From: Yun Levi <ppbuk5246@gmail.com>
To: Yury Norov <yury.norov@gmail.com>
Cc: Rasmus Villemoes <linux@rasmusvillemoes.dk>,
dushistov@mail.ru, Arnd Bergmann <arnd@arndb.de>,
Andrew Morton <akpm@linux-foundation.org>,
"Gustavo A. R. Silva" <gustavo@embeddedor.com>,
William Breathitt Gray <vilhelm.gray@gmail.com>,
richard.weiyang@linux.alibaba.com, joseph.qi@linux.alibaba.com,
skalluru@marvell.com, Josh Poimboeuf <jpoimboe@redhat.com>,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
linux-arch@vger.kernel.org,
Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Subject:
Date: Thu, 3 Dec 2020 03:22:42 +0900 [thread overview]
Message-ID: <CAM7-yPRBPP6SFzdmwWF5Y99g+aWcp=OY9Uvp-5h1MSDPmsORNw@mail.gmail.com> (raw)
In-Reply-To: <CAAH8bW-jUeFVU-0OrJzK-MuGgKJgZv38RZugEQzFRJHSXFRRDA@mail.gmail.com>
On Thu, Dec 3, 2020 at 2:26 AM Yury Norov <yury.norov@gmail.com> wrote:
> Also look at lib/find_bit_benchmark.c
Thanks. I'll see.
> We need find_next_*_bit() because find_first_*_bit() can start searching only at word-aligned
> bits. In the case of find_last_*_bit(), we can start at any bit. So, if my understanding is correct,
> for the purpose of reverse traversing we can go with already existing find_last_bit(),
Thank you. I haven't thought that way.
But I think if we implement reverse traversing using find_last_bit(),
we have a problem.
Suppose the last bit 0, 1, 2, is set.
If we start
find_last_bit(bitmap, 3) ==> return 2;
find_last_bit(bitmap, 2) ==> return 1;
find_last_bit(bitmap, 1) ==> return 0;
find_last_bit(bitmap, 0) ===> return 0? // here we couldn't
distinguish size 0 input or 0 is set
and the for_each traverse routine prevent above case by returning size
(nbits) using find_next_bit.
So, for compatibility and the same expected return value like next traversing,
I think we need to find_prev_*_bit routine. if my understanding is correct.
> I think this patch has some good catches. We definitely need to implement
> find_last_zero_bit(), as it is used by fs/ufs, and their local implementation is not optimal.
>
> We also should consider adding reverse traversing macros based on find_last_*_bit(),
> if there are proposed users.
Not only this, I think 'steal_from_bitmap_to_front' can be improved
using ffind_prev_zero_bit
like
diff --git a/fs/btrfs/free-space-cache.c b/fs/btrfs/free-space-cache.c
index af0013d3df63..9debb9707390 100644
--- a/fs/btrfs/free-space-cache.c
+++ b/fs/btrfs/free-space-cache.c
@@ -2372,7 +2372,6 @@ static bool steal_from_bitmap_to_front(struct
btrfs_free_space_ctl *ctl,
u64 bitmap_offset;
unsigned long i;
unsigned long j;
- unsigned long prev_j;
u64 bytes;
bitmap_offset = offset_to_bitmap(ctl, info->offset);
@@ -2388,20 +2387,15 @@ static bool steal_from_bitmap_to_front(struct
btrfs_free_space_ctl *ctl,
return false;
i = offset_to_bit(bitmap->offset, ctl->unit, info->offset) - 1;
- j = 0;
- prev_j = (unsigned long)-1;
- for_each_clear_bit_from(j, bitmap->bitmap, BITS_PER_BITMAP) {
- if (j > i)
- break;
- prev_j = j;
- }
- if (prev_j == i)
+ j = find_prev_zero_bit(bitmap->bitmap, BITS_PER_BITMAP, i);
+
+ if (j == i)
return false;
- if (prev_j == (unsigned long)-1)
+ if (j == BITS_PER_BITMAP)
bytes = (i + 1) * ctl->unit;
else
- bytes = (i - prev_j) * ctl->unit;
+ bytes = (i - j) * ctl->unit;
info->offset -= bytes;
info->bytes += bytes;
Thanks.
HTH
Levi.
next prev parent reply other threads:[~2020-12-02 18:23 UTC|newest]
Thread overview: 23+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-12-02 1:10 [PATCH] lib/find_bit: Add find_prev_*_bit functions Yun Levi
2020-12-02 9:47 ` Andy Shevchenko
2020-12-02 10:04 ` Rasmus Villemoes
2020-12-02 11:50 ` Yun Levi
2020-12-02 12:06 ` Andy Shevchenko
[not found] ` <CAAH8bW-jUeFVU-0OrJzK-MuGgKJgZv38RZugEQzFRJHSXFRRDA@mail.gmail.com>
2020-12-02 17:37 ` Andy Shevchenko
2020-12-02 18:27 ` Yun Levi
2020-12-02 18:51 ` your mail Andy Shevchenko
2020-12-02 18:56 ` Andy Shevchenko
2020-12-02 23:16 ` Yun Levi
2020-12-02 18:22 ` Yun Levi [this message]
2020-12-02 21:26 ` Yury Norov
2020-12-02 22:51 ` Yun Levi
2020-12-03 1:23 ` Yun Levi
2020-12-03 8:33 ` Rasmus Villemoes
2020-12-03 9:47 ` Re: Yun Levi
2020-12-03 18:46 ` Re: Yury Norov
2020-12-03 18:52 ` Re: Willy Tarreau
2020-12-04 1:36 ` Re: Yun Levi
2020-12-04 18:14 ` Re: Yury Norov
2020-12-05 0:45 ` Re: Yun Levi
2020-12-05 11:10 ` Re: Rasmus Villemoes
2020-12-05 18:20 ` Re: Yury Norov
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