linux-kernel - Re: [PATCH v12 04/20] mm: Allow page fault handlers to perform the COW

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Date:	Sun, 8 Feb 2015 13:48:14 +0200
From:	Yigal Korman <yigal@...xistor.com>
To:	Matthew Wilcox <willy@...ux.intel.com>
Cc:	Matthew Wilcox <matthew.r.wilcox@...el.com>,
	linux-fsdevel@...r.kernel.org,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH v12 04/20] mm: Allow page fault handlers to perform the COW

On Thu, Feb 5, 2015 at 11:39 PM, Matthew Wilcox <willy@...ux.intel.com> wrote:
>
> On Thu, Feb 05, 2015 at 11:16:53AM +0200, Yigal Korman wrote:
> > I have a question on a related issue (I think).
> > I've noticed that for pfn-only mappings (VM_FAULT_NOPAGE)
> > do_shared_fault only maps the pfn with r/o permissions.
> > So if I use DAX to write the mmap()-ed pfn I get two faults - first
> > handled by do_shared_fault and then again for making it r/w in
> > do_wp_page.
> > Is this simply a missing optimization like was done here with the
> > cow_page? or am I missing something?
>
> I have also noticed this behaviour.  I tracked down why it's happening:
>
> DAX calls:
>         error = vm_insert_mixed(vma, vaddr, pfn);
> which calls:
>         return insert_pfn(vma, addr, pfn, vma->vm_page_prot);
>
> If you insert some debugging, you'll notice here that vm_page_prot does
> not include PROT_WRITE.
>
> That got cleared during mmap_region() where it does:
>
>         if (vma_wants_writenotify(vma)) {
>                 pgprot_t pprot = vma->vm_page_prot;
> ...
>                 vma->vm_page_prot = vm_get_page_prot(vm_flags & ~VM_SHARED);
>
>
> And why do we want writenotify (according to the VM)?  Because we have:
>
>         /* The backer wishes to know when pages are first written to? */
>         if (vma->vm_ops && vma->vm_ops->page_mkwrite)
>                 return 1;
>
> We don't really want to be notified on a first write; we want the page to be
> inserted write-enabled.  But in the case where we've covered a hole with a
> read-only zero page, we need to be notified so we can allocate a page of
> storage.
>
> So, how to fix?  We could adjust vm_page_prot to include PROT_WRITE.
> I think that should work, since we'll only insert zeroed pages for read
> faults, and so the maybe_mkwrite() won't be called in do_set_pte().
> I'm just not entirely sure where to set it.  Perhaps a MM person could
> make a helpful suggestion?

I was thinking that do_shared_fault should simply call maybe_mkwrite()
in case of VM_FAULT_NOPAGE.
I think it's what do_wp_page does afterwards anyway:

entry = maybe_mkwrite(pte_mkdirty(entry), vma);

But I'm sure it's not the whole picture...
Help from MM would indeed be appreciated.

Y
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