linux-hardening - Re: [PATCH -next] mm: usercopy: add a debugfs interface to bypass the vmalloc check.

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Message-ID: <Z1ALDmybeJfpIqge@pc636>
Date: Wed, 4 Dec 2024 08:55:58 +0100
From: Uladzislau Rezki <urezki@...il.com>
To: Matthew Wilcox <willy@...radead.org>,
	Kefeng Wang <wangkefeng.wang@...wei.com>, zuoze <zuoze1@...wei.com>
Cc: Uladzislau Rezki <urezki@...il.com>,
	Kefeng Wang <wangkefeng.wang@...wei.com>, zuoze <zuoze1@...wei.com>,
	gustavoars@...nel.org, akpm@...ux-foundation.org,
	linux-hardening@...r.kernel.org, linux-mm@...ck.org,
	keescook@...omium.org
Subject: Re: [PATCH -next] mm: usercopy: add a debugfs interface to bypass
 the vmalloc check.

On Tue, Dec 03, 2024 at 07:56:34PM +0000, Matthew Wilcox wrote:
> On Tue, Dec 03, 2024 at 08:02:26PM +0100, Uladzislau Rezki wrote:
> 
> I think there are a few other things we can try here.
> 
> First, if the copy is small (and I still don't have an answer to that
> ...), we can skip the vmalloc lookup if the copy doesn't cross a page
> boundary.
> 
> Second, we could try storing this in a maple tree rather than an rbtree.
> That gives us RCU protected lookups rather than under a spinlock.
> 
> It might even be worth going to a rwlock first, in case the problem is
> that there's severe lock contention.
> 
> But I've asked for data on spinlock contention and not received an
> answer on that either, so I don't know what to suggest.
> 
I think, it is not about contention. It is about the extra "attached
load" when a data is heavily copied force and back. On each copy path
you need to do a scan. Maple tree is not that something can help here :)

Indeed, no contention data. Zuoze, please share this if you can.

--
Uladzislau Rezki