Re: test_bitmap fails on ppc/ppc64 on kernels v7.1.3, v7.2-rc2

["Christophe Leroy (CS GROUP)" <chleroy@kernel.org>]

Le 08/07/2026 à 09:43, Andy Shevchenko a écrit :
> On Wed, Jul 08, 2026 at 01:25:01AM +0200, Erhard Furtner wrote:
> 
> ...
> 
>>> So there is something with your config
>>
>> Interesting! As I get the same test failure on my Talos II too.
>>
>> Anyhow, I was able to bisect the issue. Offending commit is:
>>
>>   # git bisect bad
>> 6b5a4b68736798df1031404a2fad06d031253ef7 is the first bad commit
>> commit 6b5a4b68736798df1031404a2fad06d031253ef7 (HEAD)
>> Author: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
>> Date:   Thu Feb 26 12:16:44 2026 +0100
>>
>>      bitmap: Add test for out-of-boundary modifications for scatter & gather
>>
>>      Make sure that bitmap_scatter() and bitmap_gather() do not modify
>>      the bits outside of the given nbits span.
>>
>>      Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
>>      Signed-off-by: Yury Norov <ynorov@nvidia.com>
>>
>>   lib/test_bitmap.c | 10 +++++++---
>>   1 file changed, 7 insertions(+), 3 deletions(-)
>>
>>
>> Reverting this commit on top of v7.2-rc2 lets the test pass:
>>
>> test_bitmap: loaded.
>> test_bitmap: parselist('0-2047:128/256'):	888
>> test_bitmap: scnprintf("%*pbl", '0-32767'):	6074
>> test_bitmap: test_bitmap_read_perf:		1190938
>> test_bitmap: test_bitmap_write_perf:		1259471
>> test_bitmap: all 208655 tests passed
>>
>> Your hint about my config made me check a few options and I found the
>> offending one, which is INIT_STACK_ALL_PATTERN=y. On a kernel built with
>> INIT_STACK_ALL_ZERO=y the issue does not show up.
> 
> Oh, this is nice. So, there are two (more?) options I see to mitigate
> the issue:
> - carefully copy the garbage from the stack to the expected values
>    (effectively merge the whatever is on stack with the expected value)
> - allocate buffers on heap
> 
> The latter seems the easiest and right thing to do (since we can't really
> predict if the stack pattern is the same or bitmap APIs scatters the bits
> just on top of the respective set or clear ones over that pattern).
> 
> I will send a patch, thanks for the report and analysis!
> 
> 

The following change fixes the issue for me:

diff --git a/lib/test_bitmap.c b/lib/test_bitmap.c
index 69813c10e6c0b..448c3eb48a4a8 100644
--- a/lib/test_bitmap.c
+++ b/lib/test_bitmap.c
@@ -392,6 +392,7 @@ static void __init test_bitmap_sg(void)

  	/* Scatter/gather relationship */
  	bitmap_zero(bmap_tmp, 100);
+	bitmap_zero(bmap_res, 100);
  	bitmap_gather(bmap_tmp, bmap_scatter, sg_mask, nbits);
  	bitmap_scatter(bmap_res, bmap_tmp, sg_mask, nbits);
  	expect_eq_bitmap(bmap_scatter, bmap_res, 100);


Christophe