From: Johannes Sixt <j6t@kdbg.org>
To: Johannes Schindelin <johannes.schindelin@gmx.de>
Cc: Junio C Hamano <gitster@pobox.com>,
git@vger.kernel.org, Sven Strickroth <email@cs-ware.de>
Subject: Re: [PATCH 2/3] mmap(win32): avoid copy-on-write when it is unnecessary
Date: Tue, 26 Apr 2016 20:53:23 +0200 [thread overview]
Message-ID: <571FB923.9040808@kdbg.org> (raw)
In-Reply-To: <3e2a45e60e2905f52f962604cf19a0e5e39b9b1b.1461335463.git.johannes.schindelin@gmx.de>
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 <johannes.schindelin@gmx.de>
> ---
> 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
next prev parent reply other threads:[~2016-04-26 18:53 UTC|newest]
Thread overview: 8+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-04-22 14:31 [PATCH 0/3] Improve the mmap() emulation on Windows Johannes Schindelin
2016-04-22 14:31 ` [PATCH 1/3] win32mmap: set errno appropriately Johannes Schindelin
2016-04-22 14:31 ` [PATCH 2/3] mmap(win32): avoid copy-on-write when it is unnecessary Johannes Schindelin
2016-04-26 18:53 ` Johannes Sixt [this message]
2016-04-27 6:43 ` Johannes Schindelin
2016-04-27 18:51 ` Johannes Sixt
2016-04-28 8:03 ` Johannes Schindelin
2016-04-22 14:31 ` [PATCH 3/3] mmap(win32): avoid expensive fstat() call Johannes Schindelin
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