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Date: Tue, 28 Nov 2023 16:52:31 +0100
From: Christian Brauner <brauner@kernel.org>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: kernel test robot <oliver.sang@intel.com>, oe-lkp@lists.linux.dev,
	lkp@intel.com, linux-kernel@vger.kernel.org,
	Jann Horn <jannh@google.com>, linux-doc@vger.kernel.org,
	linuxppc-dev@lists.ozlabs.org, intel-gfx@lists.freedesktop.org,
	linux-fsdevel@vger.kernel.org, gfs2@lists.linux.dev,
	bpf@vger.kernel.org, ying.huang@intel.com, feng.tang@intel.com,
	fengwei.yin@intel.com
Subject: Re: [linus:master] [file] 0ede61d858: will-it-scale.per_thread_ops
 -2.9% regression
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On Mon, Nov 27, 2023 at 09:10:54AM -0800, Linus Torvalds wrote:
> On Mon, 27 Nov 2023 at 02:27, Christian Brauner <brauner@kernel.org> wrote:
> >
> > So I've picked up your patch (vfs.misc). It's clever alright so thanks
> > for the comments in there otherwise I would've stared at this for far
> > too long.
> 
> Note that I should probably have commented on one other thing: that
> whole "just load from fd[0] is always safe, because the fd[] array
> always exists".

I added a comment to that effect in the code.

> 
> IOW, that whole "load and mask" thing only works when you know the
> array exists at all.
> 
> Doing that "just mask the index" wouldn't be valid if "size = 0" is an
> option and might mean that we don't have an array at all (ie if "->fd"
> itself could be NULL.
> 
> But we never have a completely empty file descriptor array, and
> fdp->fd is never NULL.  At a minimum 'max_fds' is NR_OPEN_DEFAULT.
> 
> (The whole 'tsk->files' could be NULL, but only for kernel threads or
> when exiting, so fget_task() will check for *that*, but it's a
> separate thing)

Yep.

> 
> So that's why it's safe to *entirely* remove the whole
> 
>                 if (unlikely(fd >= fdt->max_fds))
> 
> test, and do it *all* with just "mask the index, and mask the resulting load".

Yep.