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

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Message-ID: <20210209002314.GC78818@xz-x1>
Date:   Mon, 8 Feb 2021 19:23:14 -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 07:26:25PM -0400, Jason Gunthorpe wrote:
> On Mon, Feb 08, 2021 at 05:02:59PM -0500, Peter Xu wrote:
> > 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
> 
> It doesn't? I thought DAX cases did?

I'm not familiar with dax at all.. but it seems so: e.g. dax_iomap_pte_fault()
looks like the general fault handler for dax mappings, in which there's calls
to things like vmf_insert_mixed_mkwrite() trying to install ptes with raw pfn.
Or I could also be missing something very important..  Thanks,

-- 
Peter Xu