Re: [PATCH v7 0/4] use bin_attribute to break the size limitation of cpumap ABI

Yury Norov
2021-07-28T14:54:46Z

On Wed, Jul 28, 2021 at 03:41:00PM +0200, Greg KH wrote:
> On Thu, Jul 15, 2021 at 11:58:52PM +1200, Barry Song wrote:
> > v7:
> > - update doc in code for new APIs according to the comments of
> > Andy Shevchenko;
> > - other minor cleanup and commit log fix according to the comments
> > of Andy Shevchenko
>
> I'm lost to tell if this is the latest version or if there are more
> changes? Can you send this again with the latest changes so I can
> review it?

Barry, Greg,

If you decide to keep bitmap_print_to_buf in lib/bitmap.c, could you
please add the following patch to the series.

Thanks,
Yury

From 58602766dc2877d2103a334db6c2c2e1e6b8c89b Mon Sep 17 00:00:00 2001
From: Yury Norov <yury.norov@gmail.com>
Date: Wed, 28 Jul 2021 07:39:30 -0700
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@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() 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