linux-kernel - Re: [Issue] mprotect+madvise may be better than mmap for permission changes and page zeroing

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Message-ID: <c9b4dfc0-19b9-4e19-b681-e2fab63f41e8@redhat.com>
Date: Thu, 25 Jan 2024 13:44:36 +0100
From: David Hildenbrand <david@...hat.com>
To: Lance Yang <ioworker0@...il.com>, linux-kernel@...r.kernel.org
Cc: Michal Hocko <mhocko@...e.com>, linux-mm@...ck.org
Subject: Re: [Issue] mprotect+madvise may be better than mmap for permission
 changes and page zeroing

On 25.01.24 13:28, Lance Yang wrote:
> Hello Everyone,
> 
> I've noticed that using mprotect(PROT_NONE) with
> madvise(MADV_DONTNEED) is much faster than
> mmap(PROT_NONE, MAP_FIXED) alone for changing
> permissions and zeroing pages.
> 
> I have maintained a chunk-allocator internally at the
> company. It allocates a chunk using
> mmap(PROT_READ|PROT_WRITE, MAP_ANON|MAP_PRIVATE, -1, 0),
> releases a chunk using mmap(PROT_NONE, MAP_ANON|MAP_FIXED|MAP_PRIVATE, -1, 0),
> and reuses a chunk using mprotect(PROT_READ|PROT_WRITE).
> Recently, I replaced mmap(PROT_NONE, MAP_FIXED) with
> mprotect(PROT_NONE) + madvise(MADV_DONTNEED) to
> reduce the latency of releasing chunks.
> 
> Test code:
> https://github.com/ioworker0/mmapvsmprotect/blob/main/test2.c
> 
> Here are the test results on my machine:
> CPU: AMD EPYC 7R13 Processor
> Kernel: 6.2.0
> Elapsed Time for mprotect+madvise: 3670 nanoseconds
> Elapsed Time for mmap: 5520 nanoseconds

mprotect+madvise won't free page tables, mmap will. That's the biggest 
difference.

-- 
Cheers,

David / dhildenb