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

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Message-ID: <Z06FAoq59x07-hoN@casper.infradead.org>
Date: Tue, 3 Dec 2024 04:11:46 +0000
From: Matthew Wilcox <willy@...radead.org>
To: Ze Zuo <zuoze1@...wei.com>
Cc: gustavoars@...nel.org, akpm@...ux-foundation.org,
	linux-hardening@...r.kernel.org, linux-mm@...ck.org,
	keescook@...omium.org, urezki@...il.com, wangkefeng.wang@...wei.com
Subject: Re: [PATCH -next] mm: usercopy: add a debugfs interface to bypass
 the vmalloc check.

On Tue, Dec 03, 2024 at 10:31:59AM +0800, Ze Zuo wrote:
> The commit 0aef499f3172 ("mm/usercopy: Detect vmalloc overruns") introduced
> vmalloc check for usercopy. However, in subsystems like networking, when
> memory allocated using vmalloc or vmap is subsequently copied using
> functions like copy_to_iter/copy_from_iter, the check is triggered. This
> adds overhead in the copy path, such as the cost of searching the
> red-black tree, which increases the performance burden.
> 
> We found that after merging this patch, network bandwidth performance in
> the XDP scenario significantly dropped from 25 Gbits/sec to 8 Gbits/sec,
> the hardened_usercopy is enabled by default.

What is "the XDP scenario", exactly?  Are these large or small packets?
What's taking the time in find_vmap_area()?  Is it lock contention?