Re: [PATCH] misc: Use allocate_once in getmntent

[Adhemerval Zanella <adhemerval dot zanella at linaro dot org>]

On 21/08/2019 11:40, Florian Weimer wrote:
> Both the buffer and struct mntent are now allocated on the heap.
> This results in a slight reduction of RSS usage.
> 
> 2019-08-21  Florian Weimer  <fweimer@redhat.com>
> 
> 	* misc/mntent.c (struct mntent_buffer): Define.
> 	(mntent_buffer): Adjust type to void *.
> 	(allocate): Adjust for allocate_once.
> 	(deallocate): New function.
> 	(getmntent): Call allocate_once.
> 
> diff --git a/misc/mntent.c b/misc/mntent.c
> index 980ea40967..8fae6064c6 100644
> --- a/misc/mntent.c
> +++ b/misc/mntent.c
> @@ -18,36 +18,41 @@
>  
>  #include <mntent.h>
>  #include <stdlib.h>
> -#include <libc-lock.h>
> +#include <allocate_once.h>
> +
> +struct mntent_buffer
> +{
> +  struct mntent m;
> +  char buffer[4096];
> +};

The struct mntent is just 24 bytes for 32 bits or 40 bytes for 64 bits,
I would focus on optimizing the required buffer instead. 

We need just to read just one line at the, so one option could be to move 
getmntent_r to misc/mntent.c and add a logic to call getline if the buffer
is equal a sentinel value, meaning it was called from getmnent. The
getline would be the one responsible to lock the stream and reallocate the
buffer if required, allowing having a buffer size with the maximum size
actually required.

>  
>  /* We don't want to allocate the static buffer all the time since it
> -   is not always used (in fact, rather infrequently).  Accept the
> -   extra cost of a `malloc'.  */
> -libc_freeres_ptr (static char *getmntent_buffer);
> -
> -/* This is the size of the buffer.  This is really big.  */
> -#define BUFFER_SIZE	4096
> +   is not always used (in fact, rather infrequently).  */
> +libc_freeres_ptr (static void *mntent_buffer);
>  
> +static void *
> +allocate (void *closure)
> +{
> +  return malloc (sizeof (struct mntent_buffer));
> +}
>  
>  static void
> -allocate (void)
> +deallocate (void *closure, void *ptr)
>  {
> -  getmntent_buffer = (char *) malloc (BUFFER_SIZE);
> +  free (ptr);
>  }
>  
> -
>  struct mntent *
>  getmntent (FILE *stream)
>  {
> -  static struct mntent m;
> -  __libc_once_define (static, once);
> -  __libc_once (once, allocate);
> -
> -  if (getmntent_buffer == NULL)
> +  struct mntent_buffer *buffer = allocate_once (&mntent_buffer,
> +						allocate, deallocate, NULL);
> +  if (buffer == NULL)
>      /* If no core is available we don't have a chance to run the
>         program successfully and so returning NULL is an acceptable
>         result.  */
>      return NULL;
>  
> -  return __getmntent_r (stream, &m, getmntent_buffer, BUFFER_SIZE);
> +  return __getmntent_r (stream, &buffer->m,
> +			buffer->buffer, sizeof (buffer->buffer));
>  }
>