Re: [PATCH 2/2] selftests/cgroup: test_zswap: fix implicit unsigned promotion bug in test_no_kmem_bypass

Michal Koutný <[email protected]> Thu, 6 Aug 2026 10:32:49 +0200

Newsgroups	org.kernel.vger.linux-kselftest,org.kernel.vger.cgroups,org.kvack.linux-mm
Message-ID	<[email protected]>

On Tue, Aug 04, 2026 at 04:20:45AM +0000, Wilson Felipe Pereira <[email protected]> wrote:
> In test_no_kmem_bypass(), delta (stored_pages * page_size - zswapped) is
> checked against stored_pages * page_size / 4 to verify that the pages
> pushed to zswap belong to the test memory cgroup.
> 
> Due to slight stat update timing differences, delta can evaluate to a small
> negative number (e.g. -5MB out of 1GB). Because delta is declared as a
> signed int and stored_pages is an unsigned size_t, C's usual arithmetic
> conversions implicitly promote a negative delta to a large unsigned 64-bit
> integer, causing `delta < stored_pages * page_size / 4` to falsely evaluate
> to 0 and fail the test.

Thanks for the breakdown.

> Fix this by checking abs(delta), ensuring the test correctly compares the
> absolute difference between system zswap and cgroup zswapped bytes.

I still think the delta quantity has some meaning here and taking the
abs() changes the semantics of the checked inequality.

What about making both zswapped and delta `long`s? (Under similar
reasoning, there's already a possible loss after squashing
cg_read_key_long() result into the `int`.)

In any way, also
Fixes: a549f9f31561a ("selftests: cgroup: add test_zswap with no kmem bypass test")

Thanks,
Michal

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