linux-kernel - Re: [PATCH v2 15/24] proc/fd: In proc_readfd_common use task_lookup_next_fd

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Date:   Mon, 7 Dec 2020 23:29:00 +0000
From:   Al Viro <viro@...iv.linux.org.uk>
To:     "Eric W. Biederman" <ebiederm@...ssion.com>
Cc:     linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        criu@...nvz.org, bpf@...r.kernel.org,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Christian Brauner <christian.brauner@...ntu.com>,
        Oleg Nesterov <oleg@...hat.com>,
        Cyrill Gorcunov <gorcunov@...il.com>,
        Jann Horn <jann@...jh.net>, Kees Cook <keescook@...omium.org>,
        Daniel P . Berrangé <berrange@...hat.com>,
        Jeff Layton <jlayton@...hat.com>,
        Miklos Szeredi <miklos@...redi.hu>,
        Matthew Wilcox <willy@...radead.org>,
        "J. Bruce Fields" <bfields@...ldses.org>,
        Trond Myklebust <trond.myklebust@...merspace.com>,
        Chris Wright <chrisw@...hat.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        Andrii Nakryiko <andriin@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...omium.org>,
        Andy Lavr <andy.lavr@...il.com>
Subject: Re: [PATCH v2 15/24] proc/fd: In proc_readfd_common use
 task_lookup_next_fd_rcu

On Fri, Nov 20, 2020 at 05:14:32PM -0600, Eric W. Biederman wrote:
> When discussing[1] exec and posix file locks it was realized that none
> of the callers of get_files_struct fundamentally needed to call
> get_files_struct, and that by switching them to helper functions
> instead it will both simplify their code and remove unnecessary
> increments of files_struct.count.  Those unnecessary increments can
> result in exec unnecessarily unsharing files_struct which breaking
> posix locks, and it can result in fget_light having to fallback to
> fget reducing system performance.
> 
> Using task_lookup_next_fd_rcu simplifies proc_readfd_common, by moving
> the checking for the maximum file descritor into the generic code, and
> by remvoing the need for capturing and releasing a reference on
> files_struct.
> 
> As task_lookup_fd_rcu may update the fd ctx->pos has been changed
> to be the fd +2 after task_lookup_fd_rcu returns.


> +	for (fd = ctx->pos - 2;; fd++) {
>  		struct file *f;
>  		struct fd_data data;
>  		char name[10 + 1];
>  		unsigned int len;
>  
> -		f = files_lookup_fd_rcu(files, fd);
> +		f = task_lookup_next_fd_rcu(p, &fd);

Ugh...  That makes for a massive cacheline pingpong on task_lock -
instead of grabbing/dropping task_lock() once in the beginning, we do
that for every damn descriptor.

I really don't like this one.  If anything, I would rather have
a helper that would collect a bunch of pairs (fd,mode) into an
array and have lookups batched into it.  With the loop in that
sucker grabbing a reasonable amount into a local array, then
doing proc_fill_cache() for each collected.