linux-kernel - Re: [PATCH v2 0/2] close

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Message-ID: <20190524183903.GB2658@avx2>
Date:   Fri, 24 May 2019 21:39:03 +0300
From:   Alexey Dobriyan <adobriyan@...il.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Christian Brauner <christian@...uner.io>,
        Linux List Kernel Mailing <linux-kernel@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>
Subject: Re: [PATCH v2 0/2] close_range()

On Thu, May 23, 2019 at 02:34:31PM -0700, Linus Torvalds wrote:
> On Thu, May 23, 2019 at 11:22 AM Alexey Dobriyan <adobriyan@...il.com> wrote:
> >
> > > This is v2 of this patchset.
> >
> > We've sent fdmap(2) back in the day:
> 
> Well, if the main point of the exercise is performance, then fdmap()
> is clearly inferior.

This is not true because there are other usecases.

Current equivalent is readdir() where getdents is essentially bulk fdmap()
with pretty-printing. glibc does getdents into 32KB buffer.

There was a bulk taskstats patch long before meltdown fiasco.

Unfortunately closerange() only closes ranges.
This is why I didn't even tried to send closefrom(2) from OpenBSD.

> Sadly, with all the HW security mitigation, system calls are no longer cheap.
> 
> Would there ever be any other reason to traverse unknown open files
> than to close them?

This is what lsof(1) does:

3140  openat(AT_FDCWD, "/proc/29499/fd", O_RDONLY|O_NONBLOCK|O_CLOEXEC|O_DIRECTORY) = 4
3140  fstat(4, {st_mode=S_IFDIR|0500, st_size=0, ...}) = 0
3140  getdents(4, /* 6 entries */, 32768) = 144
3140  readlink("/proc/29499/fd/0", "/dev/pts/4", 4096) = 10
3140  lstat("/proc/29499/fd/0", {st_mode=S_IFLNK|0700, st_size=64, ...}) = 0
3140  stat("/proc/29499/fd/0", {st_mode=S_IFCHR|0600, st_rdev=makedev(136, 4), ...}) = 0
3140  openat(AT_FDCWD, "/proc/29499/fdinfo/0", O_RDONLY) = 7
3140  fstat(7, {st_mode=S_IFREG|0400, st_size=0, ...}) = 0
3140  read(7, "pos:\t0\nflags:\t02002\nmnt_id:\t24\n", 1024) = 31
3140  read(7, "", 1024)                 = 0
3140  close(7)
	...

Once fdmap(2) or equivalent is in, more bulk system calls operating on
descriptors can pop up. But closefrom() will remain closefrom().