netdev - Re: [RFC v2 01/13] mm: export zap_page

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Message-ID: <20201223081324.GA21558@infradead.org>
Date:   Wed, 23 Dec 2020 08:13:24 +0000
From:   Christoph Hellwig <hch@...radead.org>
To:     Yongji Xie <xieyongji@...edance.com>
Cc:     Christoph Hellwig <hch@...radead.org>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Jason Wang <jasowang@...hat.com>,
        Stefan Hajnoczi <stefanha@...hat.com>, sgarzare@...hat.com,
        Parav Pandit <parav@...dia.com>, akpm@...ux-foundation.org,
        Randy Dunlap <rdunlap@...radead.org>,
        Matthew Wilcox <willy@...radead.org>, viro@...iv.linux.org.uk,
        axboe@...nel.dk, bcrl@...ck.org, corbet@....net,
        virtualization@...ts.linux-foundation.org, netdev@...r.kernel.org,
        kvm@...r.kernel.org, linux-aio@...ck.org,
        linux-fsdevel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [RFC v2 01/13] mm: export zap_page_range() for driver use

On Wed, Dec 23, 2020 at 02:32:07PM +0800, Yongji Xie wrote:
> Now I want to map/unmap some pages in an userland vma dynamically. The
> vm_insert_page() is being used for mapping. In the unmapping case, it
> looks like the zap_page_range() does what I want. So I export it.
> Otherwise, we need some ways to notify userspace to trigger it with
> madvise(MADV_DONTNEED), which might not be able to meet all our needs.
> For example, unmapping some pages in a memory shrinker function.
> 
> So I'd like to know what's the limitation to use zap_page_range() in a
> module. And if we can't use it in a module, is there any acceptable
> way to achieve that?

I think the anser is: don't play funny games with unmapped outside of
munmap.  Especially as synchronization is very hard to get right.