Subject: Re: [PATCH 2/3] mmap(win32): avoid copy-on-write when it is unnecessary

From: Johannes Sixt <[email-masked]>
Date: Tue, 26 Apr 2016 20:53:23 +0200

Am 22.04.2016 um 16:31 schrieb Johannes Schindelin:
> Often we are mmap()ing read-only. In those cases, it is wasteful to map in
> copy-on-write mode. Even worse: it can cause errors where we run out of
> space in the page file.
>
> So let's be extra careful to map files in read-only mode whenever
> possible.
>
> Signed-off-by: Johannes Schindelin <[email-masked]>
> ---
>   compat/win32mmap.c | 5 +++--
>   1 file changed, 3 insertions(+), 2 deletions(-)
>
> diff --git a/compat/win32mmap.c b/compat/win32mmap.c
> index 3a39f0f..b836169 100644
> --- a/compat/win32mmap.c
> +++ b/compat/win32mmap.c
> @@ -22,14 +22,15 @@ void *git_mmap(void *start, size_t length, int prot, int flags, int fd, off_t of
>   		die("Invalid usage of mmap when built with USE_WIN32_MMAP");
>
>   	hmap = CreateFileMapping((HANDLE)_get_osfhandle(fd), NULL,
> -		PAGE_WRITECOPY, 0, 0, NULL);
> +		prot == PROT_READ ? PAGE_READONLY : PAGE_WRITECOPY, 0, 0, NULL);

As long as we use this implementation with MAP_PRIVATE, PAGE_WRITECOPY 
is the right setting. Should we insert a check for MAP_PRIVATE to catch 
unexpected use-cases (think of the index-helper daemon effort)?

>
>   	if (!hmap) {
>   		errno = EINVAL;
>   		return MAP_FAILED;
>   	}
>
> -	temp = MapViewOfFileEx(hmap, FILE_MAP_COPY, h, l, length, start);
> +	temp = MapViewOfFileEx(hmap, prot == PROT_READ ?
> +			FILE_MAP_READ : FILE_MAP_COPY, h, l, length, start);

Same here: FILE_MAP_COPY is the right choice for MAP_SHARED mmaps.

>
>   	if (!CloseHandle(hmap))
>   		warning("unable to close file mapping handle");
>

Except for these mental notes, I've no comments on this series. Looks good.

-- Hannes