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

[Yury Norov <yury.norov@gmail.com>]

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@gmail.com>
> > 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@gmail.com>
> > > > > 
> > > > > 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

From 2881714db497aed103e310865da075e7b0ce7e1a Mon Sep 17 00:00:00 2001
From: Yury Norov <yury.norov@gmail.com>
Date: Mon, 27 Feb 2023 09:21:59 -0800
Subject: [PATCH] lib/bitmap: drop optimization of bitmap_{from,to}_arr64

bitmap_{from,to}_arr64() optimization is overly optimistic on 32-bit LE
architectures when it's wired to bitmap_copy_clear_tail().

bitmap_copy_clear_tail() takes care of unused bits in the bitmap up to
the next word boundary. But on 32-bit machines when copying bits from
bitmap to array of 64-bit words, it's expected that the unused part of
a recipient array must be cleared up to 64-bit boundary, so the last 4
bytes may stay untouched.

While the copying part of the optimization works correct, that clear-tail
trick makes corresponding tests reasonably fail when nbits % 64 <= 32:

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

Fix it by removing bitmap_{from,to}_arr64() optimization for 32-bit LE
arches.

Reported-by: Guenter Roeck <linux@roeck-us.net>
Fixes: 0a97953fd2210 ("lib: add bitmap_{from,to}_arr64")
Signed-off-by: Yury Norov <yury.norov@gmail.com>
---
 include/linux/bitmap.h | 8 +++-----
 lib/bitmap.c           | 2 +-
 2 files changed, 4 insertions(+), 6 deletions(-)

diff --git a/include/linux/bitmap.h b/include/linux/bitmap.h
index 40e53a2ecc0d..5abc993903fb 100644
--- a/include/linux/bitmap.h
+++ b/include/linux/bitmap.h
@@ -302,12 +302,10 @@ void bitmap_to_arr32(u32 *buf, const unsigned long *bitmap,
 #endif
 
 /*
- * On 64-bit systems bitmaps are represented as u64 arrays internally. On LE32
- * machines the order of hi and lo parts of numbers match the bitmap structure.
- * In both cases conversion is not needed when copying data from/to arrays of
- * u64.
+ * On 64-bit systems bitmaps are represented as u64 arrays internally. So,
+ * conversion is not needed when copying data from/to arrays of u64.
  */
-#if (BITS_PER_LONG == 32) && defined(__BIG_ENDIAN)
+#if BITS_PER_LONG == 32
 void bitmap_from_arr64(unsigned long *bitmap, const u64 *buf, unsigned int nbits);
 void bitmap_to_arr64(u64 *buf, const unsigned long *bitmap, unsigned int nbits);
 #else
diff --git a/lib/bitmap.c b/lib/bitmap.c
index 1c81413c51f8..ddb31015e38a 100644
--- a/lib/bitmap.c
+++ b/lib/bitmap.c
@@ -1495,7 +1495,7 @@ void bitmap_to_arr32(u32 *buf, const unsigned long *bitmap, unsigned int nbits)
 EXPORT_SYMBOL(bitmap_to_arr32);
 #endif
 
-#if (BITS_PER_LONG == 32) && defined(__BIG_ENDIAN)
+#if BITS_PER_LONG == 32
 /**
  * bitmap_from_arr64 - copy the contents of u64 array of bits to bitmap
  *	@bitmap: array of unsigned longs, the destination bitmap
-- 
2.34.1