Re:

Yury Norov
2020-12-02T21:27:41Z

On Wed, Dec 2, 2020 at 10:22 AM Yun Levi <ppbuk5246@gmail.com> wrote:
>
> 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

If you traverse backward and reach bit #0, you're done. No need to continue.

>
> 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);

This one may be implemented with find_last_zero_bit() as well:

unsigned log j = find_last_zero_bit(bitmap, BITS_PER_BITMAP);
if (j <= i || j >= BITS_PER_BITMAP)
return false;

I believe the latter version is better because find_last_*_bit() is simpler in
implementation (and partially exists), has less parameters, and therefore
simpler for users, and doesn't introduce functionality duplication.

The only consideration I can imagine to advocate find_prev*() is the performance
advantage in the scenario when we know for sure that first N bits of
bitmap are all
set/clear, and we can bypass traversing that area. But again, in this
case we can pass the
bitmap address with the appropriate offset, and stay with find_last_*()

> +
> + 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.