RE: [PATCH] bitmap: extend comment to bitmap_print_to_buf

[Song Bao Hua (Barry Song)]

> -----Original Message-----
> From: Yury Norov [mailto:yury.norov@xxxxxxxxx]
> Sent: Thursday, July 29, 2021 6:59 AM
> To: linux-kernel@xxxxxxxxxxxxxxx; gregkh@xxxxxxxxxxxxxxxxxxx; Song Bao Hua
> (Barry Song) <song.bao.hua@xxxxxxxxxxxxx>
> Cc: Yury Norov <yury.norov@xxxxxxxxx>; dave.hansen@xxxxxxxxx;
> linux@xxxxxxxxxxxxxxxxxx; rafael@xxxxxxxxxx; rdunlap@xxxxxxxxxxxxx;
> agordeev@xxxxxxxxxxxxx; sbrivio@xxxxxxxxxx; jianpeng.ma@xxxxxxxxx;
> valentin.schneider@xxxxxxx; peterz@xxxxxxxxxxxxx; bristot@xxxxxxxxxx;
> guodong.xu@xxxxxxxxxx; tangchengchang <tangchengchang@xxxxxxxxxx>; Zengtao (B)
> <prime.zeng@xxxxxxxxxxxxx>; yangyicong <yangyicong@xxxxxxxxxx>;
> tim.c.chen@xxxxxxxxxxxxxxx; Linuxarm <linuxarm@xxxxxxxxxx>;
> akpm@xxxxxxxxxxxxxxxxxxxx; andriy.shevchenko@xxxxxxxxxxxxxxx
> 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@xxxxxxxxx>
> ---

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