Subject: Re: [PATCH v3 2/3] Introduce GIT_MMAP_LIMIT to allow testing expected mmap size

From: Junio C Hamano <[email-masked]>
Date: Thu, 21 Aug 2014 15:26:27 -0700

Steffen Prohaska <[email-masked]> writes:

> Similar to testing expectations about malloc with GIT_ALLOC_LIMIT (see
> commit d41489), it can be useful to test expectations about mmap.
>
> This introduces a new environment variable GIT_MMAP_LIMIT to limit the
> largest allowed mmap length (in KB).  xmmap() is modified to check the
> limit.  Together with GIT_ALLOC_LIMIT tests can now easily confirm
> expectations about memory consumption.
>
> GIT_ALLOC_LIMIT will be used in the next commit to test that data will

I smell the need for s/ALLOC/MMAP/ here, but perhaps you did mean
ALLOC (I won't know until I check 3/3 ;-)

> be streamed to an external filter without mmaping the entire file.
>
> [commit d41489]: d41489a6424308dc9a0409bc2f6845aa08bd4f7d Add more large
>     blob test cases
>
> Signed-off-by: Steffen Prohaska <[email-masked]>
> ---
>  sha1_file.c | 17 ++++++++++++++++-
>  1 file changed, 16 insertions(+), 1 deletion(-)
>
> diff --git a/sha1_file.c b/sha1_file.c
> index 00c07f2..88d64c0 100644
> --- a/sha1_file.c
> +++ b/sha1_file.c
> @@ -663,10 +663,25 @@ void release_pack_memory(size_t need)
>  		; /* nothing */
>  }
>  
> +static void mmap_limit_check(size_t length)
> +{
> +	static int limit = -1;

Perhaps you want ssize_t here?  I see mmap() as a tool to handle a
lot more data than a single malloc() typically would ;-) so previous
mistakes by other people would not be a good excuse.

> +	if (limit == -1) {
> +		const char *env = getenv("GIT_MMAP_LIMIT");
> +		limit = env ? atoi(env) * 1024 : 0;
> +	}
> +	if (limit && length > limit)
> +		die("attempting to mmap %"PRIuMAX" over limit %d",
> +		    (intmax_t)length, limit);
> +}
> +
>  void *xmmap(void *start, size_t length,
>  	int prot, int flags, int fd, off_t offset)
>  {
> -	void *ret = mmap(start, length, prot, flags, fd, offset);
> +	void *ret;
> +
> +	mmap_limit_check(length);
> +	ret = mmap(start, length, prot, flags, fd, offset);
>  	if (ret == MAP_FAILED) {
>  		if (!length)
>  			return NULL;