linux-kernel - Re: [linus:master] [file] 0ede61d858: will-it-scale.per_thread

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Message-ID: <20231128-serpentinen-sinnieren-e186ea8742e9@brauner>
Date:   Tue, 28 Nov 2023 16:52:31 +0100
From:   Christian Brauner <brauner@...nel.org>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     kernel test robot <oliver.sang@...el.com>, oe-lkp@...ts.linux.dev,
        lkp@...el.com, linux-kernel@...r.kernel.org,
        Jann Horn <jannh@...gle.com>, linux-doc@...r.kernel.org,
        linuxppc-dev@...ts.ozlabs.org, intel-gfx@...ts.freedesktop.org,
        linux-fsdevel@...r.kernel.org, gfs2@...ts.linux.dev,
        bpf@...r.kernel.org, ying.huang@...el.com, feng.tang@...el.com,
        fengwei.yin@...el.com
Subject: Re: [linus:master] [file] 0ede61d858: will-it-scale.per_thread_ops
 -2.9% regression

On Mon, Nov 27, 2023 at 09:10:54AM -0800, Linus Torvalds wrote:
> On Mon, 27 Nov 2023 at 02:27, Christian Brauner <brauner@...nel.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.