linux-kernel - Re: [PATCH RFC v2 0/4] Add support for sharing page tables across processes (Previously mshare)

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Date:   Mon, 31 Jul 2023 18:48:47 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Rongwei Wang <rongwei.wang@...ux.alibaba.com>,
        linux-arch@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org,
        "xuyu@...ux.alibaba.com" <xuyu@...ux.alibaba.com>
Subject: Re: [PATCH RFC v2 0/4] Add support for sharing page tables across
 processes (Previously mshare)

On 31.07.23 18:38, Matthew Wilcox wrote:
> On Mon, Jul 31, 2023 at 06:30:22PM +0200, David Hildenbrand wrote:
>> Assume we do do the page table sharing at mmap time, if the flags are right.
>> Let's focus on the most common:
>>
>> mmap(memfd, PROT_READ | PROT_WRITE, MAP_SHARED)
>>
>> And doing the same in each and every process.
> 
> That may be the most common in your usage, but for a database, you're
> looking at two usage scenarios.  Postgres calls mmap() on the database
> file itself so that all processes share the kernel page cache.
> Some Commercial Databases call mmap() on a hugetlbfs file so that all
> processes share the same userspace buffer cache.  Other Commecial
> Databases call shmget() / shmat() with SHM_HUGETLB for the exact
> same reason.

I remember you said that postgres might be looking into using shmem as 
well, maybe I am wrong.

memfd/hugetlb/shmem could all be handled alike, just "arbitrary 
filesystems" would require more work.

> 
> This is why I proposed mshare().  Anyone can use it for anything.
> We have such a diverse set of users who want to do stuff with shared
> page tables that we should not be tying it to memfd or any other
> filesystem.  Not to mention that it's more flexible; you can map
> individual 4kB files into it and still get page table sharing.

That's not what the current proposal does, or am I wrong?

Also, I'm curious, is that a real requirement in the database world?

-- 
Cheers,

David / dhildenb