/*
     Process recently freed or remaindered chunks, taking one only if
     it is exact fit, or, if this a small request, the chunk is remainder from
     the most recent non-exact fit.  Place other traversed chunks in
     bins.  Note that this step is the only place in any routine where
     chunks are placed in bins.

     The outer loop here is needed because we might not realize until
     near the end of malloc that we should have consolidated, so must
     do so and retry. This happens at most once, and only when we would
     otherwise need to expand memory to service a "small" request.
   */

  for (;; )
    {
      int iters = 0;
      while ((victim = unsorted_chunks (av)->bk) != unsorted_chunks (av))
        {
          bck = victim->bk;
          if (__builtin_expect (victim->size <= 2 * SIZE_SZ, 0)
              || __builtin_expect (victim->size > av->system_mem, 0))
            malloc_printerr (check_action, "malloc(): memory corruption",
                             chunk2mem (victim), av);
          size = chunksize (victim);

          /*
             If a small request, try to use last remainder if it is the
             only chunk in unsorted bin.  This helps promote locality for
             runs of consecutive small requests. This is the only
             exception to best-fit, and applies only when there is
             no exact fit for a small chunk.
           */

          if (in_smallbin_range (nb) &&
              bck == unsorted_chunks (av) &&
              victim == av->last_remainder &&
              (unsigned long) (size) > (unsigned long) (nb + MINSIZE))
            {
              /* split and reattach remainder */
              remainder_size = size - nb;
              remainder = chunk_at_offset (victim, nb);
              unsorted_chunks (av)->bk = unsorted_chunks (av)->fd = remainder;
              av->last_remainder = remainder;
              remainder->bk = remainder->fd = unsorted_chunks (av);
              if (!in_smallbin_range (remainder_size))
                {
                  remainder->fd_nextsize = NULL;
                  remainder->bk_nextsize = NULL;
                }

              set_head (victim, nb | PREV_INUSE |
                        (av != &main_arena ? NON_MAIN_ARENA : 0));
              set_head (remainder, remainder_size | PREV_INUSE);
              set_foot (remainder, remainder_size);

              check_malloced_chunk (av, victim, nb);
              void *p = chunk2mem (victim);
              alloc_perturb (p, bytes);
              return p;
            }

          /* remove from unsorted list */
          unsorted_chunks (av)->bk = bck;
          bck->fd = unsorted_chunks (av);

          /* Take now instead of binning if exact fit */

          if (size == nb)
            {
              set_inuse_bit_at_offset (victim, size);
              if (av != &main_arena)
                victim->size |= NON_MAIN_ARENA;
              check_malloced_chunk (av, victim, nb);
              void *p = chunk2mem (victim);
              alloc_perturb (p, bytes);
              return p;
            }

          /* place chunk in bin */

          if (in_smallbin_range (size))
            {
              victim_index = smallbin_index (size);
              bck = bin_at (av, victim_index);
              fwd = bck->fd;
            }
          else
            {
              victim_index = largebin_index (size);
              bck = bin_at (av, victim_index);
              fwd = bck->fd;

              /* maintain large bins in sorted order */
              if (fwd != bck)
                {
                  /* Or with inuse bit to speed comparisons */
                  size |= PREV_INUSE;
                  /* if smaller than smallest, bypass loop below */
                  assert ((bck->bk->size & NON_MAIN_ARENA) == 0);
                  if ((unsigned long) (size) < (unsigned long) (bck->bk->size))
                    {
                      fwd = bck;
                      bck = bck->bk;

                      victim->fd_nextsize = fwd->fd;
                      victim->bk_nextsize = fwd->fd->bk_nextsize;
                      fwd->fd->bk_nextsize = victim->bk_nextsize->fd_nextsize = victim;
                    }
                  else
                    {
                      assert ((fwd->size & NON_MAIN_ARENA) == 0);
                      while ((unsigned long) size < fwd->size)
                        {
                          fwd = fwd->fd_nextsize;
                          assert ((fwd->size & NON_MAIN_ARENA) == 0);
                        }

                      if ((unsigned long) size == (unsigned long) fwd->size)
                        /* Always insert in the second position.  */
                        fwd = fwd->fd;
                      else
                        {
                          victim->fd_nextsize = fwd;
                          victim->bk_nextsize = fwd->bk_nextsize;
                          fwd->bk_nextsize = victim;
                          victim->bk_nextsize->fd_nextsize = victim;
                        }
                      bck = fwd->bk;
                    }
                }
              else
                victim->fd_nextsize = victim->bk_nextsize = victim;
            }

          mark_bin (av, victim_index);
          victim->bk = bck;
          victim->fd = fwd;
          fwd->bk = victim;
          bck->fd = victim;

#define MAX_ITERS       10000
          if (++iters >= MAX_ITERS)
            break;
        }

      /*
         If a large request, scan through the chunks of current bin in
         sorted order to find smallest that fits.  Use the skip list for this.
       */

      if (!in_smallbin_range (nb))
        {
          bin = bin_at (av, idx);

          /* skip scan if empty or largest chunk is too small */
          if ((victim = first (bin)) != bin &&
              (unsigned long) (victim->size) >= (unsigned long) (nb))
            {
              victim = victim->bk_nextsize;
              while (((unsigned long) (size = chunksize (victim)) <
                      (unsigned long) (nb)))
                victim = victim->bk_nextsize;

              /* Avoid removing the first entry for a size so that the skip
                 list does not have to be rerouted.  */
              if (victim != last (bin) && victim->size == victim->fd->size)
                victim = victim->fd;

              remainder_size = size - nb;
              unlink (av, victim, bck, fwd);

              /* Exhaust */
              if (remainder_size < MINSIZE)
                {
                  set_inuse_bit_at_offset (victim, size);
                  if (av != &main_arena)
                    victim->size |= NON_MAIN_ARENA;
                }
              /* Split */
              else
                {
                  remainder = chunk_at_offset (victim, nb);
                  /* We cannot assume the unsorted list is empty and therefore
                     have to perform a complete insert here.  */
                  bck = unsorted_chunks (av);
                  fwd = bck->fd;
	  if (__glibc_unlikely (fwd->bk != bck))
                    {
                      errstr = "malloc(): corrupted unsorted chunks";
                      goto errout;
                    }
                  remainder->bk = bck;
                  remainder->fd = fwd;
                  bck->fd = remainder;
                  fwd->bk = remainder;
                  if (!in_smallbin_range (remainder_size))
                    {
                      remainder->fd_nextsize = NULL;
                      remainder->bk_nextsize = NULL;
                    }

                  set_head (victim, nb | PREV_INUSE |
                            (av != &main_arena ? NON_MAIN_ARENA : 0));
                  set_head (remainder, remainder_size | PREV_INUSE);
                  set_foot (remainder, remainder_size);
                }
              check_malloced_chunk (av, victim, nb);
              void *p = chunk2mem (victim);
              alloc_perturb (p, bytes);
              return p;
            }
        }

      /*
         Search for a chunk by scanning bins, starting with next largest
         bin. This search is strictly by best-fit; i.e., the smallest
         (with ties going to approximately the least recently used) chunk
         that fits is selected.

         The bitmap avoids needing to check that most blocks are nonempty.
         The particular case of skipping all bins during warm-up phases
         when no chunks have been returned yet is faster than it might look.
       */

      ++idx;
      bin = bin_at (av, idx);
      block = idx2block (idx);
      map = av->binmap[block];
      bit = idx2bit (idx);

      for (;; )
        {
          /* Skip rest of block if there are no more set bits in this block.  */
          if (bit > map || bit == 0)
            {
              do
                {
                  if (++block >= BINMAPSIZE) /* out of bins */
                    goto use_top;
                }
              while ((map = av->binmap[block]) == 0);

              bin = bin_at (av, (block << BINMAPSHIFT));
              bit = 1;
            }

          /* Advance to bin with set bit. There must be one. */
          while ((bit & map) == 0)
            {
              bin = next_bin (bin);
              bit <<= 1;
              assert (bit != 0);
            }

          /* Inspect the bin. It is likely to be non-empty */
          victim = last (bin);

          /*  If a false alarm (empty bin), clear the bit. */
          if (victim == bin)
            {
              av->binmap[block] = map &= ~bit; /* Write through */
              bin = next_bin (bin);
              bit <<= 1;
            }

          else
            {
              size = chunksize (victim);

              /*  We know the first chunk in this bin is big enough to use. */
              assert ((unsigned long) (size) >= (unsigned long) (nb));

              remainder_size = size - nb;

              /* unlink */
              unlink (av, victim, bck, fwd);

              /* Exhaust */
              if (remainder_size < MINSIZE)
                {
                  set_inuse_bit_at_offset (victim, size);
                  if (av != &main_arena)
                    victim->size |= NON_MAIN_ARENA;
                }

              /* Split */
              else
                {
                  remainder = chunk_at_offset (victim, nb);

                  /* We cannot assume the unsorted list is empty and therefore
                     have to perform a complete insert here.  */
                  bck = unsorted_chunks (av);
                  fwd = bck->fd;
	  if (__glibc_unlikely (fwd->bk != bck))
                    {
                      errstr = "malloc(): corrupted unsorted chunks 2";
                      goto errout;
                    }
                  remainder->bk = bck;
                  remainder->fd = fwd;
                  bck->fd = remainder;
                  fwd->bk = remainder;

                  /* advertise as last remainder */
                  if (in_smallbin_range (nb))
                    av->last_remainder = remainder;
                  if (!in_smallbin_range (remainder_size))
                    {
                      remainder->fd_nextsize = NULL;
                      remainder->bk_nextsize = NULL;
                    }
                  set_head (victim, nb | PREV_INUSE |
                            (av != &main_arena ? NON_MAIN_ARENA : 0));
                  set_head (remainder, remainder_size | PREV_INUSE);
                  set_foot (remainder, remainder_size);
                }
              check_malloced_chunk (av, victim, nb);
              void *p = chunk2mem (victim);
              alloc_perturb (p, bytes);
              return p;
            }
        }

    use_top:
      /*
         If large enough, split off the chunk bordering the end of memory
         (held in av->top). Note that this is in accord with the best-fit
         search rule.  In effect, av->top is treated as larger (and thus
         less well fitting) than any other available chunk since it can
         be extended to be as large as necessary (up to system
         limitations).

         We require that av->top always exists (i.e., has size >=
         MINSIZE) after initialization, so if it would otherwise be
         exhausted by current request, it is replenished. (The main
         reason for ensuring it exists is that we may need MINSIZE space
         to put in fenceposts in sysmalloc.)
       */

      victim = av->top;
      size = chunksize (victim);

      if ((unsigned long) (size) >= (unsigned long) (nb + MINSIZE))
        {
          remainder_size = size - nb;
          remainder = chunk_at_offset (victim, nb);
          av->top = remainder;
          set_head (victim, nb | PREV_INUSE |
                    (av != &main_arena ? NON_MAIN_ARENA : 0));
          set_head (remainder, remainder_size | PREV_INUSE);

          check_malloced_chunk (av, victim, nb);
          void *p = chunk2mem (victim);
          alloc_perturb (p, bytes);
          return p;
        }

      /* When we are using atomic ops to free fast chunks we can get
         here for all block sizes.  */
      else if (have_fastchunks (av))
        {
          malloc_consolidate (av);
          /* restore original bin index */
          if (in_smallbin_range (nb))
            idx = smallbin_index (nb);
          else
            idx = largebin_index (nb);
        }

      /*
         Otherwise, relay to handle system-dependent cases
       */
      else
        {
          void *p = sysmalloc (nb, av);
          if (p != NULL)
            alloc_perturb (p, bytes);
          return p;
        }
    }
}

if (in_smallbin_range (nb) &&             // 1. 你申请的是个小块 (Small Bin 范围)
    bck == unsorted_chunks (av) &&       // 2. Unsorted Bin 里【只有一个】chunk
    victim == av->last_remainder &&      // 3. 这个 chunk 正好是“上次剩下的”
    (unsigned long) (size) > (unsigned long) (nb + MINSIZE)) // 4. 它足够大