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

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Date:   Wed, 22 Apr 2020 08:35:23 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Eric Dumazet' <eric.dumazet@...il.com>,
        Nate Karstens <nate.karstens@...min.com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        "Jeff Layton" <jlayton@...nel.org>,
        "J. Bruce Fields" <bfields@...ldses.org>,
        "Arnd Bergmann" <arnd@...db.de>,
        Richard Henderson <rth@...ddle.net>,
        "Ivan Kokshaysky" <ink@...assic.park.msu.ru>,
        Matt Turner <mattst88@...il.com>,
        "James E.J. Bottomley" <James.Bottomley@...senPartnership.com>,
        Helge Deller <deller@....de>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
        "linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
        "linux-alpha@...r.kernel.org" <linux-alpha@...r.kernel.org>,
        "linux-parisc@...r.kernel.org" <linux-parisc@...r.kernel.org>,
        "sparclinux@...r.kernel.org" <sparclinux@...r.kernel.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
CC:     Changli Gao <xiaosuo@...il.com>
Subject: RE: [PATCH 1/4] fs: Implement close-on-fork

From: Eric Dumazet
> Sent: 20 April 2020 11:26
> 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.
...
> 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.

How about only allocating the 'close on fork' bitmap the first time
a process sets a bit in it?

Off hand I can't imagine the use case.
I thought posix always shared fd tables across fork().

	David

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