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From: John Hubbard <jhubbard@nvidia.com>
To: Peter Collingbourne <pcc@google.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Catalin Marinas <catalin.marinas@arm.com>
Cc: Evgenii Stepanov <eugenis@google.com>,
	<linux-arm-kernel@lists.infradead.org>, <linux-mm@kvack.org>
Subject: Re: [PATCH] mm: introduce reference pages
Date: Sun, 2 Aug 2020 20:28:08 -0700	[thread overview]
Message-ID: <92fa4a71-d8dc-0f07-832c-cbceca43e537@nvidia.com> (raw)
In-Reply-To: <20200731203241.50427-1-pcc@google.com>

On 7/31/20 1:32 PM, Peter Collingbourne wrote:
...

Hi,

I can see why you want to do this. A few points to consider, below.

btw, the patch would *not* apply for me, via `git am`. I finally used
patch(1) and that worked. Probably good to mention which tree and branch
this applies to, as a first step to avoiding that, but I'm not quite sure
what else went wrong. Maybe use stock git, instead of
2.28.0.163.g6104cc2f0b6-goog? Just guessing.

> @@ -1684,9 +1695,33 @@ static inline int accountable_mapping(struct file *file, vm_flags_t vm_flags)
>   	return (vm_flags & (VM_NORESERVE | VM_SHARED | VM_WRITE)) == VM_WRITE;
>   }
>   
> +static vm_fault_t refpage_fault(struct vm_fault *vmf)
> +{
> +	struct page *page;
> +
> +	if (get_user_pages((unsigned long)vmf->vma->vm_private_data, 1, 0,
> +			   &page, 0) != 1)
> +		return VM_FAULT_SIGSEGV;
> +

This will end up overflowing the page->_refcount in some situations.

Some thoughts:

In order to implement this feature, the reference pages need to be made
at least a little bit more special, and probably little bit more like
zero pages. At one extreme, for example, zero pages could be a special
case of reference pages, although I'm not sure of a clean way to
implement that.


The reason that more special-ness is required, is that things such as
reference counting and locking can be special-cased with zero pages.
Doing so allows avoiding page->_refcount overflows, for example. Your
patch here, however, allows normal pages to be treated *almost* like a
zero page, in that it's a page full of constant value data. But because
a refpage can be any page, not just a special one that is defined at a
single location, that leads to problems with refcounts.


> +	vmf->page = page;
> +	return VM_FAULT_LOCKED;

Is the page really locked, or is this a case of "the page is special and
we can safely claim it is locked"? Maybe I'm just confused about the use
of VM_FAULT_LOCKED: I thought you only should set it after locking the
page.


> +}
> +
> +static void refpage_close(struct vm_area_struct *vma)
> +{
> +	/* This function exists only to prevent is_mergeable_vma from allowing a
> +	 * reference page mapping to be merged with an anonymous mapping.
> +	 */

While it is true that implementing a vma's .close() method will prevent
vma merging, this is an abuse of that function: it depends on how that
function is implemented. And given that refpages represent significant
new capability, I think they deserve their own "if" clause (and perhaps
a VMA flag) in is_mergeable_vma(), instead of this kind of minor hack.



thanks,
-- 
John Hubbard
NVIDIA


  reply	other threads:[~2020-08-03  3:28 UTC|newest]

Thread overview: 10+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-07-31 20:32 [PATCH] mm: introduce reference pages Peter Collingbourne
2020-08-03  3:28 ` John Hubbard [this message]
2020-08-03  3:51   ` Matthew Wilcox
2020-08-13 22:03     ` Peter Collingbourne
2020-08-13 22:03   ` Peter Collingbourne
2020-08-03  9:32 ` Kirill A. Shutemov
2020-08-03 12:01   ` Catalin Marinas
2020-08-04  0:50     ` Peter Collingbourne
2020-08-04 15:27       ` Catalin Marinas
2020-08-04 15:48         ` Kirill A. Shutemov

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