linux-kernel - Re: [RFC PATCH 0/5] madvise MADV

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Date:   Mon, 16 Aug 2021 15:27:55 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     David Hildenbrand <david@...hat.com>
Cc:     Khalid Aziz <khalid.aziz@...cle.com>,
        "Longpeng (Mike, Cloud Infrastructure Service Product Dept.)" 
        <longpeng2@...wei.com>, Steven Sistare <steven.sistare@...cle.com>,
        Anthony Yznaga <anthony.yznaga@...cle.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "Gonglei (Arei)" <arei.gonglei@...wei.com>
Subject: Re: [RFC PATCH 0/5] madvise MADV_DOEXEC

On Mon, Aug 16, 2021 at 04:10:28PM +0200, David Hildenbrand wrote:
> > > > Until recently, the CPUs only having 4 1GB TLB entries.  I'm sure we
> > > > still have customers using that generation of CPUs.  2MB pages perform
> > > > better than 1GB pages on the previous generation of hardware, and I
> > > > haven't seen numbers for the next generation yet.
> > > 
> > > I read that somewhere else before, yet we have heavy 1 GiB page users,
> > > especially in the context of VMs and DPDK.
> > 
> > I wonder if those users actually benchmarked.  Or whether the memory
> > savings worked out so well for them that the loss of TLB performance
> > didn't matter.
> 
> These applications are extremely performance sensitive (i.e., RT workloads),

"real time does not mean real fast".  it means predictable latency.

> > > I will rephrase my previous statement "hugetlbfs just doesn't raise these
> > > problems because we are special casing it all over the place already". For
> > > example, not allowing to swap such pages. Disallowing MADV_DONTNEED. Special
> > > hugetlbfs locking.
> > 
> > Sure, that's why I want to drag this feature out of "oh this is a
> > hugetlb special case" and into "this is something Linux supports".
> 
> I would have understood the move to optimize SHMEM internally - similar to
> how we seem to optimize hugetlbfs SHMEM right now internally. (although
> sharing page tables for shmem can still be quite tricky)
> 
> I did not follow why we have to play games with MAP_PRIVATE, and having
> private anonymous pages shared between processes that don't COW, introducing
> new syscalls etc.

It's not about SHMEM, it's about file-backed pages on regular
filesystems.  I don't want to have XFS, ext4 and btrfs all with their
own implementations of ARCH_WANT_HUGE_PMD_SHARE.