucket, h_number, true);
+    }
+    else
+    {
+        decommit_succeeded_p = virtual_decommit (page_start, decommit_size, bucket, h_number);
+    }
     bool require_clearing_memory_p = !decommit_succeeded_p || use_large_pages_p;
     dprintf (REGIONS_LOG, ("decommitted region %p(%p-%p) (%zu bytes) - success: %d",
         region,

src/coreclr/gc/regions_segments.cpp

@@ -1522,6 +1522,12 @@ size_t gc_heap::decommit_heap_segment_pages_worker (heap_segment* seg,
 //decommit all pages except one or 2
 void gc_heap::decommit_heap_segment (heap_segment* seg)
 {
+    // For large pages, VirtualDecommit is a no-op so skip the decommit entirely
+    // to avoid lowering committed/used bookkeeping while memory retains stale data.
+    if (use_large_pages_p)
+    {
+        return;
+    }
 #ifdef USE_REGIONS
     if (!dt_high_memory_load_p())
     {
@@ -1814,32 +1820,38 @@ void gc_heap::distribute_free_regions()
         while (decommit_step(DECOMMIT_TIME_STEP_MILLISECONDS))
         {
         }
-#ifdef MULTIPLE_HEAPS
-        for (int i = 0; i < n_heaps; i++)
+        // For large pages, VirtualDecommit on in-use regions is a no-op so the
+        // memory is never actually returned to the OS. Skip the tail decommit
+        // entirely to avoid misleading bookkeeping and unnecessary memclr overhead.
+        if (!use_large_pages_p)
         {
-            gc_heap* hp = g_heaps[i];
-            int hn = i;
+#ifdef MULTIPLE_HEAPS
+            for (int i = 0; i < n_heaps; i++)
+            {
+                gc_heap* hp = g_heaps[i];
+                int hn = i;
 #else //MULTIPLE_HEAPS
-        {
-            gc_heap* hp = pGenGCHeap;
-            int hn  = 0;
-#endif //MULTIPLE_HEAPS
-            for (int i = 0; i < total_generation_count; i++)
             {
-                generation* generation = hp->generation_of (i);
-                heap_segment* region = heap_segment_rw (generation_start_segment (generation));
-                while (region != nullptr)
+                gc_heap* hp = pGenGCHeap;
+                int hn  = 0;
+#endif //MULTIPLE_HEAPS
+                for (int i = 0; i < total_generation_count; i++)
                 {
-                    uint8_t* aligned_allocated = align_on_page (heap_segment_allocated (region));
-                    size_t end_space = heap_segment_committed (region) - aligned_allocated;
-                    if (end_space > 0)
+                    generation* generation = hp->generation_of (i);
+                    heap_segment* region = heap_segment_rw (generation_start_segment (generation));
+                    while (region != nullptr)
                     {
-                        virtual_decommit (aligned_allocated, end_space, gen_to_oh (i), hn, heap_segment_used (region));
-                        heap_segment_committed (region) = aligned_allocated;
-                        heap_segment_used (region) = min (heap_segment_used (region), heap_segment_committed (region));
-                        assert (heap_segment_committed (region) > heap_segment_mem (region));
+                        uint8_t* aligned_allocated = align_on_page (heap_segment_allocated (region));
+                        size_t end_space = heap_segment_committed (region) - aligned_allocated;
+                        if (end_space > 0)
+                        {
+                            virtual_decommit (aligned_allocated, end_space, gen_to_oh (i), hn);
+                            heap_segment_committed (region) = aligned_allocated;
+                            heap_segment_used (region) = min (heap_segment_used (region), heap_segment_committed (region));
+                            assert (heap_segment_committed (region) > heap_segment_mem (region));
+                        }
+                        region = heap_segment_next_rw (region);
                     }
-                    region = heap_segment_next_rw (region);
                 }
             }
         }

src/tests/GC/API/GC/Collect_Aggressive_LargePages.cs

@@ -0,0 +1,96 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+using System;
+using System.Collections.Concurrent;
+using System.Runtime.CompilerServices;
+using System.Threading;
+using Xunit;
+
+// Regression test for https://github.com/dotnet/runtime/issues/126903
+// Verifies that aggressive GC does not corrupt the heap under (fake) large pages.
+// The fake large pages mode (DOTNET_GCLargePagesFakeMode=1) exercises the same
+// GC code paths as real large pages without requiring OS-level large page setup.
+public class AggressiveCollectLargePages
+{
+    const int DurationMs = 3000;
+    const int WriterCount = 4;
+
+    [Fact]
+    public static int TestEntryPoint()
+    {
+        var dict = new ConcurrentDictionary<int, byte[]>();
+        var cts = new CancellationTokenSource(DurationMs);
+        var token = cts.Token;
+        int errors = 0;
+
+        Thread[] writers = new Thread[WriterCount];
+        for (int t = 0; t < WriterCount; t++)
+        {
+            int tid = t;
+            writers[t] = new Thread(() =>
+            {
+                try
+                {
+                    int i = tid * 1_000_000;
+                    while (!token.IsCancellationRequested)
+                    {
+                        dict[i] = new byte[100];
+                        i++;
+                        if ((i % 1000) == 0)
+                        {
+                            dict.Clear();
+                        }
+                    }
+                }
+                catch (Exception ex)
+                {
+                    Console.WriteLine($"Writer {tid} caught: {ex.GetType().Name}: {ex.Message}");
+                    Interlocked.Increment(ref errors);
+                }
+            });
+            writers[t].IsBackground = true;
+            writers[t].Start();
+        }
+
+        Thread gcThread = new Thread(() =>
+        {
+            while (!token.IsCancellationRequested)
+            {
+                CreateGarbage();
+                GC.Collect(2, GCCollectionMode.Aggressive, blocking: true, compacting: true);
+                Thread.Sleep(50);
+            }
+        });
+        gcThread.IsBackground = true;
+        gcThread.Start();
+
+        gcThread.Join();
+        for (int t = 0; t < WriterCount; t++)
+        {
+            writers[t].Join();
+        }
+
+        if (errors > 0)
+        {
+            Console.WriteLine($"FAIL: {errors} writer(s) hit exceptions (heap corruption).");
+            return 101;
+        }
+
+        Console.WriteLine("PASS: No heap corruption detected.");
+        return 100;
+    }
+
+    [MethodImpl(MethodImplOptions.NoInlining)]
+    static void CreateGarbage()
+    {
+        byte[][] small = new byte[500][];
+        for (int i = 0; i < small.Length; i++)
+        {
+            small[i] = new byte[4000];
+        }
+        byte[] large = new byte[8 * 1024 * 1024];
+        GC.KeepAlive(small);
+        GC.KeepAlive(large);
+    }
+}

src/tests/GC/API/GC/Collect_Aggressive_LargePages.csproj

@@ -0,0 +1,18 @@
+<Project Sdk="Microsoft.NET.Sdk">
+  <PropertyGroup>
+    <!-- Needed for CLRTestEnvironmentVariable -->
+    <RequiresProcessIsolation>true</RequiresProcessIsolation>
+    <CLRTestTargetUnsupported Condition="'$(RuntimeFlavor)' != 'coreclr'">true</CLRTestTargetUnsupported>
+    <CLRTestPriority>0</CLRTestPriority>
+  </PropertyGroup>
+  <PropertyGroup>
+    <DebugType>PdbOnly</DebugType>
+  </PropertyGroup>
+  <ItemGroup>
+    <Compile Include="Collect_Aggressive_LargePages.cs" />
+  </ItemGroup>
+  <ItemGroup>
+    <CLRTestEnvironmentVariable Include="DOTNET_GCLargePages" Value="2" />
+    <CLRTestEnvironmentVariable Include="DOTNET_GCHeapHardLimit" Value="0xC0000000" />
+  </ItemGroup>
+</Project>