Re: [PATCH 1/4] fs: Implement close-on-fork

Eric Dumazet
2020-04-20T10:25:54Z

On 4/20/20 12:15 AM, Nate Karstens wrote:
> The close-on-fork flag causes the file descriptor to be closed
> atomically in the child process before the child process returns
> from fork(). Implement this feature and provide a method to
> get/set the close-on-fork flag using fcntl(2).
>
> This functionality was approved by the Austin Common Standards
> Revision Group for inclusion in the next revision of the POSIX
> standard (see issue 1318 in the Austin Group Defect Tracker).

Oh well... yet another feature slowing down a critical path.

>
> Co-developed-by: Changli Gao <xiaosuo@gmail.com>
> Signed-off-by: Changli Gao <xiaosuo@gmail.com>
> Signed-off-by: Nate Karstens <nate.karstens@garmin.com>
> ---
> fs/fcntl.c | 2 ++
> fs/file.c | 50 +++++++++++++++++++++++++-
> include/linux/fdtable.h | 7 ++++
> include/linux/file.h | 2 ++
> include/uapi/asm-generic/fcntl.h | 5 +--
> tools/include/uapi/asm-generic/fcntl.h | 5 +--
> 6 files changed, 66 insertions(+), 5 deletions(-)
>
> diff --git a/fs/fcntl.c b/fs/fcntl.c
> index 2e4c0fa2074b..23964abf4a1a 100644
> --- a/fs/fcntl.c
> +++ b/fs/fcntl.c
> @@ -335,10 +335,12 @@ static long do_fcntl(int fd, unsigned int cmd, unsigned long arg,
> break;
> case F_GETFD:
> err = get_close_on_exec(fd) ? FD_CLOEXEC : 0;
> + err |= get_close_on_fork(fd) ? FD_CLOFORK : 0;
> break;
> case F_SETFD:
> err = 0;
> set_close_on_exec(fd, arg & FD_CLOEXEC);
> + set_close_on_fork(fd, arg & FD_CLOFORK);
> break;
> case F_GETFL:
> err = filp->f_flags;
> diff --git a/fs/file.c b/fs/file.c
> index c8a4e4c86e55..de7260ba718d 100644
> --- a/fs/file.c
> +++ b/fs/file.c
> @@ -57,6 +57,8 @@ static void copy_fd_bitmaps(struct fdtable *nfdt, struct fdtable *ofdt,
> memset((char *)nfdt->open_fds + cpy, 0, set);
> memcpy(nfdt->close_on_exec, ofdt->close_on_exec, cpy);
> memset((char *)nfdt->close_on_exec + cpy, 0, set);
> + memcpy(nfdt->close_on_fork, ofdt->close_on_fork, cpy);
> + memset((char *)nfdt->close_on_fork + cpy, 0, set);
>

I suggest we group the two bits of a file (close_on_exec, close_on_fork) together,
so that we do not have to dirty two separate cache lines.

Otherwise we will add yet another cache line miss at every file opening/closing for processes
with big file tables.

Ie having a _single_ bitmap array, even bit for close_on_exec, odd bit for close_on_fork

static inline void __set_close_on_exec(unsigned int fd, struct fdtable *fdt)
{
__set_bit(fd * 2, fdt->close_on_fork_exec);
}

static inline void __set_close_on_fork(unsigned int fd, struct fdtable *fdt)
{
__set_bit(fd * 2 + 1, fdt->close_on_fork_exec);
}

Also the F_GETFD/F_SETFD implementation must use a single function call,
to not acquire the spinlock twice.