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Start with a debug kernel that has CONFIG_KCSAN=y. Add CONFIG_KCSAN_STRICT=y for the strictest Linux-Kernel-Memory-Model (LKMM) checking and CONFIG_KCSAN_WEAK_MEMORY=y when you suspect missing barriers or release/acquire ordering. Tune kcsan.skip_watch, kcsan.udelay_task, and, for interrupt code, kcsan.udelay_interrupt to make races more observable. Then combine KCSAN with LKMM litmus tests, lockdep/RCU validation, fault injection, and memory-lifetime detectors such as CONFIG_DEBUG_PAGEALLOC and KASAN.
The settings to enable first
Page-cache bugs can belong to several different classes: two CPUs accessing ordinary data without a valid synchronization rule, a missing memory barrier, an invalid lock or RCU contract, an allocation or I/O failure path that was never exercised, or memory corruption that only looks like a race. No single kernel option detects all of them. The most effective setup uses one instrumented kernel for dynamic testing and separate, proof-oriented tests for ordering assumptions.
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| Setting or tool | Best at finding | Important qualification |
|---|---|---|
CONFIG_KCSAN=y |
Runtime plain-data races through compiler instrumentation and sampled watchpoints | Sampling means a clean run does not prove that no race exists |
CONFIG_KCSAN_STRICT=y |
Stricter checking against the Linux kernel memory-consistency model | Use the target tree’s Kconfig help; dependencies and defaults vary by release |
CONFIG_KCSAN_WEAK_MEMORY=y |
Some missing ordering involving load/store buffering and barriers | Its model covers only a subset of possible ordering failures |
kcsan.skip_watch |
Sampling frequency | Lower values install watchpoints more often but increase overhead |
kcsan.udelay_task |
Races between task contexts | Larger delays widen the race window and can make the system much slower |
kcsan.udelay_interrupt |
Races involving interrupt context | Keep this delay conservative when interrupt latency matters |
Lockdep and CONFIG_PROVE_RCU |
Lock dependency, lock ordering, and RCU dereference-contract violations | They do not replace KCSAN for unsynchronized plain accesses |
| Fault-injection controls | Allocation, slab, copy, and block-I/O error paths | Failures must be driven while a workload is exercising the relevant filesystem path |
CONFIG_DEBUG_PAGEALLOC, KASAN, UBSAN |
Use-after-free, out-of-bounds, freed-page, and selected undefined-behavior defects | These diagnose memory corruption or lifetime errors rather than proving a race |
Build a KCSAN kernel for page-cache testing
Enable the baseline detector
Set CONFIG_KCSAN=y in the configuration for the kernel tree you are testing, then rebuild and boot that kernel. KCSAN is described by the Linux Kernel documentation as a dynamic race detector: compile-time instrumentation marks accesses and a watchpoint-based sampler looks for conflicting operations at runtime.
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CONFIG_KCSAN=y
CONFIG_KCSAN_STRICT=y
CONFIG_KCSAN_WEAK_MEMORY=yDo not assume that this fragment is valid without checking the source tree’s Kconfig help. Option dependencies, architecture support, and defaults can change between kernel releases. Keep the exact commit, configuration, compiler, filesystem, and workload with every report so another developer can reproduce the observation.
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Choose strict and weak-memory modes for the question you are asking
CONFIG_KCSAN_STRICT=y applies the strictest KCSAN rules and aims to follow LKMM as closely as possible. It is a good default for a dedicated race-hunting build when additional runtime cost is acceptable.
CONFIG_KCSAN_WEAK_MEMORY=y is specifically useful when the suspected defect is not merely two unsynchronized accesses, but an ordering mistake: for example, a producer publishes page-cache state before the data is visible, or a consumer observes a flag without the acquire operation required by the design. The documented model simulates load or store buffering and can expose some missing smp_mb(), smp_wmb(), smp_rmb(), smp_store_release(), and equivalent atomic ordering. It is not a complete model of every weak-memory failure, so a quiet run cannot establish that all barriers are correct.
Tune KCSAN so rare interleavings occur more often
Sampling with kcsan.skip_watch
KCSAN does not watch every access continuously. kcsan.skip_watch specifies how many per-CPU operations are skipped before another watchpoint is installed. Lowering it increases the chance that a short page-cache interleaving is observed, at the cost of more instrumentation overhead and slowdown.
Inspect the live value and, when supported by the build, change it through the module-parameter interface:
cat /sys/module/kcsan/parameters/skip_watch
echo 0 > /sys/module/kcsan/parameters/skip_watchUse a value appropriate for the experiment rather than leaving an aggressive setting enabled on a general-purpose machine. Record the value in test logs because changing it changes the probability and timing of observations.
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Widen task-context races with kcsan.udelay_task
After KCSAN places a task watchpoint, kcsan.udelay_task adds a delay. Increasing it gives another CPU or task more time to touch the same location, which is often valuable for page-cache invalidation, writeback, reclaim, and truncate races. The trade-off is substantial slowdown and a workload that may no longer resemble production timing.
cat /sys/module/kcsan/parameters/udelay_task
echo 100 > /sys/module/kcsan/parameters/udelay_taskThe accepted units and range are defined by the target kernel; verify them in that tree before choosing a number.
Cover interrupt paths with kcsan.udelay_interrupt
kcsan.udelay_interrupt provides the analogous delay for interrupt-context watchpoints. It can help when completion, invalidation, or block-layer callbacks race with page-cache state, but excessive delay can distort interrupt latency and trigger unrelated timeouts. Keep it smaller than task delay unless the test specifically targets an interrupt race.
KCSAN also exposes a debugfs interface, normally under /sys/kernel/debug/kcsan, for statistics, on/off control, and filtering. Mount debugfs before using it if it is not already mounted, and consult the files exported by the running kernel rather than assuming every release has identical names.
Exercise the real page-cache paths
Combine operations that share page state
A detector is useful only if the workload reaches the code where the bug lives. Run concurrent buffered reads and writes while also driving operations that invalidate, create, or migrate cache pages:
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- truncate and hole-punch operations racing with reads, writes, and memory mappings;
- mmap faults concurrent with writeback, reclaim, and page invalidation;
- direct transitions between dirty, writeback, and clean states under memory pressure;
- remount or filesystem lifecycle operations while file descriptors continue to issue I/O;
- filesystem error paths, including delayed writeback failures and failed page allocation.
Use multiple processes or threads pinned to different CPUs when possible, and repeat the same schedule often enough to distinguish a deterministic failure from a rare report. The Linux Kernel documentation does not prescribe one universal page-cache test; the workload has to match the filesystem implementation and kernel tree under investigation.
Make failures reproducible
Log the kernel commit, complete .config, boot parameters, filesystem type and mount options, CPU topology, memory pressure, KCSAN parameters, and the operation sequence. Preserve the KCSAN report and the smallest workload that still reproduces it. A report seen only with extreme delays may still be real, but it should be re-tested with less intrusive timing before being treated as a production-equivalent failure.
Use LKMM when the suspected bug is a missing barrier
KCSAN is a runtime sampler. If the central question is whether a particular release/acquire or barrier pattern permits an unwanted outcome, reduce the code to a minimal Linux Kernel Memory Model litmus test. The LKMM documentation describes herd7 as exhaustively exploring the state space of small tests and klitmus7 as a way to turn a test into a kernel module for execution.
- Identify the smallest shared variables and the publication, consumption, and invalidation events involved in the page-cache path.
- Express the competing CPU operations and the forbidden outcome in a litmus-test file.
- Run the model with the Linux configuration, for example
herd7 -conf linux-kernel.cfg test.litmus. - If the model permits the outcome, compare the result with the actual code and test a candidate barrier change.
- Use
klitmus7when an in-kernel execution is needed to check the behavior on real hardware.
A litmus test proves properties of the modeled operations, not of the entire filesystem. Keep the full KCSAN workload: it can reveal an unmodeled access, an incorrect annotation, or an interaction with locking and reference counts that the small test intentionally omitted.
Check lock and RCU contracts separately
Lockdep
Enable the lockdep-related options required by the target tree, including CONFIG_PROVE_LOCKING and CONFIG_DEBUG_LOCK_ALLOC where available. Lockdep checks dependency graphs and ordering rules, so it can catch an inversion or an acquisition sequence that page-cache code violates only under reclaim or writeback. It does not detect every plain data race: code can access the same field without a lock and still evade lockdep if no lock contract is declared.
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RCU validation
Enable CONFIG_PROVE_RCU to validate RCU dereference and read-side usage. This is valuable when page or address-space metadata is published and reclaimed through RCU. Treat an RCU warning as a contract violation to fix, not as a substitute for checking the memory ordering of the publication itself.
Force allocation, copy, and I/O failures
Rare error paths often contain the interleaving that normal I/O never reaches. Mount debugfs and use the documented fault-injection controls:
fail_page_allocto force page-allocation failures;failslabto exercise slab-allocation failures;fail_usercopyto force selected user-copy failures;fail_make_requestto inject block-I/O request failures.
Each control has parameters exposed by the running kernel. Inspect its debugfs directory and configure probability, interval, or other available limits for the experiment; do not assume a setting from one release has the same interface in another. Boot-time forms and the helper tools/testing/fault-injection/failcmd.sh are also documented in the kernel source.
Run the injector while buffered I/O, writeback, reclaim, truncate, and remapping are active. Verify that the filesystem recovers cleanly, that failed pages do not remain incorrectly marked dirty or uptodate, and that subsequent reads do not observe stale data. Keep failure rates bounded: an injector that fails every operation may test only a permanent-outage path, not the retry and cleanup races that occur with intermittent errors.
Separate races from memory-lifetime corruption
CONFIG_DEBUG_PAGEALLOC
Enable CONFIG_DEBUG_PAGEALLOC when a suspected page-cache race may actually be an access to a page after it was freed or returned to the allocator. The debug_pagealloc= boot parameter enables freed-page protection, while debug_guardpage_minorder= controls the order of guarded pages. These mechanisms are expensive and should be used on a dedicated debug boot.
The kernel command-line documentation warns that DMA or firmware writes which bypass the CPU’s MMU can evade this protection. A clean DEBUG_PAGEALLOC run therefore does not rule out corruption from a device or firmware path.
KASAN and UBSAN
KASAN targets out-of-bounds accesses and use-after-free bugs; UBSAN covers selected forms of undefined behavior. Enable them when a KCSAN report points at a location whose lifetime, bounds, or type assumptions are uncertain. KASAN may impose more overhead than a KCSAN-only build, so use a separate configuration if the combined instrumentation prevents the workload from reaching the failing path.
Choose the detector by bug class
| Suspected defect | Primary check | What to combine it with | Main blind spot |
|---|---|---|---|
| Unsynchronized ordinary access | KCSAN | Lockdep and a focused reproducer | Sampling can miss an execution |
| Missing barrier or release/acquire edge | KCSAN weak-memory mode and LKMM | herd7 model, then klitmus7 or a real workload |
Weak-memory modeling is partial; a small model omits surrounding code |
| Lock dependency or ordering error | Lockdep with CONFIG_PROVE_LOCKING |
KCSAN for accesses outside the declared lock contract | No warning when the race involves no recognized lock |
| RCU misuse | CONFIG_PROVE_RCU |
KCSAN and LKMM for publication ordering | Does not prove unrelated lifetime rules |
| Allocation, copy, or request failure cleanup | Fault-injection controls | KCSAN, lockdep, and filesystem-specific tests | Only the injected failure modes are exercised |
| Use-after-free or bounds corruption | KASAN and CONFIG_DEBUG_PAGEALLOC |
UBSAN and KCSAN if concurrency is also involved | High overhead; DMA and firmware writes may evade page protection |
A practical investigation sequence
- Build and boot a kernel with
CONFIG_KCSAN=y; add strict and weak-memory modes when the performance budget allows. - Capture a baseline workload using buffered reads, writes, mmap faults, truncate, hole punching, writeback, reclaim, and remount operations.
- Lower
kcsan.skip_watchand raise task delay for short task-context races. Add interrupt delay only for paths that actually run in interrupt context. - Run lockdep and RCU validation in the same or a companion debug build to identify contract violations early.
- When a report suggests ordering rather than a plain race, create the smallest LKMM test and evaluate it with
herd7. - Repeat the workload with controlled
fail_page_alloc,failslab,fail_usercopy, andfail_make_requestinjection. - If symptoms include corrupted contents, stale pointers, or crashes after reclamation, switch to a KASAN or DEBUG_PAGEALLOC build instead of treating every symptom as a race.
- Retest the fix with normal KCSAN sampling and without artificial delays, then on the real filesystem and hardware configuration that matters.
How to interpret a clean run
A clean KCSAN run means that no conflicting accesses were observed during that sampled execution. It does not demonstrate the absence of page-cache races, and it says even less about all possible weak-memory outcomes. Likewise, a litmus test that forbids an outcome says only that the modeled operations obey LKMM; it does not verify every caller, error path, or filesystem-specific invariant. Confidence comes from combining dynamic observation, exhaustive analysis of small ordering patterns, synchronization checks, injected failures, and memory-corruption detectors.
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Bottom line
For page-cache concurrency work, make KCSAN the first-line runtime detector, tune its sampling and delays deliberately, and enable weak-memory mode when barriers are in doubt. Use LKMM tools to analyze ordering claims, lockdep and RCU checks to validate synchronization contracts, fault injection to reach rare cleanup paths, and DEBUG_PAGEALLOC or KASAN to distinguish lifetime corruption from a true race. Verify every option against the kernel release you are testing and preserve the exact configuration and workload with each result.
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