[PATCH] bitmap: extend comment to bitmap_print_to_buf

From: Yury Norov <yury.norov@gmail.com>
To: linux-kernel@vger.kernel.org, gregkh@linuxfoundation.org,
	Barry Song <song.bao.hua@hisilicon.com>
Cc: Yury Norov <yury.norov@gmail.com>,
	dave.hansen@intel.com, linux@rasmusvillemoes.dk,
	rafael@kernel.org, rdunlap@infradead.org, agordeev@linux.ibm.com,
	sbrivio@redhat.com, jianpeng.ma@intel.com,
	valentin.schneider@arm.com, peterz@infradead.org,
	bristot@redhat.com, guodong.xu@linaro.org,
	tangchengchang@huawei.com, prime.zeng@hisilicon.com,
	yangyicong@huawei.com, tim.c.chen@linux.intel.com,
	linuxarm@huawei.com, akpm@linux-foundation.org,
	andriy.shevchenko@linux.intel.com
Subject: [PATCH] bitmap: extend comment to bitmap_print_to_buf
Date: Wed, 28 Jul 2021 11:58:31 -0700	[thread overview]
Message-ID: <20210728185831.215079-1-yury.norov@gmail.com> (raw)
In-Reply-To: <20210715115856.11304-1-song.bao.hua@hisilicon.com>

Extend comment to new function to warn potential users about caveats.

Signed-off-by: Yury Norov <yury.norov@gmail.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

