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From: Jann Horn <>
To: Linus Torvalds <>
Cc: kernel test robot <>,
	Miklos Szeredi <>,
	LKML <>,, kernel test robot <>,
	"Huang, Ying" <>,
	Feng Tang <>,
	Zhengjun Xing <>,
Subject: Re: [fget] 054aa8d439: will-it-scale.per_thread_ops -5.7% regression
Date: Sat, 11 Dec 2021 00:29:32 +0100	[thread overview]
Message-ID: <> (raw)
In-Reply-To: <>

On Fri, Dec 10, 2021 at 10:59 PM Linus Torvalds
<> wrote:
> On Fri, Dec 10, 2021 at 1:25 PM Linus Torvalds
> <> wrote:
> >
> > We could make a special light-weight version of files_lookup_fd_raw(),
> > I guess. We don't need the *whole* "look it up again".  We don't need
> > to re-check the array bounds, and we don't need to do the nospec
> > lookup - we would have triggered a NULL file pointer if that happened
> > the first time around.
> >
> > So all we'd need to do is "check that fdt is the same, and check that
> > fdt->fd[fd] is the same".
> This is an ENTIRELY UNTESTED patch to do that.
> It basically rewrites __fget_files() from scratch: it really wants to
> do the fd array lookup by hand, in order to cache the intermediate fdt
> pointer, and in order to cache the intermediate speculation-safe fd
> array index etc.
> It's not a very complicated function, and rewriting it actually cleans
> up the loop to not need the ugly goto.
> I made it use a helper wrapper function for the rcu locking, so that
> the "meat" of the function can just use plain "return NULL" for the
> error cases.
> However, not only is it entirely untested, this rewrite also means
> that gcc has now decided that the result is so simple and clear that
> it will inline it into all the callers.
> I guess that's a good sign - writing the code in a way that makes the
> compiler say "now it's so trivial that it should be inlined" is
> certainly not a bad thing. But it makes it hard to really compare the
> asm.
> I did try a version with "noinline" just to make it more comparable,
> and hey, it all looked sane to me there too.
> I added more comments about what is going on.
> Again - this is UNTESTED. I've looked at the code, I've looked at the
> diff, and I've looked at the code it generates. It all looks fine to
> me. But I've looked at it so much that I suspect that I'd be entirely
> blind to any completely obvious bug by now.
> Comments?

One nit: The original implementation is using rcu_dereference_raw()
because it can run in different contexts, but here plain
rcu_dereference() would probably be more appropriate?

(I was wondering for a bit whether we should also change the
get_mm_exe_file() path, but I guess that's fine because it can only
ever happen for regular executable files and currently there's also no
path to pull out the mm->exe_file and use it for some other syscall?)

> Oliver, does this make any difference in the performance department?
>                  Linus

  reply	other threads:[~2021-12-10 23:30 UTC|newest]

Thread overview: 13+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-12-10  5:37 [fget] 054aa8d439: will-it-scale.per_thread_ops -5.7% regression kernel test robot
2021-12-10 18:33 ` Linus Torvalds
2021-12-10 20:29   ` Jann Horn
2021-12-10 21:25     ` Linus Torvalds
2021-12-10 21:59       ` Linus Torvalds
2021-12-10 23:29         ` Jann Horn [this message]
2021-12-11  1:01           ` Linus Torvalds
2021-12-11  1:32         ` Linus Torvalds
2021-12-13  8:31         ` [LKP] " Carel Si
2021-12-13 18:37           ` Linus Torvalds
2021-12-13 19:44             ` Linus Torvalds
2021-12-15 12:54               ` Greg KH
2021-12-13 10:57   ` Carel Si

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