ext2: Use memory offset instead of block id for mmaping info

ext2's mmaping information is used to perform mmap() in crosvm's main
process. Previously, the struct has a block id field so we needed to
calculate the offset by multiplying block size.
Instead, the struct now contains memory offset for convenience.

BUG=b:329359333
TEST=run VM on workstation with pmem-ext2

Change-Id: I845fc021c715a4b490e80142823855832fe3bc41
Reviewed-on: https://chromium-review.googlesource.com/c/crosvm/crosvm/+/5771636
Commit-Queue: Keiichi Watanabe <keiichiw@chromium.org>
Reviewed-by: Takaya Saeki <takayas@chromium.org>

ext2/src/arena.rs

@@ -168,8 +168,8 @@ impl BlockId {
 
 /// Information on how to mmap a host file to ext2 blocks.
 pub struct FileMappingInfo {
-    /// The ext2 disk block id that the memory region maps to.
-    pub start_block: BlockId,
+    /// Offset in the memory that a file is mapped to.
+    pub mem_offset: usize,
     /// The file to be mmap'd.
     pub file: File,
     /// The length of the mapping.
@@ -205,18 +205,17 @@ impl<'a> Arena<'a> {
     }
 
     /// A helper function to mark a region as reserved.
-    fn reserve(&self, block: BlockId, block_offset: usize, len: usize) -> Result<()> {
-        let offset = u32::from(block) as usize * self.block_size + block_offset;
-        let mem_end = offset.checked_add(len).context("mem_end overflow")?;
+    fn reserve(&self, mem_offset: usize, len: usize) -> Result<()> {
+        let mem_end = mem_offset.checked_add(len).context("mem_end overflow")?;
 
         if mem_end > self.mem.size() {
             bail!(
-                "out of memory region: {offset} + {len} > {}",
+                "out of memory region: {mem_offset} + {len} > {}",
                 self.mem.size()
             );
         }
 
-        self.regions.borrow_mut().allocate(offset, len)?;
+        self.regions.borrow_mut().allocate(mem_offset, len)?;
 
         Ok(())
     }
@@ -226,14 +225,14 @@ impl<'a> Arena<'a> {
     /// `into_mapping_info()` to retrieve the mapping information and call mmap later instead.
     pub fn reserve_for_mmap(
         &self,
-        start_block: BlockId,
+        mem_offset: usize,
         length: usize,
         file: File,
         file_offset: usize,
     ) -> Result<()> {
-        self.reserve(start_block, 0, length)?;
+        self.reserve(mem_offset, length)?;
         self.mappings.borrow_mut().push(FileMappingInfo {
-            start_block,
+            mem_offset,
             length,
             file: file.try_clone()?,
             file_offset,
@@ -250,9 +249,9 @@ impl<'a> Arena<'a> {
         block_offset: usize,
         len: usize,
     ) -> Result<&'a mut [u8]> {
-        self.reserve(block, block_offset, len)?;
-
         let offset = u32::from(block) as usize * self.block_size + block_offset;
+        self.reserve(offset, len)?;
+
         let new_addr = (self.mem.as_ptr() as usize)
             .checked_add(offset)
             .context("address overflow")?;

ext2/src/fs.rs

@@ -482,10 +482,11 @@ impl<'a> Ext2<'a> {
             }
             let length = std::cmp::min(remaining, BLOCK_SIZE * blocks.len());
             let start_block = blocks[0];
+            let mem_offset = u32::from(start_block) as usize * BLOCK_SIZE;
             // Reserve the region in arena to prevent from overwriting metadata.
             arena
                 .reserve_for_mmap(
-                    start_block,
+                    mem_offset,
                     length,
                     file.try_clone().context("failed to clone file")?,
                     file_offset,
@@ -814,14 +815,14 @@ pub fn create_ext2_region(cfg: &Config, src_dir: Option<&Path>) -> Result<Memory
     mem.msync()?;
     let mut mmap_arena = MemoryMappingArena::from(mem);
     for FileMappingInfo {
-        start_block,
+        mem_offset,
         file,
         length,
         file_offset,
     } in file_mappings
     {
         mmap_arena.add_fd_mapping(
-            u32::from(start_block) as usize * BLOCK_SIZE,
+            mem_offset,
             length,
             &file,
             file_offset as u64, /* fd_offset */