[PATCH 1/2] malloc: Improve mmap interface

[Adhemerval Zanella Netto]

On 02/10/25 13:25, Wilco Dijkstra wrote:
> 
> Add mmap_set_chunk() to create a new chunk from an mmap block.
> Remove set_mmap_is_hp() since it is done inside mmap_set_chunk().
> Rename prev_size_mmap() to mmap_base_offset().  Cleanup comments.
> 
> Passes regress, OK for commit?

LGTM, thanks.

Reviewed-by: Adhemerval Zanella  <adhemerval.zanella@linaro.org>

> 
> ---
> 
> diff --git a/malloc/malloc.c b/malloc/malloc.c
> index efe136f2d13cfcbc1cffdcbde70704bbd43831bc..2992ef5cff377b59329b22b1bfbac95ef30c1a18 100644
> --- a/malloc/malloc.c
> +++ b/malloc/malloc.c
> @@ -1477,26 +1477,19 @@ tag_new_usable (void *ptr)
>    return ptr;
>  }
>  
> -/* HP page used for an mmap()'ed chunk. */
> +/* Huge page used for an mmap chunk.  */
>  #define MMAP_HP 0x1
>  
> -/* Check for HP usage from an mmap()'ed chunk. */
> +/* Return whether an mmap chunk uses huge pages.  */
>  static __always_inline bool
>  mmap_is_hp (mchunkptr p)
>  {
>    return prev_size (p) & MMAP_HP;
>  }
>  
> -/* Set HP advised field for an mmap()'ed chunk. */
> -static __always_inline void
> -set_mmap_is_hp (mchunkptr p)
> -{
> -  prev_size (p) |= MMAP_HP;
> -}
> -
> -/* Get an mmap()ed chunk's offset, ignoring huge page bits. */
> +/* Return the mmap chunk's offset from mmap base.  */
>  static __always_inline size_t
> -prev_size_mmap (mchunkptr p)
> +mmap_base_offset (mchunkptr p)
>  {
>    return prev_size (p) & ~MMAP_HP;
>  }
> @@ -1505,14 +1498,24 @@ prev_size_mmap (mchunkptr p)
>  static __always_inline void *
>  mmap_base (mchunkptr p)
>  {
> -  return (char *) p - prev_size_mmap (p);
> +  return (char *) p - mmap_base_offset (p);
>  }
>  
>  /* Return total mmap size of a chunk with IS_MMAPPED set.  */
>  static __always_inline size_t
>  mmap_size (mchunkptr p)
>  {
> -  return prev_size_mmap (p) + chunksize (p) + CHUNK_HDR_SZ;
> +  return mmap_base_offset (p) + chunksize (p) + CHUNK_HDR_SZ;
> +}
> +
> +/* Return a new chunk from an mmap.  */
> +static __always_inline mchunkptr
> +mmap_set_chunk (void *mmap_base, size_t mmap_size, size_t offset, bool is_hp)
> +{
> +  mchunkptr p = (mchunkptr) ((char *)mmap_base + offset);
> +  prev_size (p) = offset | (is_hp ? MMAP_HP : 0);
> +  set_head (p, (mmap_size - offset - CHUNK_HDR_SZ) | IS_MMAPPED);
> +  return p;
>  }
>  
>  /*
> @@ -2462,15 +2465,7 @@ sysmalloc_mmap (INTERNAL_SIZE_T nb, size_t pagesize, int extra_flags)
>  
>    __set_vma_name (mm, size, " glibc: malloc");
>  
> -  /* Store offset to start of mmap in prev_size.  */
> -  mchunkptr p = (mchunkptr) (mm + padding);
> -  set_prev_size (p, padding);
> -  set_head (p, (size - padding - CHUNK_HDR_SZ) | IS_MMAPPED);
> -
> -  /* Must also check whether huge pages were used in the mmap call
> -     and this is not the fallback call after using huge pages failed */
> -  if (__glibc_unlikely (extra_flags & mp_.hp_flags))
> -    set_mmap_is_hp (p);
> +  mchunkptr p = mmap_set_chunk (mm, size, padding, extra_flags != 0);
>  
>    /* update statistics */
>    int new = atomic_fetch_add_relaxed (&mp_.n_mmaps, 1) + 1;
> @@ -3049,8 +3044,9 @@ munmap_chunk (mchunkptr p)
>  static mchunkptr
>  mremap_chunk (mchunkptr p, size_t new_size)
>  {
> -  size_t pagesize = mmap_is_hp (p) ? mp_.hp_pagesize : GLRO (dl_pagesize);
> -  INTERNAL_SIZE_T offset = prev_size_mmap (p);
> +  bool is_hp = mmap_is_hp (p);
> +  size_t pagesize = is_hp ? mp_.hp_pagesize : GLRO (dl_pagesize);
> +  INTERNAL_SIZE_T offset = mmap_base_offset (p);
>    INTERNAL_SIZE_T size = chunksize (p);
>    char *cp;
>  
> @@ -3078,12 +3074,7 @@ mremap_chunk (mchunkptr p, size_t new_size)
>  
>    madvise_thp (cp, new_size);
>  
> -  p = (mchunkptr) (cp + offset);
> -
> -  assert (!misaligned_chunk (p));
> -
> -  assert (prev_size_mmap (p) == offset);
> -  set_head (p, (new_size - offset - CHUNK_HDR_SZ) | IS_MMAPPED);
> +  p = mmap_set_chunk (cp, new_size, offset, is_hp);
>  
>    INTERNAL_SIZE_T new;
>    new = atomic_fetch_add_relaxed (&mp_.mmapped_mem, new_size - size - offset)
>