linux-kernel - Re: [RFC PATCH v3 1/2] mempinfd: Add new syscall to provide memory pin

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Date:   Mon, 8 Feb 2021 01:30:56 +0000
From:   Matthew Wilcox <willy@...radead.org>
To:     "Song Bao Hua (Barry Song)" <song.bao.hua@...ilicon.com>
Cc:     "Wangzhou (B)" <wangzhou1@...ilicon.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "linux-api@...r.kernel.org" <linux-api@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        "gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
        "jgg@...pe.ca" <jgg@...pe.ca>,
        "kevin.tian@...el.com" <kevin.tian@...el.com>,
        "jean-philippe@...aro.org" <jean-philippe@...aro.org>,
        "eric.auger@...hat.com" <eric.auger@...hat.com>,
        "Liguozhu (Kenneth)" <liguozhu@...ilicon.com>,
        "zhangfei.gao@...aro.org" <zhangfei.gao@...aro.org>,
        "chensihang (A)" <chensihang1@...ilicon.com>
Subject: Re: [RFC PATCH v3 1/2] mempinfd: Add new syscall to provide memory
 pin

On Sun, Feb 07, 2021 at 10:24:28PM +0000, Song Bao Hua (Barry Song) wrote:
> > > In high-performance I/O cases, accelerators might want to perform
> > > I/O on a memory without IO page faults which can result in dramatically
> > > increased latency. Current memory related APIs could not achieve this
> > > requirement, e.g. mlock can only avoid memory to swap to backup device,
> > > page migration can still trigger IO page fault.
> > 
> > Well ... we have two requirements.  The application wants to not take
> > page faults.  The system wants to move the application to a different
> > NUMA node in order to optimise overall performance.  Why should the
> > application's desires take precedence over the kernel's desires?  And why
> > should it be done this way rather than by the sysadmin using numactl to
> > lock the application to a particular node?
> 
> NUMA balancer is just one of many reasons for page migration. Even one
> simple alloc_pages() can cause memory migration in just single NUMA
> node or UMA system.
> 
> The other reasons for page migration include but are not limited to:
> * memory move due to CMA
> * memory move due to huge pages creation
> 
> Hardly we can ask users to disable the COMPACTION, CMA and Huge Page
> in the whole system.

You're dodging the question.  Should the CMA allocation fail because
another application is using SVA?

I would say no.  The application using SVA should take the one-time
performance hit from having its memory moved around.