diff --git a/src/bin/cache_check.rs b/src/bin/cache_check.rs
new file mode 100644
index 0000000..4fed20c
--- /dev/null
+++ b/src/bin/cache_check.rs
@@ -0,0 +1,62 @@
+extern crate clap;
+extern crate thinp;
+
+use clap::{App, Arg};
+use std::path::Path;
+use thinp::cache::check::{check, CacheCheckOptions};
+
+//------------------------------------------
+
+fn main() {
+    let parser = App::new("cache_check")
+        .version(thinp::version::TOOLS_VERSION)
+        .arg(
+            Arg::with_name("INPUT")
+                .help("Specify the input device to check")
+                .required(true)
+                .index(1),
+        )
+        .arg(
+            Arg::with_name("SB_ONLY")
+                .help("Only check the superblock.")
+                .long("super-block-only")
+                .value_name("SB_ONLY"),
+        )
+        .arg(
+            Arg::with_name("SKIP_MAPPINGS")
+                .help("Don't check the mapping array")
+                .long("skip-mappings")
+                .value_name("SKIP_MAPPINGS"),
+        )
+        .arg(
+            Arg::with_name("SKIP_HINTS")
+                .help("Don't check the hint array")
+                .long("skip-hints")
+                .value_name("SKIP_HINTS"),
+        )
+        .arg(
+            Arg::with_name("SKIP_DISCARDS")
+                .help("Don't check the discard bitset")
+                .long("skip-discards")
+                .value_name("SKIP_DISCARDS"),
+        );
+
+    let matches = parser.get_matches();
+    let input_file = Path::new(matches.value_of("INPUT").unwrap());
+
+    let opts = CacheCheckOptions {
+        dev: &input_file,
+        async_io: false,
+        sb_only: matches.is_present("SB_ONLY"),
+        skip_mappings: matches.is_present("SKIP_MAPPINGS"),
+        skip_hints: matches.is_present("SKIP_HINTS"),
+        skip_discards: matches.is_present("SKIP_DISCARDS"),
+    };
+
+    if let Err(reason) = check(opts) {
+        eprintln!("{}", reason);
+        std::process::exit(1);
+    }
+}
+
+//------------------------------------------
diff --git a/src/cache/check.rs b/src/cache/check.rs
new file mode 100644
index 0000000..92895e1
--- /dev/null
+++ b/src/cache/check.rs
@@ -0,0 +1,166 @@
+use anyhow::anyhow;
+use std::marker::PhantomData;
+use std::path::Path;
+use std::sync::{Arc, Mutex};
+use std::collections::*;
+
+use crate::io_engine::{AsyncIoEngine, IoEngine, SyncIoEngine};
+use crate::cache::hint::*;
+use crate::cache::mapping::*;
+use crate::cache::superblock::*;
+use crate::pdata::array_walker::*;
+
+//------------------------------------------
+
+const MAX_CONCURRENT_IO: u32 = 1024;
+
+//------------------------------------------
+
+struct CheckMappingVisitor {
+    allowed_flags: u32,
+    seen_oblocks: Arc<Mutex<BTreeSet<u64>>>,
+}
+
+impl CheckMappingVisitor {
+    fn new(metadata_version: u32) -> CheckMappingVisitor {
+        let mut flags: u32 = MappingFlags::Valid as u32;
+        if metadata_version == 1 {
+            flags |= MappingFlags::Dirty as u32;
+        }
+        CheckMappingVisitor {
+            allowed_flags: flags,
+            seen_oblocks: Arc::new(Mutex::new(BTreeSet::new())),
+        }
+    }
+
+    fn seen_oblock(&self, b: u64) -> bool {
+        let seen_oblocks = self.seen_oblocks.lock().unwrap();
+        return seen_oblocks.contains(&b);
+    }
+
+    fn record_oblock(&self, b: u64) {
+        let mut seen_oblocks = self.seen_oblocks.lock().unwrap();
+        seen_oblocks.insert(b);
+    }
+
+    // FIXME: is it possible to validate flags at an early phase?
+    // e.g., move to ctor of Mapping?
+    fn has_unknown_flags(&self, m: &Mapping) -> bool {
+        return (m.flags & self.allowed_flags) != 0;
+    }
+}
+
+impl ArrayBlockVisitor<Mapping> for CheckMappingVisitor {
+    fn visit(&self, _index: u64, m: Mapping) -> anyhow::Result<()> {
+        if !m.is_valid() {
+            return Ok(());
+        }
+
+        if self.seen_oblock(m.oblock) {
+            return Err(anyhow!("origin block already mapped"));
+        }
+
+        self.record_oblock(m.oblock);
+
+        if !self.has_unknown_flags(&m) {
+            return Err(anyhow!("unknown flags in mapping"));
+        }
+
+        Ok(())
+    }
+}
+
+//------------------------------------------
+
+struct CheckHintVisitor<Width> {
+    _not_used: PhantomData<Width>,
+}
+
+impl<Width> CheckHintVisitor<Width> {
+    fn new() -> CheckHintVisitor<Width> {
+        CheckHintVisitor {
+            _not_used: PhantomData,
+        }
+    }
+}
+
+impl<Width: typenum::Unsigned> ArrayBlockVisitor<Hint<Width>> for CheckHintVisitor<Width> {
+    fn visit(&self, _index: u64, _hint: Hint<Width>) -> anyhow::Result<()> {
+        // TODO: check hints
+        Ok(())
+    }
+}
+
+//------------------------------------------
+
+// TODO: ignore_non_fatal, clear_needs_check, auto_repair
+pub struct CacheCheckOptions<'a> {
+    pub dev: &'a Path,
+    pub async_io: bool,
+    pub sb_only: bool,
+    pub skip_mappings: bool,
+    pub skip_hints: bool,
+    pub skip_discards: bool,
+}
+
+// TODO: thread pool, report
+struct Context {
+    engine: Arc<dyn IoEngine + Send + Sync>,
+}
+
+fn mk_context(opts: &CacheCheckOptions) -> anyhow::Result<Context> {
+    let engine: Arc<dyn IoEngine + Send + Sync>;
+
+    if opts.async_io {
+        engine = Arc::new(AsyncIoEngine::new(
+            opts.dev,
+            MAX_CONCURRENT_IO,
+            false,
+        )?);
+    } else {
+        let nr_threads = std::cmp::max(8, num_cpus::get() * 2);
+        engine = Arc::new(SyncIoEngine::new(opts.dev, nr_threads, false)?);
+    }
+
+    Ok(Context {
+        engine,
+    })
+}
+
+pub fn check(opts: CacheCheckOptions) -> anyhow::Result<()> {
+    let ctx = mk_context(&opts)?;
+
+    let engine = &ctx.engine;
+
+    let sb = read_superblock(engine.as_ref(), SUPERBLOCK_LOCATION)?;
+
+    if opts.sb_only {
+        return Ok(());
+    }
+
+    // TODO: factor out into check_mappings()
+    if !opts.skip_mappings {
+        let w = ArrayWalker::new(engine.clone(), false);
+        let c = Box::new(CheckMappingVisitor::new(sb.version));
+        w.walk(c, sb.mapping_root)?;
+
+        if sb.version >= 2 {
+           // TODO: check dirty bitset
+        }
+    }
+
+    if !opts.skip_hints && sb.hint_root != 0 {
+        let w = ArrayWalker::new(engine.clone(), false);
+        type Width = typenum::U4; // FIXME: check sb.policy_hint_size
+        let c = Box::new(CheckHintVisitor::<Width>::new());
+        w.walk(c, sb.hint_root)?;
+    }
+
+    if !opts.skip_discards {
+        // TODO: check discard bitset
+    }
+
+    Ok(())
+}
+
+//------------------------------------------
diff --git a/src/cache/mod.rs b/src/cache/mod.rs
index f1cf219..a120ce6 100644
--- a/src/cache/mod.rs
+++ b/src/cache/mod.rs
@@ -1,3 +1,4 @@
+pub mod check;
 pub mod hint;
 pub mod mapping;
 pub mod superblock;