linux-kernel - Re: [PATCH v5 2/5] lib/test_bitmap: add tests for bitmap

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Date:   Mon, 25 Sep 2023 15:09:04 +0200
From:   Alexander Potapenko <glider@...gle.com>
To:     Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
Cc:     catalin.marinas@....com, will@...nel.org, pcc@...gle.com,
        andreyknvl@...il.com, linux@...musvillemoes.dk,
        yury.norov@...il.com, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, eugenis@...gle.com,
        syednwaris@...il.com, william.gray@...aro.org
Subject: Re: [PATCH v5 2/5] lib/test_bitmap: add tests for bitmap_{read,write}()

On Mon, Sep 25, 2023 at 2:23 PM Andy Shevchenko
<andriy.shevchenko@...ux.intel.com> wrote:
>
> On Mon, Sep 25, 2023 at 02:16:37PM +0200, Alexander Potapenko wrote:
>
> ...
>
> > > +/*
> > > + * Test bitmap should be big enough to include the cases when start is not in
> > > + * the first word, and start+nbits lands in the following word.
> > > + */
> > > +#define TEST_BIT_LEN (1000)
> >
> > Dunno why this didn't fire previously, but CONFIG_CPU_BIG_ENDIAN=y
> > kernel reports mismatches here, presumably because the last quad word
> > ends up partially initialized.
>
> Hmm... But if designed and used correctly it shouldn't be the issue,
> and 1000, I believe, is carefully chosen to be specifically not dividable
> by pow-of-2 value.
>

The problem manifests already right after initialization:

static void __init test_bit_len_1000(void)
{
        DECLARE_BITMAP(bitmap, TEST_BIT_LEN);
        DECLARE_BITMAP(exp_bitmap, TEST_BIT_LEN);
        memset(bitmap, 0x00, TEST_BYTE_LEN);
        memset(exp_bitmap, 0x00, TEST_BYTE_LEN);
        expect_eq_bitmap(exp_bitmap, bitmap, TEST_BIT_LEN);
}

...
[   29.601614][    T1] test_bitmap: [lib/test_bitmap.c:1250] bitmaps
contents differ: expected
"960-963,966-967,969,971-973,976,978-979,981", got "963"
...

So it's probably expect_eq_bitmap() that is incorrectly rounding up
the bitmap length somewhere (or maybe it is not supposed to be used
for non-aligned bitmaps?)
Looking further...