Re: [PATCH v5 2/2] selftests/cgroup: test_zswap: fix implicit unsigned promotion bug in test_no_kmem_bypass

[Michal Koutný]

On Tue, Sep 01, 2026 at 05:22:28AM +0000, Wilson Felipe Pereira <wfelipe@xxxxxxxxxx> 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.
> 
> Fix this by declaring zswapped and delta as signed long long and comparing
> against a signed threshold, ensuring negative deltas correctly evaluate
> to true.
> 
> Fixes: a549f9f31561a ("selftests: cgroup: add test_zswap with no kmem bypass test")
> Signed-off-by: Wilson Felipe Pereira <wfelipe@xxxxxxxxxx>
> Acked-by: Michal Koutný <mkoutny@xxxxxxxx>

(The long long helper is a non-trivial change, so the ack should be
stripped on this version (unless you can convince me ;-).)

I'm afraid this bitness propagated too broadly.

The long vs long long difference is only relevant on 32b, right?
And the bit-width issue only appears because of the product, not the
values read from memory.stat [1].

I'd consider three ways forward:
a) Consolidate the helpers into one cg_read_key_s64() (same explicit size
   regardless of arch),
b) keep single arch-dependent helper and tackle product(s) only,
c) do not bother with 32b in these selftests (I have a hunch that
   they're not so relevant on such archs).

WDYT?

Michal


[1] At least the values derived from available memory, memory events
    could grow indefinitely. It's true that kernel uses explicit u64 for
    all of the entries.

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