Re: [PATCH 3/5] lib/bitmap: add test for bitmap_{from,to}_arr64
From: Guenter Roeck
Date: Mon Feb 27 2023 - 15:12:11 EST
On 2/27/23 11:24, Yury Norov wrote:
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
From 2881714db497aed103e310865da075e7b0ce7e1a Mon Sep 17 00:00:00 2001
From: Yury Norov <yury.norov@xxxxxxxxx>
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@xxxxxxxxxxxx>
Fixes: 0a97953fd2210 ("lib: add bitmap_{from,to}_arr64")
Signed-off-by: Yury Norov <yury.norov@xxxxxxxxx>
Tested with 32-bit i386 image. With this patch on top of
v6.2-12765-g982818426a0f, the log messages are gone. Without this patch,
they are still seen.
Tested-by: Guenter Roeck <linux@xxxxxxxxxxxx>
Guenter
---
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