linux-kernel - Re: [PATCH 0/2] KVM: do not assume PTE is writable after follow

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Message-ID: <20210208220259.GA71523@xz-x1>
Date:   Mon, 8 Feb 2021 17:02:59 -0500
From:   Peter Xu <peterx@...hat.com>
To:     Jason Gunthorpe <jgg@...pe.ca>
Cc:     dan.j.williams@...el.com, Paolo Bonzini <pbonzini@...hat.com>,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 0/2] KVM: do not assume PTE is writable after follow_pfn

On Mon, Feb 08, 2021 at 02:51:33PM -0400, Jason Gunthorpe wrote:
> On Fri, Feb 05, 2021 at 01:14:11PM -0500, Peter Xu wrote:
> 
> > But I do have a question on why dax as the only user needs to pass in the
> > notifier to follow_pte() for initialization.
> 
> Not sure either, why does DAX opencode something very much like
> page_mkclean() with dax_entry_mkclean()?
> 
> Also it looks like DAX uses the wrong notifier, it calls
> MMU_NOTIFY_CLEAR but page_mkclean_one() uses
> MMU_NOTIFY_PROTECTION_PAGE for the same PTE modification sequence??
> 
> page_mkclean() has some technique to make the notifier have the right
> size without becoming entangled in the PTL locks..

Right.  I guess it's because dax doesn't have "struct page*" on the back, so it
can't directly use page_mkclean().  However the whole logic does look very
similar, so maybe they can be merged in some way.

Thanks,

-- 
Peter Xu