linux-kernel - Re: [RFC v3 PATCH 4/5] mm: mmap: zap pages with read mmap

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Message-Id: <20180629201547.5322cfc4b52d19a0443daec2@linux-foundation.org>
Date:   Fri, 29 Jun 2018 20:15:47 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Yang Shi <yang.shi@...ux.alibaba.com>
Cc:     mhocko@...nel.org, willy@...radead.org, ldufour@...ux.vnet.ibm.com,
        peterz@...radead.org, mingo@...hat.com, acme@...nel.org,
        alexander.shishkin@...ux.intel.com, jolsa@...hat.com,
        namhyung@...nel.org, tglx@...utronix.de, hpa@...or.com,
        linux-mm@...ck.org, x86@...nel.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC v3 PATCH 4/5] mm: mmap: zap pages with read mmap_sem for
 large mapping

On Fri, 29 Jun 2018 19:28:15 -0700 Yang Shi <yang.shi@...ux.alibaba.com> wrote:

> 
> 
> > we're adding a bunch of code to 32-bit kernels which will never be
> > executed.
> >
> > I'm thinking it would be better to be much more explicit with "#ifdef
> > CONFIG_64BIT" in this code, rather than relying upon the above magic.
> >
> > But I tend to think that the fact that we haven't solved anything on
> > locked vmas or on uprobed mappings is a shostopper for the whole
> > approach :(
> 
> I agree it is not that perfect. But, it still could improve the most use 
> cases.

Well, those unaddressed usecases will need to be fixed at some point. 
What's our plan for that?

Would one of your earlier designs have addressed all usecases?  I
expect the dumb unmap-a-little-bit-at-a-time approach would have?

> For the locked vmas and hugetlb vmas, unmapping operations need modify 
> vm_flags. But, I'm wondering we might be able to separate unmap and 
> vm_flags update. Because we know they will be unmapped right away, the 
> vm_flags might be able to be updated in write mmap_sem critical section 
> before the actual unmap is called or after it. This is just off the top 
> of my head.
> 
> For uprobed mappings, I'm not sure how vital it is to this case.
> 
> Thanks,
> Yang
> 
> >