diff --git a/native/core/src/execution/memory_pools/fair_pool.rs b/native/core/src/execution/memory_pools/fair_pool.rs
index 347c3d8ef69..993194ee303 100644
--- a/native/core/src/execution/memory_pools/fair_pool.rs
+++ b/native/core/src/execution/memory_pools/fair_pool.rs
@@ -16,6 +16,7 @@
 // under the License.
 
 use std::{
+    collections::HashMap,
     fmt::{Debug, Display, Formatter, Result as FmtResult},
     sync::Arc,
 };
@@ -27,7 +28,7 @@ use datafusion::common::resources_err;
 use datafusion::execution::memory_pool::MemoryConsumer;
 use datafusion::{
     common::DataFusionError,
-    execution::memory_pool::{MemoryPool, MemoryReservation},
+    execution::memory_pool::{MemoryLimit, MemoryPool, MemoryReservation},
 };
 use parking_lot::Mutex;
 
@@ -37,11 +38,16 @@ pub struct CometFairMemoryPool {
     task_memory_manager_handle: Arc<Global<JObject<'static>>>,
     pool_size: usize,
     state: Mutex<CometFairPoolState>,
+    #[cfg(test)]
+    test_memory_manager: Option<Arc<tests::TestMemoryManager>>,
 }
 
+#[derive(Default)]
 struct CometFairPoolState {
     used: usize,
-    num: usize,
+    // new_empty(), split(), and take() share a registered consumer but have independent sizes.
+    // All sibling reservations must count against the same fair share.
+    consumer_usage: HashMap<usize, usize>,
 }
 
 impl Debug for CometFairMemoryPool {
@@ -50,7 +56,7 @@ impl Debug for CometFairMemoryPool {
         f.debug_struct("CometFairMemoryPool")
             .field("pool_size", &self.pool_size)
             .field("used", &state.used)
-            .field("num", &state.num)
+            .field("num", &state.consumer_usage.len())
             .finish()
     }
 }
@@ -63,11 +69,17 @@ impl CometFairMemoryPool {
         Self {
             task_memory_manager_handle,
             pool_size,
-            state: Mutex::new(CometFairPoolState { used: 0, num: 0 }),
+            state: Mutex::new(CometFairPoolState::default()),
+            #[cfg(test)]
+            test_memory_manager: None,
         }
     }
 
     fn acquire(&self, additional: usize) -> CometResult<i64> {
+        #[cfg(test)]
+        if let Some(manager) = &self.test_memory_manager {
+            return manager.acquire(additional);
+        }
         let handle = self.task_memory_manager_handle.as_obj();
         JVMClasses::with_env(|env| unsafe {
             jni_call!(env,
@@ -76,6 +88,11 @@ impl CometFairMemoryPool {
     }
 
     fn release(&self, size: usize) -> CometResult<()> {
+        #[cfg(test)]
+        if let Some(manager) = &self.test_memory_manager {
+            manager.release(size);
+            return Ok(());
+        }
         let handle = self.task_memory_manager_handle.as_obj();
         JVMClasses::with_env(|env| unsafe {
             jni_call!(env, comet_task_memory_manager(handle).release_memory(size as i64) -> ())
@@ -89,7 +106,9 @@ impl Display for CometFairMemoryPool {
         write!(
             f,
             "CometFairMemoryPool(pool_size={}, used={}, num={})",
-            self.pool_size, state.used, state.num
+            self.pool_size,
+            state.used,
+            state.consumer_usage.len()
         )
     }
 }
@@ -102,63 +121,74 @@ impl MemoryPool for CometFairMemoryPool {
         "CometFairMemoryPool"
     }
 
-    fn register(&self, _: &MemoryConsumer) {
-        let mut state = self.state.lock();
-        state.num = state
-            .num
-            .checked_add(1)
-            .expect("unexpected amount of register happened");
+    fn register(&self, consumer: &MemoryConsumer) {
+        assert!(
+            self.state
+                .lock()
+                .consumer_usage
+                .insert(consumer.id(), 0)
+                .is_none(),
+            "memory consumer was registered more than once"
+        );
     }
 
-    fn unregister(&self, _: &MemoryConsumer) {
-        let mut state = self.state.lock();
-        state.num = state
-            .num
-            .checked_sub(1)
-            .expect("unexpected amount of unregister happened");
+    fn unregister(&self, consumer: &MemoryConsumer) {
+        let usage = self.state.lock().consumer_usage.remove(&consumer.id());
+        assert_eq!(
+            usage,
+            Some(0),
+            "consumer must release its reservations before unregistering"
+        );
     }
 
-    fn grow(&self, _reservation: &MemoryReservation, additional: usize) {
-        self.try_grow(_reservation, additional).unwrap();
+    fn grow(&self, reservation: &MemoryReservation, additional: usize) {
+        self.try_grow(reservation, additional).unwrap();
     }
 
-    fn shrink(&self, _reservation: &MemoryReservation, subtractive: usize) {
+    fn shrink(&self, reservation: &MemoryReservation, subtractive: usize) {
         if subtractive > 0 {
             let mut state = self.state.lock();
-            // We don't use reservation.size() here because DataFusion 53+ decrements
-            // the reservation's atomic size before calling pool.shrink(), so it would
-            // reflect the post-shrink value rather than the pre-shrink value.
-            if state.used < subtractive {
-                panic!(
-                    "Failed to release {subtractive} bytes where only {} bytes tracked by pool",
-                    state.used
-                )
-            }
+            let usage = state.consumer_usage[&reservation.consumer().id()];
+            assert!(
+                usage >= subtractive,
+                "consumer released more bytes than it reserved"
+            );
             self.release(subtractive)
                 .unwrap_or_else(|_| panic!("Failed to release {subtractive} bytes"));
-            state.used = state.used.checked_sub(subtractive).unwrap();
+            *state
+                .consumer_usage
+                .get_mut(&reservation.consumer().id())
+                .unwrap() -= subtractive;
+            state.used -= subtractive;
         }
     }
 
     fn try_grow(
         &self,
-        _reservation: &MemoryReservation,
+        reservation: &MemoryReservation,
         additional: usize,
     ) -> Result<(), DataFusionError> {
         if additional > 0 {
             let mut state = self.state.lock();
-            let num = state.num;
-            let limit = self
-                .pool_size
-                .checked_div(num)
-                .expect("overflow in checked_div");
-            // We use state.used instead of reservation.size() because DataFusion 53+
-            // calls pool.try_grow() before incrementing the reservation's atomic size,
-            // so reservation.size() would not include prior grows.
-            let used = state.used;
-            if limit < used + additional {
+            // Preserve the policy of sharing among all registered consumers. Spillability
+            // annotations and sharing only among spillable consumers are a separate change.
+            let num = state.consumer_usage.len();
+            let limit = self.pool_size / num;
+            let used = state.consumer_usage[&reservation.consumer().id()];
+            if used
+                .checked_add(additional)
+                .is_none_or(|requested| requested > limit)
+            {
                 return resources_err!(
-                    "Failed to acquire {additional} bytes where {used} bytes already reserved and the fair limit is {limit} bytes, {num} registered"
+                    "Failed to acquire {additional} bytes where {used} bytes already reserved by this consumer and the fair limit is {limit} bytes, {num} registered"
+                );
+            }
+            // Existing allocations may exceed their new fair share when another consumer
+            // registers. A per-consumer bound alone cannot enforce the configured pool size.
+            if additional > self.pool_size.saturating_sub(state.used) {
+                return resources_err!(
+                    "Failed to acquire {additional} bytes where {} bytes already reserved pool-wide and the pool limit is {} bytes",
+                    state.used, self.pool_size
                 );
             }
 
@@ -176,6 +206,10 @@ impl MemoryPool for CometFairMemoryPool {
                     state.used
                 );
             }
+            *state
+                .consumer_usage
+                .get_mut(&reservation.consumer().id())
+                .unwrap() += additional;
             state.used = state
                 .used
                 .checked_add(additional)
@@ -187,4 +221,187 @@ impl MemoryPool for CometFairMemoryPool {
     fn reserved(&self) -> usize {
         self.state.lock().used
     }
+
+    fn memory_limit(&self) -> MemoryLimit {
+        MemoryLimit::Finite(self.pool_size)
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use std::sync::atomic::{AtomicBool, AtomicUsize, Ordering::SeqCst};
+
+    // Replace only JNI grants/releases so the tests exercise actual MemoryPool admission and
+    // DataFusion reservation lifetimes without requiring a running Spark task.
+    #[derive(Default)]
+    pub(super) struct TestMemoryManager {
+        used: AtomicUsize,
+        acquires: AtomicUsize,
+        partial_grant: AtomicBool,
+        fail_acquire: AtomicBool,
+    }
+
+    impl TestMemoryManager {
+        pub(super) fn acquire(&self, requested: usize) -> CometResult<i64> {
+            self.acquires.fetch_add(1, SeqCst);
+            if self.fail_acquire.load(SeqCst) {
+                return Err(crate::errors::CometError::Internal(
+                    "test acquire failure".into(),
+                ));
+            }
+            let granted = if self.partial_grant.load(SeqCst) {
+                requested / 2
+            } else {
+                requested
+            };
+            self.used.fetch_add(granted, SeqCst);
+            Ok(granted as i64)
+        }
+
+        pub(super) fn release(&self, bytes: usize) {
+            self.used
+                .fetch_update(SeqCst, SeqCst, |used| used.checked_sub(bytes))
+                .unwrap();
+        }
+    }
+
+    fn pool(size: usize) -> (Arc<dyn MemoryPool>, Arc<TestMemoryManager>) {
+        let manager = Arc::new(TestMemoryManager::default());
+        let mut pool = CometFairMemoryPool::new(Arc::new(Global::null()), size);
+        pool.test_memory_manager = Some(Arc::clone(&manager));
+        (Arc::new(pool), manager)
+    }
+
+    #[test]
+    fn each_consumer_can_use_its_fair_share() {
+        let (pool, manager) = pool(32);
+        let other = MemoryConsumer::new("other").register(&pool);
+        let requesting = MemoryConsumer::new("requesting").register(&pool);
+        other.try_grow(10).unwrap();
+        requesting.try_grow(6).unwrap();
+        requesting.try_grow(10).unwrap();
+        assert!(requesting.try_grow(1).is_err());
+        other.try_grow(6).unwrap();
+        assert_eq!(pool.reserved(), 32);
+        assert_eq!(manager.used.load(SeqCst), 32);
+        assert!(matches!(pool.memory_limit(), MemoryLimit::Finite(32)));
+    }
+
+    #[test]
+    fn sibling_reservations_share_their_consumers_limit() {
+        let (pool, manager) = pool(100);
+        let parent = MemoryConsumer::new("same name")
+            .with_can_spill(true)
+            .register(&pool);
+        let first = parent.new_empty();
+        let second = parent.new_empty();
+        let other = MemoryConsumer::new("same name")
+            .with_can_spill(true)
+            .register(&pool);
+        first.try_grow(30).unwrap();
+        second.try_grow(20).unwrap();
+        assert!(parent.try_grow(1).is_err());
+        assert!(second.try_grow(1).is_err());
+        // Rejected growth never asks Spark for memory, even though the pool has free capacity.
+        assert_eq!(manager.acquires.load(SeqCst), 2);
+        other.try_grow(50).unwrap();
+        drop(parent);
+        drop(first);
+        second.try_grow(30).unwrap();
+        assert_eq!(pool.reserved(), 100);
+        drop(second);
+        drop(other);
+        assert_eq!(pool.reserved(), 0);
+        assert_eq!(manager.used.load(SeqCst), 0);
+    }
+
+    #[test]
+    fn split_and_take_do_not_create_another_allowance() {
+        let (pool, manager) = pool(100);
+        let mut parent = MemoryConsumer::new("consumer").register(&pool);
+        let other = MemoryConsumer::new("other").register(&pool);
+        parent.try_grow(50).unwrap();
+        let split = parent.split(20);
+        let taken = parent.take();
+        assert_eq!(parent.size(), 0);
+        assert_eq!(split.size(), 20);
+        assert_eq!(taken.size(), 30);
+        for reservation in [&parent, &split, &taken] {
+            assert!(reservation.try_grow(1).is_err());
+        }
+        assert_eq!(manager.acquires.load(SeqCst), 1);
+        split.shrink(10);
+        parent.try_grow(10).unwrap();
+        drop(parent);
+        drop(split);
+        taken.try_grow(20).unwrap();
+        other.try_grow(50).unwrap();
+        assert_eq!(pool.reserved(), 100);
+        drop(taken);
+        drop(other);
+        assert_eq!(pool.reserved(), 0);
+        assert_eq!(manager.used.load(SeqCst), 0);
+    }
+
+    #[test]
+    fn registration_after_allocation_cannot_exceed_pool_capacity() {
+        let (pool, manager) = pool(100);
+        let first = MemoryConsumer::new("first").register(&pool);
+        first.try_grow(100).unwrap();
+        let second = MemoryConsumer::new("second").register(&pool);
+        assert!(second.try_grow(1).is_err());
+        assert_eq!(manager.acquires.load(SeqCst), 1);
+        assert_eq!(pool.reserved(), 100);
+        first.shrink(50);
+        second.try_grow(50).unwrap();
+        assert_eq!(pool.reserved(), 100);
+    }
+
+    #[test]
+    fn mixed_consumers_keep_the_existing_sharing_policy() {
+        let (pool, _) = pool(100);
+        let fixed = MemoryConsumer::new("fixed").register(&pool);
+        let spilling = MemoryConsumer::new("spilling")
+            .with_can_spill(true)
+            .register(&pool);
+        assert!(spilling.try_grow(51).is_err());
+        spilling.try_grow(50).unwrap();
+        fixed.try_grow(50).unwrap();
+        assert_eq!(pool.reserved(), 100);
+        fixed.free();
+        assert!(spilling.try_grow(1).is_err());
+        drop(fixed);
+        spilling.try_grow(50).unwrap();
+        assert_eq!(pool.reserved(), 100);
+    }
+
+    #[test]
+    fn failed_acquisition_does_not_change_consumer_or_pool_usage() {
+        let (pool, manager) = pool(100);
+        let reservation = MemoryConsumer::new("consumer").register(&pool);
+        manager.partial_grant.store(true, SeqCst);
+        assert!(reservation.try_grow(100).is_err());
+        assert_eq!(pool.reserved(), 0);
+        assert_eq!(manager.used.load(SeqCst), 0);
+        manager.partial_grant.store(false, SeqCst);
+        manager.fail_acquire.store(true, SeqCst);
+        assert!(reservation.try_grow(100).is_err());
+        assert_eq!(pool.reserved(), 0);
+        manager.fail_acquire.store(false, SeqCst);
+        reservation.try_grow(100).unwrap();
+        reservation.free();
+        assert_eq!(pool.reserved(), 0);
+        assert_eq!(manager.used.load(SeqCst), 0);
+    }
+
+    #[test]
+    fn overflowing_request_is_rejected_before_acquisition() {
+        let (pool, manager) = pool(100);
+        let reservation = MemoryConsumer::new("consumer").register(&pool);
+        reservation.try_grow(1).unwrap();
+        assert!(reservation.try_grow(usize::MAX).is_err());
+        assert_eq!(pool.reserved(), 1);
+        assert_eq!(manager.acquires.load(SeqCst), 1);
+    }
 }