[no subject]

[Yun Levi]

On Thu, Dec 3, 2020 at 6:26 AM Yury Norov <yury.norov@xxxxxxxxx> wrote:
>
> On Wed, Dec 2, 2020 at 10:22 AM Yun Levi <ppbuk5246@xxxxxxxxx> wrote:
> >
> > On Thu, Dec 3, 2020 at 2:26 AM Yury Norov <yury.norov@xxxxxxxxx> 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.
I think the case when I consider the this macro like

#define for_each_clear_bit_reverse(bit, addr, size)
    for ((bit) = find_last_zero_bit((addr), (size))
          (bit) < (size);
          (bit) = find_prev_zero_bit((addr), (size), (bit)))

If we implement the above macro only with find_last_zero_bit,
I think there is no way without adding any additional variable to finish loop.
But I don't want to add additional variable to sustain same format
with for_each_clear_bit,
That's why i decide to implement find_prev_*_bit series.

I don't know it's correct thinking or biased. Am I wrong?

>
> >
> > 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;
>
Actually, in that code, we don't need to check the bit after i.
Originally, if my understanding is correct, former code tries to find
the last 0 bit before i.
and if all bits are fully set before i, it use next one as i + 1

that's why i think the if condition should be
   if (j >= i)

But above condition couldn't the discern the case when all bits are
fully set before i.
Also, I think we don't need to check the bit after i and In this case,
find_prev_zero_bit which
specifies the start point is clear to show the meaning of the code.


> 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.

Thanks but