linux-kernel - RE: [PATCH] bitmap: extend comment to bitmap_print_to

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Message-ID: <eec4210a5d724cccb2931a3f4403e5be@hisilicon.com>
Date:   Thu, 29 Jul 2021 06:04:09 +0000
From:   "Song Bao Hua (Barry Song)" <song.bao.hua@...ilicon.com>
To:     Yury Norov <yury.norov@...il.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>
CC:     "dave.hansen@...el.com" <dave.hansen@...el.com>,
        "linux@...musvillemoes.dk" <linux@...musvillemoes.dk>,
        "rafael@...nel.org" <rafael@...nel.org>,
        "rdunlap@...radead.org" <rdunlap@...radead.org>,
        "agordeev@...ux.ibm.com" <agordeev@...ux.ibm.com>,
        "sbrivio@...hat.com" <sbrivio@...hat.com>,
        "jianpeng.ma@...el.com" <jianpeng.ma@...el.com>,
        "valentin.schneider@....com" <valentin.schneider@....com>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "bristot@...hat.com" <bristot@...hat.com>,
        "guodong.xu@...aro.org" <guodong.xu@...aro.org>,
        tangchengchang <tangchengchang@...wei.com>,
        "Zengtao (B)" <prime.zeng@...ilicon.com>,
        yangyicong <yangyicong@...wei.com>,
        "tim.c.chen@...ux.intel.com" <tim.c.chen@...ux.intel.com>,
        Linuxarm <linuxarm@...wei.com>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        "andriy.shevchenko@...ux.intel.com" 
        <andriy.shevchenko@...ux.intel.com>
Subject: RE: [PATCH] bitmap: extend comment to bitmap_print_to_buf



> -----Original Message-----
> From: Yury Norov [mailto:yury.norov@...il.com]
> Sent: Thursday, July 29, 2021 6:59 AM
> To: linux-kernel@...r.kernel.org; gregkh@...uxfoundation.org; Song Bao Hua
> (Barry Song) <song.bao.hua@...ilicon.com>
> Cc: Yury Norov <yury.norov@...il.com>; dave.hansen@...el.com;
> linux@...musvillemoes.dk; rafael@...nel.org; rdunlap@...radead.org;
> agordeev@...ux.ibm.com; sbrivio@...hat.com; jianpeng.ma@...el.com;
> valentin.schneider@....com; peterz@...radead.org; bristot@...hat.com;
> guodong.xu@...aro.org; tangchengchang <tangchengchang@...wei.com>; Zengtao (B)
> <prime.zeng@...ilicon.com>; yangyicong <yangyicong@...wei.com>;
> tim.c.chen@...ux.intel.com; Linuxarm <linuxarm@...wei.com>;
> akpm@...ux-foundation.org; andriy.shevchenko@...ux.intel.com
> Subject: [PATCH] bitmap: extend comment to bitmap_print_to_buf
> 
> Extend comment to new function to warn potential users about caveats.
> 
> Signed-off-by: Yury Norov <yury.norov@...il.com>
> ---

Looks awesome. Thanks, Yury. I have integrated your patch into
the latest series v8.

>  lib/bitmap.c | 18 ++++++++++++++++++
>  1 file changed, 18 insertions(+)
> 
> diff --git a/lib/bitmap.c b/lib/bitmap.c
> index 56bcffe2fa8c..b9f557ca668c 100644
> --- a/lib/bitmap.c
> +++ b/lib/bitmap.c
> @@ -545,6 +545,24 @@ EXPORT_SYMBOL(bitmap_print_to_pagebuf);
>   * mainly serves bin_attribute which doesn't work with exact one page, and it
>   * can break the size limit of converted decimal list and hexadecimal bitmask.
>   *
> + * WARNING!
> + *
> + * This function is not a replacement for sprintf() or
> bitmap_print_to_pagebuf().
> + * It is intended to workaround sysfs limitations discussed above and should
> be
> + * used carefully in general case for the following reasons:
> + *  - Time complexity is O(nbits^2/count), comparing to O(nbits) for snprintf().
> + *  - Memory complexity is O(nbits), comparing to O(1) for snprintf().
> + *  - @off and @count are NOT offset and number of bits to print.
> + *  - If printing part of bitmap as list, the resulting string is not a correct
> + *    list representation of bitmap. Particularly, some bits within or out of
> + *    related interval may be erroneously set or unset. The format of the string
> + *    may be broken, so bitmap_parselist-like parser may fail parsing it.
> + *  - If printing the whole bitmap as list by parts, user must ensure the order
> + *    of calls of the function such that the offset is incremented linearly.
> + *  - If printing the whole bitmap as list by parts, user must keep bitmap
> + *    unchanged between the very first and very last call. Otherwise concatenated
> + *    result may be incorrect, and format may be broken.
> + *
>   * Returns the number of characters actually printed to @buf
>   */
>  int bitmap_print_to_buf(bool list, char *buf, const unsigned long *maskp,
> --
> 2.30.2

Thanks
Barry