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

Karstens, Nate
2020-05-01T14:45:23Z

Eric,

Thanks for the suggestion. I looked into it and noticed that do_close_on_exec() appears to have some optimizations as well:

> set = fdt->close_on_exec[i];
> if (!set)
> continue;

If we interleave the close-on-exec and close-on-fork flags then this optimization will have to be removed. Do you have a sense of which optimization provides the most benefit?

I noticed a couple of other issues with the original patch that I will need to investigate or rework:

1) I'm not sure dup_fd() is the best place to check the close-on-fork flag. For example, the ksys_unshare() > unshare_fd() > dup_fd() execution path seems suspect. I will either add a parameter to the function indicating if the flag should be checked or do a separate function, like do_close_on_fork().
2) If the close-on-fork flag is set, then __clear_open_fd() should be called instead of just __clear_bit(). This will ensure that fdt->full_fds_bits() is updated.
3) Need to investigate if the close-on-fork (or close-on-exec) flags need to be cleared when the file is closed as part of the close-on-fork execution path.

Others -- I will respond to feedback outside of implementation details in a separate message.

Thanks,

Nate

-----Original Message-----
From: Eric Dumazet <eric.dumazet@gmail.com>
Sent: Monday, April 20, 2020 05:26
To: Karstens, Nate <Nate.Karstens@garmin.com>; Alexander Viro <viro@zeniv.linux.org.uk>; Jeff Layton <jlayton@kernel.org>; J. Bruce Fields <bfields@fieldses.org>; Arnd Bergmann <arnd@arndb.de>; Richard Henderson <rth@twiddle.net>; Ivan Kokshaysky <ink@jurassic.park.msu.ru>; Matt Turner <mattst88@gmail.com>; James E.J. Bottomley <James.Bottomley@HansenPartnership.com>; Helge Deller <deller@gmx.de>; David S. Miller <davem@davemloft.net>; Jakub Kicinski <kuba@kernel.org>; linux-fsdevel@vger.kernel.org; linux-arch@vger.kernel.org; linux-alpha@vger.kernel.org; linux-parisc@vger.kernel.org; sparclinux@vger.kernel.org; netdev@vger.kernel.org; linux-kernel@vger.kernel.org
Cc: Changli Gao <xiaosuo@gmail.com>
Subject: Re: [PATCH 1/4] fs: Implement close-on-fork

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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.