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

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Message-ID: <90aca1e9-61b5-88d-d28c-369e6973559e@google.com>
Date:   Sun, 7 Feb 2021 18:18:01 -0800 (PST)
From:   David Rientjes <rientjes@...gle.com>
To:     "Song Bao Hua (Barry Song)" <song.bao.hua@...ilicon.com>
cc:     Matthew Wilcox <willy@...radead.org>,
        "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, 7 Feb 2021, Song Bao Hua (Barry Song) wrote:

> 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.
> 

What about only for mlocked memory, i.e. disable 
vm.compact_unevictable_allowed?

Adding syscalls is a big deal, we can make a reasonable inference that 
we'll have to support this forever if it's merged.  I haven't seen mention 
of what other unevictable memory *should* be migratable that would be 
adversely affected if we disable that sysctl.  Maybe that gets you part of 
the way there and there are some other deficiencies, but it seems like a 
good start would be to describe how CONFIG_NUMA_BALANCING=n + 
vm.compact_unevcitable_allowed + mlock() doesn't get you mostly there and 
then look into what's missing.

If it's a very compelling case where there simply are no alternatives, it 
would make sense.  Alternative is to find a more generic way, perhaps in 
combination with vm.compact_unevictable_allowed, to achieve what you're 
looking to do that can be useful even beyond your originally intended use 
case.