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. > > 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.