feat: enable THP for guest memory

This commit adds THP for the guest memory, with a new value for the
huge_pages option.

Signed-off-by: Marco Marangoni <mamarang@amazon.com>

Author: marco-marangoni

CHANGELOG.md

@@ -10,6 +10,12 @@ and this project adheres to
 
 ### Added
 
+- [#6003](https://github.com/firecracker-microvm/firecracker/pull/6003): Added a
+  new option `Transparent` for the `huge_pages` setting. If set, Firecracker
+  will use transparent huge pages for the guest memory via
+  `madvise(MADV_HUGEPAGE)`. Guest memory must be a multiple of 2MB when using
+  this option.
+
 ### Changed
 
 ### Deprecated

docs/hugepages.md

@@ -1,14 +1,37 @@
 # Backing Guest Memory by Huge Pages
 
-Firecracker supports backing the guest memory of a VM by 2MB hugetlbfs pages.
-This can be enabled by setting the `huge_pages` field of `PUT` or `PATCH`
-requests to the `/machine-config` endpoint to `2M`.
-
-Backing guest memory by huge pages can bring performance improvements for
-specific workloads, due to less TLB contention and less overhead during
-virtual->physical address resolution. It can also help reduce the number of
-KVM_EXITS required to rebuild extended page tables post snapshot restore, as
-well as improve boot times (by up to 50% as measured by Firecracker's
+Firecracker supports three modes for the `huge_pages` field of `PUT` or `PATCH`
+requests to the `/machine-config` endpoint:
+
+- `None` (default): Uses regular 4K pages with no huge page behavior.
+- `Transparent`: Uses `madvise(MADV_HUGEPAGE)` to request transparent huge pages
+  for guest memory. Guest memory size must be a multiple of 2MB.
+- `2M`: Backs guest memory by 2MB hugetlbfs pages.
+
+## Transparent Huge Pages (THP)
+
+Setting `huge_pages` to `Transparent` enables transparent huge pages for guest
+memory via `madvise(MADV_HUGEPAGE)`. This allows the kernel to opportunistically
+back guest memory with 2MB pages without requiring a pre-allocated hugetlbfs
+pool.
+
+Limitations:
+
+- THP is only effective for anonymous memory (non-memfd). When vhost-user-blk
+  devices are in use, guest memory is memfd-backed and THP will not be applied.
+- THP does not integrate with UFFD; no transparent huge pages will be allocated
+  during userfault-handling while resuming from a snapshot.
+
+Please refer to the [Linux Documentation][thp_docs] for more information.
+
+## Hugetlbfs (2M)
+
+Setting `huge_pages` to `2M` backs guest memory by 2MB hugetlbfs pages. This can
+bring performance improvements for specific workloads, due to less TLB
+contention and less overhead during virtual->physical address resolution. It can
+also help reduce the number of KVM_EXITS required to rebuild extended page
+tables post snapshot restore, as well as improve boot times (by up to 50% as
+measured by Firecracker's
 [boot time performance tests](../tests/integration_tests/performance/test_boottime.py))
 
 Using hugetlbfs requires the host running Firecracker to have a pre-allocated
@@ -43,15 +66,5 @@ the device is unable to reclaim the hugepage backing of the guest and drop RSS.
 However, the balloon can still be inflated and used to restrict memory usage in
 the guest.
 
-## FAQ
-
-### Why does Firecracker not offer a transparent huge pages (THP) setting?
-
-Firecracker's guest memory can be memfd based. Linux (as of 6.1) does not offer
-a way to dynamically enable THP for such memory regions. Additionally, UFFD does
-not integrate with THP (no transparent huge pages will be allocated during
-userfaulting). Please refer to the [Linux Documentation][thp_docs] for more
-information.
-
 [hugetlbfs_docs]: https://docs.kernel.org/admin-guide/mm/hugetlbpage.html
 [thp_docs]: https://www.kernel.org/doc/html/next/admin-guide/mm/transhuge.html#hugepages-in-tmpfs-shmem

src/firecracker/src/api_server/request/machine_configuration.rs

@@ -104,6 +104,7 @@ mod tests {
 
         let huge_pages_cases = [
             ("None", HugePageConfig::None),
+            ("Transparent", HugePageConfig::Transparent),
             ("2M", HugePageConfig::Hugetlbfs2M),
         ];
 

src/firecracker/swagger/firecracker.yaml

@@ -1442,8 +1442,13 @@ definitions:
         type: string
         enum:
           - None
+          - Transparent
           - 2M
-        description: Which huge pages configuration (if any) should be used to back guest memory.
+        default: None
+        description: >-
+          Which huge pages configuration should be used to back guest memory.
+          "None" uses regular 4K pages. "Transparent" enables THP via
+          madvise(MADV_HUGEPAGE). "2M" uses explicit hugetlbfs 2MB pages.
 
   MemoryBackend:
     type: object

src/vmm/src/devices/virtio/vhost_user.rs

@@ -487,6 +487,7 @@ pub(crate) mod tests {
                 libc::MAP_PRIVATE,
                 Some(file),
                 false,
+                libc::MADV_HUGEPAGE,
             )
             .unwrap()
             .into_iter()

src/vmm/src/persist.rs

@@ -449,8 +449,13 @@ pub fn restore_from_snapshot(
                 .into());
             }
             (
-                guest_memory_from_file(mem_backend_path, mem_state, track_dirty_pages)
-                    .map_err(RestoreFromSnapshotGuestMemoryError::File)?,
+                guest_memory_from_file(
+                    mem_backend_path,
+                    mem_state,
+                    track_dirty_pages,
+                    vm_resources.machine_config.huge_pages,
+                )
+                .map_err(RestoreFromSnapshotGuestMemoryError::File)?,
                 None,
             )
         }
@@ -512,9 +517,11 @@ fn guest_memory_from_file(
     mem_file_path: &Path,
     mem_state: &GuestMemoryState,
     track_dirty_pages: bool,
+    huge_pages: HugePageConfig,
 ) -> Result<Vec<GuestRegionMmap>, GuestMemoryFromFileError> {
     let mem_file = File::open(mem_file_path)?;
-    let guest_mem = memory::snapshot_file(mem_file, mem_state.regions(), track_dirty_pages)?;
+    let guest_mem =
+        memory::snapshot_file(mem_file, mem_state.regions(), track_dirty_pages, huge_pages)?;
     Ok(guest_mem)
 }
 

src/vmm/src/resources.rs

@@ -580,6 +580,7 @@ mod tests {
     use crate::vmm_config::RateLimiterConfig;
     use crate::vmm_config::boot_source::{BootConfig, BootSource, BootSourceConfig};
     use crate::vmm_config::drive::{BlockBuilder, BlockDeviceConfig};
+    use crate::vmm_config::machine_config::HugePageConfig::{Hugetlbfs2M, Transparent};
     use crate::vmm_config::machine_config::{HugePageConfig, MachineConfig, MachineConfigError};
     use crate::vmm_config::net::{NetBuilder, NetworkInterfaceConfig};
     use crate::vmm_config::vsock::tests::default_config;
@@ -1476,6 +1477,26 @@ mod tests {
             Err(MachineConfigError::InvalidMemorySize)
         );
 
+        // Odd memory size - not supported by THP/Hugetlbfs
+        aux_vm_config.mem_size_mib = Some(1025);
+        aux_vm_config.huge_pages = Some(Transparent);
+        assert_eq!(
+            vm_resources.update_machine_config(&aux_vm_config),
+            Err(MachineConfigError::InvalidMemorySize)
+        );
+        aux_vm_config.huge_pages = Some(Hugetlbfs2M);
+        assert_eq!(
+            vm_resources.update_machine_config(&aux_vm_config),
+            Err(MachineConfigError::InvalidMemorySize)
+        );
+        // Odd size supported by HugePageConfig::None
+        aux_vm_config.huge_pages = Some(HugePageConfig::None);
+        vm_resources.update_machine_config(&aux_vm_config).unwrap();
+        assert_eq!(
+            MachineConfigUpdate::from(vm_resources.machine_config.clone()),
+            aux_vm_config
+        );
+
         // Incompatible mem_size_mib with balloon size.
         vm_resources.machine_config.mem_size_mib = 128;
         vm_resources

src/vmm/src/vmm_config/machine_config.rs

@@ -34,9 +34,11 @@ pub enum MachineConfigError {
 /// Describes the possible (huge)page configurations for a microVM's memory.
 #[derive(Clone, Copy, Debug, Default, PartialEq, Eq, Serialize, Deserialize)]
 pub enum HugePageConfig {
-    /// Do not use hugepages, e.g. back guest memory by 4K
+    /// Back guest memory by 4K pages, no hugepage behavior
     #[default]
     None,
+    /// Use madvise(MADV_HUGEPAGE) for transparent huge pages
+    Transparent,
     /// Back guest memory by 2MB hugetlbfs pages
     #[serde(rename = "2M")]
     Hugetlbfs2M,
@@ -49,6 +51,10 @@ impl HugePageConfig {
         let divisor = match self {
             // Any integer memory size expressed in MiB will be a multiple of 4096KiB.
             HugePageConfig::None => 1,
+            // Note: THP technically supports memory not 2MB aligned, however that would mean
+            // some pages at the tail would be forced to be 4k size. To avoid performance/fragmentation surprises,
+            // having a memory multiple of 2MB is wiser.
+            HugePageConfig::Transparent => 2,
             HugePageConfig::Hugetlbfs2M => 2,
         };
 
@@ -59,11 +65,20 @@ impl HugePageConfig {
     /// create a mapping backed by huge pages as described by this [`HugePageConfig`].
     pub fn mmap_flags(&self) -> libc::c_int {
         match self {
-            HugePageConfig::None => 0,
+            HugePageConfig::None | HugePageConfig::Transparent => 0,
             HugePageConfig::Hugetlbfs2M => libc::MAP_HUGETLB | libc::MAP_HUGE_2MB,
         }
     }
 
+    /// Returns the flags required to pass to [libc::madvise], after allocating anonymous guest memory.
+    /// Note: returning [libc::MADV_NORMAL] might skip the call to `madvise` entirely.
+    pub fn madvise_flags(&self) -> libc::c_int {
+        match self {
+            HugePageConfig::Transparent => libc::MADV_HUGEPAGE,
+            HugePageConfig::None | HugePageConfig::Hugetlbfs2M => libc::MADV_NORMAL,
+        }
+    }
+
     /// Returns `true` iff this [`HugePageConfig`] describes a hugetlbfs-based configuration.
     pub fn is_hugetlbfs(&self) -> bool {
         matches!(self, HugePageConfig::Hugetlbfs2M)
@@ -72,7 +87,7 @@ impl HugePageConfig {
     /// Gets the page size in bytes of this [`HugePageConfig`].
     pub fn page_size(&self) -> usize {
         match self {
-            HugePageConfig::None => 4096,
+            HugePageConfig::None | HugePageConfig::Transparent => 4096,
             HugePageConfig::Hugetlbfs2M => 2 * 1024 * 1024,
         }
     }
@@ -81,7 +96,7 @@ impl HugePageConfig {
 impl From<HugePageConfig> for Option<memfd::HugetlbSize> {
     fn from(value: HugePageConfig) -> Self {
         match value {
-            HugePageConfig::None => None,
+            HugePageConfig::None | HugePageConfig::Transparent => None,
             HugePageConfig::Hugetlbfs2M => Some(memfd::HugetlbSize::Huge2MB),
         }
     }

src/vmm/src/vstate/memory.rs

@@ -6,6 +6,7 @@
 // found in the THIRD-PARTY file.
 
 use std::fs::File;
+use std::io;
 use std::io::SeekFrom;
 use std::ops::Deref;
 use std::sync::{Arc, Mutex};
@@ -22,12 +23,12 @@ pub use vm_memory::{
 };
 use vm_memory::{GuestMemoryError, GuestMemoryRegionBytes, VolatileSlice, WriteVolatile};
 
-use crate::DirtyBitmap;
 use crate::arch::host_page_size;
 use crate::logger::error;
 use crate::utils::u64_to_usize;
 use crate::vmm_config::machine_config::HugePageConfig;
 use crate::vstate::vm::{KvmVm, VmError};
+use crate::{DirtyBitmap, warn_unrestricted};
 
 /// Type of GuestRegionMmap.
 pub type GuestRegionMmap = vm_memory::GuestRegionMmap<Option<AtomicBitmap>>;
@@ -528,6 +529,7 @@ pub fn create(
     mmap_flags: libc::c_int,
     file: Option<File>,
     track_dirty_pages: bool,
+    madvise_flags: libc::c_int,
 ) -> Result<Vec<GuestRegionMmap>, MemoryError> {
     let mut offset = 0;
     let file = file.map(Arc::new);
@@ -559,6 +561,18 @@ pub fn create(
                 start,
             )
             .ok_or(MemoryError::VmMemoryError)
+            .inspect(|region| {
+                if madvise_flags != libc::MADV_NORMAL {
+                    // SAFETY: The referenced memory was just mapped.
+                    let ret = unsafe {
+                        libc::madvise(region.as_ptr().cast(), region.size(), madvise_flags)
+                    };
+                    if ret != 0 {
+                        let e = io::Error::last_os_error();
+                        warn_unrestricted!("Madvise call failed for guest memory: {e}");
+                    }
+                }
+            })
         })
         .collect::<Result<Vec<_>, _>>()
 }
@@ -577,6 +591,7 @@ pub fn memfd_backed(
         libc::MAP_SHARED | huge_pages.mmap_flags(),
         Some(memfd_file),
         track_dirty_pages,
+        huge_pages.madvise_flags(),
     )
 }
 
@@ -591,6 +606,7 @@ pub fn anonymous(
         libc::MAP_PRIVATE | libc::MAP_ANONYMOUS | huge_pages.mmap_flags(),
         None,
         track_dirty_pages,
+        huge_pages.madvise_flags(),
     )
 }
 
@@ -600,6 +616,7 @@ pub fn snapshot_file(
     file: File,
     regions: impl Iterator<Item = (GuestAddress, usize)>,
     track_dirty_pages: bool,
+    huge_pages: HugePageConfig,
 ) -> Result<Vec<GuestRegionMmap>, MemoryError> {
     let regions: Vec<_> = regions.collect();
     let memory_size = regions
@@ -619,6 +636,7 @@ pub fn snapshot_file(
         libc::MAP_PRIVATE,
         Some(file),
         track_dirty_pages,
+        huge_pages.madvise_flags(),
     )
 }
 
@@ -951,8 +969,13 @@ mod tests {
             file.write_all(&vec![0x42u8; page_size]).unwrap();
 
             let regions = vec![(GuestAddress(0), page_size)];
-            let guest_regions =
-                snapshot_file(file, regions.into_iter(), dirty_page_tracking).unwrap();
+            let guest_regions = snapshot_file(
+                file,
+                regions.into_iter(),
+                dirty_page_tracking,
+                HugePageConfig::None,
+            )
+            .unwrap();
             assert_eq!(guest_regions.len(), 1);
             guest_regions.iter().for_each(|region| {
                 assert_eq!(region.bitmap().is_some(), dirty_page_tracking);
@@ -973,7 +996,8 @@ mod tests {
             (GuestAddress(0x10000), page_size),
             (GuestAddress(0x20000), page_size),
         ];
-        let guest_regions = snapshot_file(file, regions.into_iter(), false).unwrap();
+        let guest_regions =
+            snapshot_file(file, regions.into_iter(), false, HugePageConfig::None).unwrap();
         assert_eq!(guest_regions.len(), 3);
     }
 
@@ -985,7 +1009,7 @@ mod tests {
         file.write_all(&vec![0x42u8; page_size]).unwrap();
 
         let regions = vec![(GuestAddress(0), 2 * page_size)];
-        let result = snapshot_file(file, regions.into_iter(), false);
+        let result = snapshot_file(file, regions.into_iter(), false, HugePageConfig::None);
         assert!(matches!(result.unwrap_err(), MemoryError::OffsetTooLarge));
     }
 
@@ -1175,8 +1199,15 @@ mod tests {
         let mut memory_file = TempFile::new().unwrap().into_file();
         guest_memory.dump(&mut memory_file).unwrap();
 
-        let restored_guest_memory =
-            into_region_ext(snapshot_file(memory_file, memory_state.regions(), false).unwrap());
+        let restored_guest_memory = into_region_ext(
+            snapshot_file(
+                memory_file,
+                memory_state.regions(),
+                false,
+                HugePageConfig::None,
+            )
+            .unwrap(),
+        );
 
         // Check that the region contents are the same.
         let mut restored_region = vec![0u8; page_size * 2];
@@ -1240,8 +1271,9 @@ mod tests {
             .unwrap();
 
         // We can restore from this because this is the first dirty dump.
-        let restored_guest_memory =
-            into_region_ext(snapshot_file(file, memory_state.regions(), false).unwrap());
+        let restored_guest_memory = into_region_ext(
+            snapshot_file(file, memory_state.regions(), false, HugePageConfig::None).unwrap(),
+        );
 
         // Check that the region contents are the same.
         let mut restored_region = vec![0u8; region_size];
@@ -1465,6 +1497,7 @@ mod tests {
                 memory_file,
                 std::iter::once((GuestAddress(0), 2 * page_size)),
                 false,
+                HugePageConfig::None,
             )
             .unwrap(),
         );