From 0e72d9b5ed7a0eee2ea5ab27083dd32dcefbf132 Mon Sep 17 00:00:00 2001
From: Wei Liu <liuwe@microsoft.com>
Date: Sat, 21 Dec 2024 00:54:15 +0000
Subject: [PATCH] block: increase the size of temporary vectors

The size was set to one because without VIRTIO_BLK_F_SEG_MAX, the guest
only used one data descriptor per request.

The value 32 is empirically derived from booting a guest. This value
eliminates all SmallVec allocations observable by DHAT.

Signed-off-by: Wei Liu <liuwe@microsoft.com>
---
 block/src/lib.rs | 18 +++++++++++-------
 1 file changed, 11 insertions(+), 7 deletions(-)

diff --git a/block/src/lib.rs b/block/src/lib.rs
index 8f70805cc..904c0a9cd 100644
--- a/block/src/lib.rs
+++ b/block/src/lib.rs
@@ -220,6 +220,8 @@ fn sector<B: Bitmap + 'static>(
     mem.read_obj(addr).map_err(Error::GuestMemory)
 }
 
+const DEFAULT_DESCRIPTOR_VEC_SIZE: usize = 32;
+
 #[derive(Debug)]
 pub struct AlignedOperation {
     origin_ptr: u64,
@@ -232,10 +234,10 @@ pub struct AlignedOperation {
 pub struct Request {
     pub request_type: RequestType,
     pub sector: u64,
-    pub data_descriptors: SmallVec<[(GuestAddress, u32); 1]>,
+    pub data_descriptors: SmallVec<[(GuestAddress, u32); DEFAULT_DESCRIPTOR_VEC_SIZE]>,
     pub status_addr: GuestAddress,
     pub writeback: bool,
-    pub aligned_operations: SmallVec<[AlignedOperation; 1]>,
+    pub aligned_operations: SmallVec<[AlignedOperation; DEFAULT_DESCRIPTOR_VEC_SIZE]>,
     pub start: Instant,
 }
 
@@ -263,10 +265,10 @@ impl Request {
         let mut req = Request {
             request_type: request_type(desc_chain.memory(), hdr_desc_addr)?,
             sector: sector(desc_chain.memory(), hdr_desc_addr)?,
-            data_descriptors: SmallVec::with_capacity(1),
+            data_descriptors: SmallVec::with_capacity(DEFAULT_DESCRIPTOR_VEC_SIZE),
             status_addr: GuestAddress(0),
             writeback: true,
-            aligned_operations: SmallVec::with_capacity(1),
+            aligned_operations: SmallVec::with_capacity(DEFAULT_DESCRIPTOR_VEC_SIZE),
             start: Instant::now(),
         };
 
@@ -398,7 +400,7 @@ impl Request {
         let request_type = self.request_type;
         let offset = (sector << SECTOR_SHIFT) as libc::off_t;
 
-        let mut iovecs: SmallVec<[libc::iovec; 1]> =
+        let mut iovecs: SmallVec<[libc::iovec; DEFAULT_DESCRIPTOR_VEC_SIZE]> =
             SmallVec::with_capacity(self.data_descriptors.len());
         for (data_addr, data_len) in &self.data_descriptors {
             if *data_len == 0 {
@@ -653,7 +655,8 @@ where
         completion_list: &mut VecDeque<(u64, i32)>,
     ) -> AsyncIoResult<()> {
         // Convert libc::iovec into IoSliceMut
-        let mut slices: SmallVec<[IoSliceMut; 1]> = SmallVec::with_capacity(iovecs.len());
+        let mut slices: SmallVec<[IoSliceMut; DEFAULT_DESCRIPTOR_VEC_SIZE]> =
+            SmallVec::with_capacity(iovecs.len());
         for iovec in iovecs.iter() {
             // SAFETY: on Linux IoSliceMut wraps around libc::iovec
             slices.push(IoSliceMut::new(unsafe {
@@ -690,7 +693,8 @@ where
         completion_list: &mut VecDeque<(u64, i32)>,
     ) -> AsyncIoResult<()> {
         // Convert libc::iovec into IoSlice
-        let mut slices: SmallVec<[IoSlice; 1]> = SmallVec::with_capacity(iovecs.len());
+        let mut slices: SmallVec<[IoSlice; DEFAULT_DESCRIPTOR_VEC_SIZE]> =
+            SmallVec::with_capacity(iovecs.len());
         for iovec in iovecs.iter() {
             // SAFETY: on Linux IoSlice wraps around libc::iovec
             slices.push(IoSlice::new(unsafe {