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

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Date:   Wed, 28 Jul 2021 11:58:31 -0700
From:   Yury Norov <yury.norov@...il.com>
To:     linux-kernel@...r.kernel.org, gregkh@...uxfoundation.org,
        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@...wei.com, prime.zeng@...ilicon.com,
        yangyicong@...wei.com, tim.c.chen@...ux.intel.com,
        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>
---
 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