Hi guys:


   i found i intresting things when i fix a memory leak issue on may platform which based on musl c library.  the issue has been fixed but a puzzle leave it to me.
in the file of malloc/mallocng/malloc.c,  a function called "alloc_meta" says that as belows,so you can see the brk pointer of brk system call parmeter never decrease the brk, is not it ? did gilibc also does like this way? 
why design like this, thank you !
struct meta *alloc_meta(void)
{
        struct meta *m;
        unsigned char *p;
        if (!ctx.init_done) {
#ifndef PAGESIZE
                ctx.pagesize = get_page_size();
#endif
                ctx.secret = get_random_secret();
                ctx.init_done = 1;
        }
        size_t pagesize = PGSZ;
        if (pagesize < 4096) pagesize = 4096;
        if ((m = dequeue_head(&ctx.free_meta_head))) return m;
        if (!ctx.avail_meta_count) {
                int need_unprotect = 1;
                if (!ctx.avail_meta_area_count && ctx.brk!=-1) {
                        uintptr_t new = ctx.brk + pagesize;
                        int need_guard = 0;
                        if (!ctx.brk) {
                                need_guard = 1;
                                ctx.brk = brk(0);
                                // some ancient kernels returned _ebss
                                // instead of next page as initial brk.
                                ctx.brk += -ctx.brk & (pagesize-1);
                                new = ctx.brk + 2*pagesize;
                        }
                        if (brk(new) != new) {
                                ctx.brk = -1;
                        } else {
                                if (need_guard) mmap((void *)ctx.brk, pagesize,
                                        PROT_NONE, MAP_ANON|MAP_PRIVATE|MAP_FIXED, -1, 0);
                                ctx.brk = new;
                                ctx.avail_meta_areas = (void *)(new - pagesize);
                                ctx.avail_meta_area_count = pagesize>>12;
                                need_unprotect = 0;
                        }
                }
                if (!ctx.avail_meta_area_count) {
                        size_t n = 2UL << ctx.meta_alloc_shift;
                        p = mmap(0, n*pagesize, PROT_NONE,
                                MAP_PRIVATE|MAP_ANON, -1, 0);
                        if (p==MAP_FAILED) return 0;
                        ctx.avail_meta_areas = p + pagesize;
                        ctx.avail_meta_area_count = (n-1)*(pagesize>>12);
                        ctx.meta_alloc_shift++;
/\<brk\>                                                                                                                                                                                  63,15-43      10%