linux-kernel - Re: [PATCH] epoll: try to be a _bit

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Date: Mon, 6 May 2024 10:45:27 +0200
From: Christian Brauner <brauner@...nel.org>
To: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Al Viro <viro@...iv.linux.org.uk>, keescook@...omium.org, 
	axboe@...nel.dk, christian.koenig@....com, dri-devel@...ts.freedesktop.org, 
	io-uring@...r.kernel.org, jack@...e.cz, laura@...bott.name, linaro-mm-sig@...ts.linaro.org, 
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org, linux-media@...r.kernel.org, 
	minhquangbui99@...il.com, sumit.semwal@...aro.org, 
	syzbot+045b454ab35fd82a35fb@...kaller.appspotmail.com, syzkaller-bugs@...glegroups.com
Subject: Re: [PATCH] epoll: try to be a _bit_ better about file lifetimes

> The fact is, it's not dma-buf that is violating any rules. It's epoll.

I agree that epoll() not taking a reference on the file is at least
unexpected and contradicts the usual code patterns for the sake of
performance and that it very likely is the case that most callers of
f_op->poll() don't know this.

Note, I cleary wrote upthread that I'm ok to do it like you suggested
but raised two concerns a) there's currently only one instance of
prolonged @file lifetime in f_op->poll() afaict and b) that there's
possibly going to be some performance impact on epoll().

So it's at least worth discussing what's more important because epoll()
is very widely used and it's not that we haven't favored performance
before.

But you've already said that you aren't concerned with performance on
epoll() upthread. So afaict then there's really not a lot more to
discuss other than take the patch and see whether we get any complaints.