Re: [PATCH 3/5] lib/bitmap: add test for bitmap_{from,to}_arr64

[Yury Norov]

On Mon, Feb 27, 2023 at 06:59:12AM -0800, Guenter Roeck wrote:
> On 2/27/23 06:46, Alexander Lobakin wrote:
> > From: Yury Norov <yury.norov@xxxxxxxxx>
> > Date: Sat, 25 Feb 2023 16:06:45 -0800
> > 
> > > On Sat, Feb 25, 2023 at 04:05:02PM -0800, Yury Norov wrote:
> > > > On Sat, Feb 25, 2023 at 10:47:02AM -0800, Guenter Roeck wrote:
> > > > > Hi,
> > > > > 
> > > > > On Thu, Apr 28, 2022 at 01:51:14PM -0700, Yury Norov wrote:
> > > > > > Test newly added bitmap_{from,to}_arr64() functions similarly to
> > > > > > already existing bitmap_{from,to}_arr32() tests.
> > > > > > 
> > > > > > Signed-off-by: Yury Norov <yury.norov@xxxxxxxxx>
> > > > > 
> > > > > Ever since this test is in the tree, several of my boot tests show
> > > > > lots of messages such as
> > > > > 
> > > > > test_bitmap: bitmap_to_arr64(nbits == 1): tail is not safely cleared: 0xa5a5a5a500000001 (must be 0x0000000000000001)
> > 
> > Hmmm, the whole 4 bytes weren't touched.
> > 
> > > > > test_bitmap: bitmap_to_arr64(nbits == 2): tail is not safely cleared: 0xa5a5a5a500000001 (must be 0x0000000000000003)
> > > > > test_bitmap: bitmap_to_arr64(nbits == 3): tail is not safely cleared: 0xa5a5a5a500000001 (must be 0x0000000000000007)
> > 
> > This is where it gets worse...
> > 
> > > > > ...
> > > > > test_bitmap: bitmap_to_arr64(nbits == 927): tail is not safely cleared: 0xa5a5a5a500000000 (must be 0x000000007fffffff)
> > > > > test_bitmap: bitmap_to_arr64(nbits == 928): tail is not safely cleared: 0xa5a5a5a580000000 (must be 0x00000000ffffffff)
> > 
> > I don't see the pattern how the actual result gets generated. But the
> > problem is in the bitmap code rather than in the subtest -- "must be"s
> > are fully correct.
> > 
> > Given that the 0xa5s are present in the upper 32 bits, it is Big Endian
> > I guess? Maybe even 32-bit Big Endian? Otherwise I'd start concerning
> > how comes it doesn't reproduce on x86_64s :D
> > 
> 
> It does reproduce on 32-bit x86 builds, and as far as I can see
> it is only seen with 32-bit little endian systems.

Hi Guenter, Alexander,

I think that the reason for the failures like this:

> test_bitmap: bitmap_to_arr64(nbits == 1): tail is not safely cleared: 0xa5a5a5a500000001 (must be 0x0000000000000001)

is that bitmap_to_arr64 is overly optimized for 32-bit LE architectures.

Regarding this:

> test_bitmap: bitmap_to_arr64(nbits == 927): tail is not safely cleared: 0xa5a5a5a500000000 (must be 0x000000007fffffff)

I am not sure what happens, but because this again happens on 32-bit
LE only, I hope the following fix would help too.

Can you please check if the patch works for you? I don't have a 32-bit LE
machine in hand, and all my 32-bit VMs (arm and i386) refuse to load the
latest kernels for some weird reason, so it's only build-tested.

I'll give it a full-run when restore my 32-bit setups.

Thanks,
Yury