linux-kernel - Re: [PATCH 2/2] lib/bitmap: drop optimization of bitmap_{from,to}

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Message-ID: <fd0da26f-6437-75bf-c013-f338174325fe@intel.com>
Date:   Tue, 28 Feb 2023 11:45:18 +0100
From:   Alexander Lobakin <aleksander.lobakin@...el.com>
To:     Yury Norov <yury.norov@...il.com>
CC:     <linux-kernel@...r.kernel.org>,
        Alexander Gordeev <agordeev@...ux.ibm.com>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Christian Borntraeger <borntraeger@...ux.ibm.com>,
        Claudio Imbrenda <imbrenda@...ux.ibm.com>,
        David Hildenbrand <david@...hat.com>,
        Guenter Roeck <linux@...ck-us.net>,
        Heiko Carstens <hca@...ux.ibm.com>,
        Janosch Frank <frankja@...ux.ibm.com>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Sven Schnelle <svens@...ux.ibm.com>,
        Vasily Gorbik <gor@...ux.ibm.com>,
        <linux-s390@...r.kernel.org>, <kvm@...r.kernel.org>
Subject: Re: [PATCH 2/2] lib/bitmap: drop optimization of
 bitmap_{from,to}_arr64

From: Yury Norov <yury.norov@...il.com>
Date: Mon, 27 Feb 2023 13:45:24 -0800

> 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 when nbits % 64 <= 32.
> 
> While the copying part of the optimization works correct, that clear-tail
> trick makes corresponding tests reasonably fail:
> 
> 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@...ck-us.net>
> Link: https://lore.kernel.org/lkml/20230225184702.GA3587246@roeck-us.net/
> Fixes: 0a97953fd221 ("lib: add bitmap_{from,to}_arr64")
> Signed-off-by: Yury Norov <yury.norov@...il.com>
> Tested-by: Guenter Roeck <linux@...ck-us.net>
> ---
>  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..7d4c90eb3df4 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,
> + * the 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

For the series:

Reviewed-by: Alexander Lobakin <aleksander.lobakin@...el.com>

Thanks,
Olek