linux-kernel - Re: [RFC PATCH 0/6] Add support for shared PTEs across processes

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Message-ID: <YfBIpmxvc0+mFByf@casper.infradead.org>
Date:   Tue, 25 Jan 2022 18:59:50 +0000
From:   Matthew Wilcox <willy@...radead.org>
To:     "Kirill A. Shutemov" <kirill@...temov.name>
Cc:     Khalid Aziz <khalid.aziz@...cle.com>, akpm@...ux-foundation.org,
        longpeng2@...wei.com, arnd@...db.de, dave.hansen@...ux.intel.com,
        david@...hat.com, rppt@...nel.org, surenb@...gle.com,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [RFC PATCH 0/6] Add support for shared PTEs across processes

On Tue, Jan 25, 2022 at 09:57:05PM +0300, Kirill A. Shutemov wrote:
> On Tue, Jan 25, 2022 at 02:09:47PM +0000, Matthew Wilcox wrote:
> > > I think zero-API approach (plus madvise() hints to tweak it) is worth
> > > considering.
> > 
> > I think the zero-API approach actually misses out on a lot of
> > possibilities that the mshare() approach offers.  For example, mshare()
> > allows you to mmap() many small files in the shared region -- you
> > can't do that with zeroAPI.
> 
> Do you consider a use-case for many small files to be common? I would
> think that the main consumer of the feature to be mmap of huge files.
> And in this case zero enabling burden on userspace side sounds like a
> sweet deal.

mmap() of huge files is certainly the Oracle use-case.  With occasional
funny business like mprotect() of a single page in the middle of a 1GB
hugepage.

The approach of designating ranges of a process's address space as
sharable with other processes felt like the cleaner & frankly more
interesting approach that opens up use-cases other than "hurr, durr, we
are Oracle, we like big files, kernel get out of way now, transactions
to perform".